JP2004109700A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus and an optical scanner which can effectively correct relative color shift among respective colors and output an excellent color image with small color shift even in continuous printing as to an image forming apparatus using an optical scanner which scan laser beams projected by a plurality of light source units by a deflecting means to expose a plurality of image carriers. <P>SOLUTION: Provided is a write start position correcting means (140) which has at least one wedgelike prism in the optical path extending from a light source unit (150) to the deflecting means (130) and can change a beam spot position in a vertical scanning direction by rotating and adjusting the wedgelike prism almost on the optical axis, and beam spot positions on a photoreceptor (160Y etc.) are based on displacement data also recorded beforehand or based on displacement data measured by displacement detection means (300b). <P>COPYRIGHT: (C)2004,JPO

Description

【００３８】
ここで、本発明はプリズムの頂角α［ｄｅｇ］は以下の（式３）の関係を満足するように設定することにより、ビームスポット位置ずれ補正を行うことを特徴としている。
【００３９】
１　＜　｜　ｍ×ｆｃ×（ｎ−１）×α｝　｜　＜　３０　　　…（式３）
（３式）において、上限を超えると、光束に波面収差を発生し、ビームスポット形状が乱れたり（サイドローブの発生）、ビームスポット径の太りを生じる。下限を超えると、感度が鈍すぎ、書き込み開始位置の調整のために大きな回転角を与える必要があり、経時変化補正時などの場合に高速に応答することができない。
【００４０】
つまり、（式３）の条件を満足することにより、ビームスポット径に太りを生じることなく、また、補正の感度が鈍すぎすこともなく、書き込み開始位置の調整のために大きな回転角を与えることなく、経時変化補正時などの場合に高速に応答することができる。なお、楔形状プリズム１の回転調整は、駆動源としてステッピングモータ、超音波モータなどを用いることにより容易に駆動制御可能である。
【００４１】
［３］請求項７の説明
転写ベルト２０５（中間転写体）は図１の例では転写ベルト２０５として構成したが、ドラム状に構成することもできる。このように、中間転写体をベルト状、ドラム状の回転体として構成した場合、中間転写体における副走査ドット位置変動のイメージ図を図４（ａ）に示す。
【００４２】
このように中間転写体は周期的に副走査方向にΔＺ（前記したようにこれは副走査方向補正量と同じ）のドット位置ずれを発生する。１周期は回転体からなる中間転写体の１回転にかかる時間Ｔ_ｍに相当する。中間転写体がドラム状またはベルト状の回転体で形成されており、かかる中間転写体（回転体）の１回転の長さをＬ_ｍ、線速度をｖ_ｍとしたとき、
Ｔ_ｍ　＝　Ｌ_ｍ／ｖ_ｍ　　　　…　（式４）で表され、
以下の（式５）に示す関係を満足することが望ましい。
０．５　＜　Ｌ_ｍ／ｖ_ｍ　　＜　５　　［ｓｅｃ］　　　　…　（式５）
上記（５式）において、上限を超えると、１周期が長過ぎるため振動などの外乱の影響を受けやすく、下限を超えると、ビームスポット位置補正制御に対する高い応答速度が要求されるため、楔形状プリズム回転の速度が追従することができない。従って、上記（５式）を満足することで、振動などの外乱の影響を受けにくく、また、ビームスポット位置補正制御に対する追従性もよい。
【００４３】
図４（ｂ）に本発明の実施によるビームスポット位置補正後の副走査方向での副走査ドットの位置ずれを示す。中間転写体上のドット位置ずれを書き込み光学系のビームスポット位置で補正することにより、低周波の大きな位置ずれ成分は良好に補正できている（但し、非常に高い周期の位置ずれまでは補正できない）。
【００４４】
［４］請求項３、４の説明
本発明の実施例を図５に示す。楔形状プリズムの回転調整手段はステッピングモータを駆動源とするリードスクリュー型のアクチュエータであることを特徴としている。
【００４５】
リードスクリュー型のアクチュエータの構成を説明すると、楔形状プリズム１はプリズムホルダー５に装着されている。ここで、プリズムホルダー５はコリメートレンズの光軸Ｏ−Ｏを回転軸として回転自在に支持されていて、その一部にアーム５ａが形成されている。
【００４６】
このアーム５ａの自由端側の上面には不動部材の間に介在する伸張性のばね６の下端側が接していて、該アーム５ａを押圧している。このため、楔形状プリズム１はプリズムホルダー５と共に光軸Ｏ−Ｏを回転軸（回転中心）として図５では時計まわりのモーメントが付与されている。
【００４７】
このモーメントによる楔形状プリズム１はプリズムホルダー５の回転は、アーム５ａの自由端側の下面に接している受け部材７により阻止されている。受け部材は円柱状をした軸体でその軸線方向、図中上側の先端部が円錐状をしており、この円錐の頂点部がアーム５ａの自由端側の下面に接している。この接している部分をアクチュエータ作用点Ｐと称する。
【００４８】
一方、受け部材７の上記頂点部の反対側、図中下側の端部にはナット８が固定（あるいは雌ねじが形成）されていて、このナット８にはステッピングモータ９の回転軸と一体の雄ねじが螺合されている。このねじをリードスクリューと称する。ステッピングモータ９は不動部材に固定されている。
【００４９】
かかる構成の回転手段をリードスクリュー型のアクチュエータと称し、ステッピングモータ９を駆動することにより、楔形状プリズム１はプリズムホルダー５と共に光軸Ｏ−Ｏを回転軸（回転中心）として回転する。
【００５０】
かかるリードスクリュー型のアクチュエータは、ステッピングモータ９を駆動源としているので、楔形状プリズム１の回転角をデジタルのパルス信号として駆動制御できるため、マイコンなどで位置ずれ量を演算した後、パルス信号として容易にフィードバック制御できる。
【００５１】
ここで、リニアアクチュエータの変動量（＝ナットの変位量＝作用点Ｐの変位量）Δｘは下式（式６）で与えられる。
Δｘ＝Ｒ×ｔａｎ（Δγ）　　　　　　　　　（式６）
（ここで、Ｒ：回転中心からアクチュエータ作用点までの距離、Δγ：楔形状プリズム回転角度とする。）
一方、前述のように中間転写体の１回転に要する時間Ｔ_ｍは次式（式４）で表される。
Ｔ_ｍ　＝　Ｌ_ｍ／ｖ_ｍ　［ｓｅｃ］　　　　　　　　　　（式４）
従って、感光体上の副走査方向補正量ΔＺを制御するために必要な単位時間当たりのステッピングモータの駆動周波数Ｎは下式（式７）で与えられる。
【００５２】
Ｎ＝Δｘ／ｐ×Ｎ_０／Ｔｍ　　　　　　　　（式７）
（ここで、ｐ：リードスクリューのねじピッチ、Ｎ_０：ステッピングモータ１回転当たりのパルス数とする。）
そこで、請求項４の発明では、以下の式を満足することにより、良好に色ずれを低減することができる。
１０　＜　Ｎ　＜　２０００　［ｐｐｓ］　　　　　　（式８）
上限（２０００ｐｐｓ、好ましくは１０００ｐｐｓ）を超えると、ステッピングモータが応答できなくなるためビームスポット位置ずれ補正が追従することができない。　また下限を超えると、分解能が粗すぎるため、ビームスポット位置補正精度が不十分となる。
【００５３】
一方、ステッピングモータのトルクＴは下式で与えられる。
【００５４】
【数２】

【００５５】
（ここで、ｐ：リードスクリューのねじピッチ、Ｔ１：ばね（６）のテンションで発生するトルク、Ｒ：回転中心と作用点との距離を表す。）
ステッピングモータ最大応答パルス数Ｎｍａｘは、図６に示すような規格表（モータの特性図）を用いて、トルクＴに対するプルインの駆動周波数から読みとることができる。
【００５６】
したがって、式７における感光体上の副走査方向補正量ΔＺを制御するために必要な単位時間当たりのパルス数Ｎは、
Ｎ　＜　Ｎｍａｘ　　　（式１０）
の条件を満たす必要がある。
【００５７】
［５］請求項８の説明
図１に示したカラー画像形成装置の要部において、また、後述する図８のカラー画像形成装置において、副走査方向の色ずれを補正するためには、各色に対応する各走査光学系の走査線の位置を、それぞれについて中間転写体上に重ね合わせた時のずれがゼロに近くなるように補正することも考えられる。
【００５８】
しかしながら、４色に対応するそれぞれのレーザービームの書き込み開始位置（走査位置）を調整する際に、４つの全部をそれぞれ所定の基準位置に合せようとすると、誤って大きな回転偏心を与え、光学性能（ビームスポット径）が劣化する恐れがある。また書き込み開始位置補正手段の部品点数も多くなりコストアップを引き起こす。
【００５９】
そこで本発明は、４つのレーザービームの中の１つを基準とし、それに対して他の３つが合致するようにすることとし、その基準となる色を特定した。つまり、Ｙ（イエロー）、Ｍ（マゼンタ）、Ｃ（シアン）、Ｋ（黒）の４つの色のうち、基準色を黒色（ブラック）としたことを特徴としている。
【００６０】
走査光学系のうち黒の画像に対応するレーザービームを基準とし、その基準色のレーザービームによる走査位置に略一致するように、基準色以外の走査光学系からのレーザービームの走査位置を補正することを特徴としている。
【００６１】
これにより、４色の内、３色を調整すればよいため、楔形状プリズム１は３つ用いることで足りる。即ち、「相対的な色ずれ」の補正を行うことで、色調の変化を十分に抑えた色再現性の高い画像を得ることができる。従って、例えば、図１に示すように、書き込み開始位置補正手段１４０も３つあればよい。
【００６２】
本発明によれば、基準色を黒色としているため次のような利点を得ることができる。
黒色は他の色に対しコントラストが高いため、振動、温度変動などの外乱によるビームスポット径、ビームスポット位置変動劣化の影響が画像に現れやすい。従って、ブラックを基準色とすることで、かかるブラック用のレーザービームを扱う走査光学系の各光学部品を剛性高く固定することができ、外乱の影響を受けにくい走査光学系を実現することができる。
【００６３】
［６］請求項５の説明
図３に示したとおり、複数枚の画像を連続プリント出力するなどの場合は、光走査装置内部ではポリゴンミラー１３０駆動用のポリゴンモーター（図示せず）、光源１５０ａである半導体レーザーから発熱を生じ、光走査装置外部では、定着手段においてトナー定着時のヒーター熱などの影響により、画像形成装置内部の温度は急激に変化する。この場合、感光体上のビームスポット位置も急激に変動し、１枚目、数枚目、数十枚目と次第に出力カラー画像の色合いが変化することが大きな課題となっている。
【００６４】
そこで本例は、図１において、各感光体１６０Ｙ、１６０Ｍ、１６０Ｃ、１６０Ｋ上での走査ビームの副走査方向の相対的な位置ずれを検知する位置ずれ検知手段（ビームスポット位置検知手段と兼用）と、光源から偏向手段に至る光路中に、楔形状のプリズムを少なくとも１つ有し、該プリズムを略光軸回りに回転調整することにより、副走査方向のビームスポット位置を可変とする書き込み開始位置補正手段１４０を有し、前記位置ずれ検知手段により計測された位置ずれデータに基づき、画像データ書込み中に感光体上のビームスポット位置を制御する光走査装置を構成している。
【００６５】
本例では、位置ずれ検知手段は、光走査装置の主走査方向の書き込み領域外に設けた非平行フォトダイオードセンサーによりビームスポット位置を検知している。本素子（非平行フォトダイオード（ＰＤ））は各感光体１６０Ｙ、１６０Ｍ、１６０Ｃ、１６０Ｋのそれぞれに対する走査ビームの有効書き込み領域外（図１におけるビームスポット位置検知手段３００ａ、３００ｂなど）に設けるのが望ましく、その際、主走査方向の書き込み開始位置を決定する同期信号を検知する機能を兼ねても構わない。本例では兼ねている。
【００６６】
図７において、フォトダイオードＰＤ１（ＰＤ１’）の受光面は走査ビームに直交し、フォトダイオードＰＤ２（ＰＤ２’）の受光面はフォトダイオードＰＤ１（ＰＤ１’）の受光面に対して傾いている。この傾き角をα１とする。また、上記ヒーター熱による温度変化前の走査ビームをＬ１、温度変化後の走査ビームをＬ２としたとき、副走査方向にΔＺ（未知）ずれたとする。この場合、１対の非平行フォトダイオード間、例えば、非平行フォトダイオードＰＤ１とＰＤ２との間（或いは、非平行フォトダイオードＰＤ１’とＰＤ２’との間）を走査ビームＬ１、Ｌ２が通過する時間Ｔ１、Ｔ２を計測し、Ｔ２−Ｔ１の時間差を求めることにより、副走査方向の走査位置（書き込み開始位置）をモニター、検知する。
【００６７】
副走査方向の相対的なドット位置ずれ（＝副走査方向補正量ΔＺ）は、ＰＤ１とＰＤ２との各受光面間のなす角度α１と、時間差Ｔ２−Ｔ１が既知であるので、計算により容易に求めることができる。この補正量を、書き込み開始位置補正手段１４０により補正する。
【００６８】
本発明によれば、複数枚の画像を連続プリント出力するなどの場合に、感光体上のビームスポット位置が温度変化などにより急激に変動する場合においても、画像データ書込み中においても感光体上のビームスポット位置を補正可能である。
【００６９】
なお、フォトダイオードＰＤ１’とＰＤ１との間を走査ビームが通過するに要する時間Ｔ０の変動を検知することにより、主走査方向の倍率変動をモニターすることも可能である。
［７］請求項９の説明
前記［６］請求項５の説明において、図１の例でいえば、各感光体１６０Ｙ、１６０Ｍ、１６０Ｃ、１６０Ｋ上での走査ビームの副走査方向の相対的な位置ずれを検知する位置ずれ検知手段（ビームスポット位置検知手段３００ａ、３００ｂなど）、具体的には例えば、非平行フォトダイオードＰＤ１、ＰＤ２による、走査ビームＬ１とＬ２の検知時間Ｔ_Ｓ（位置ずれ検知を開始して、検知完了までの時間）は以下を満足することを特徴とする。
Ｔ_Ｓ　＜　１０×（Ｌ_ｐ／Ｖ_ｐ）　　　　　　（式１１）
（ここで、Ｌｐ：プリント出力用紙（記録紙）の出力方向（図１で矢印で示す転写ベルト２０５の移動方向）の長さ、Ｖｐ：感光体の線速とする。）
この（式１１）は、少なくともプリントを５枚出力する間の、走査線位置ずれを検知することにより、急激な温度変動を生じた場合でも、色調変化が肉眼で確認できないことを示しており、検知時間Ｔ_Ｓが上式の範囲を外れた場合は、色調が問題となる可能性があることを表している。
【００７０】
など、検知時間Ｔ_Ｓは、位置ずれ補正量を演算する時間も含んでいる（演算とはノイズ低減のために平均化、異常値処理などの位置ずれ検知精度を高め、走査線補正手段にフィードバックする補正量を算出する時間を表すものとする。）
［８］請求項６の説明
本発明の光走査装置を持つカラー画像形成装置の一例として、図１に示したものの他にも次の例を挙げることができる。図７には、複数の感光体（像担持体）上を光走査装置で露光して静電潜像を形成したのち現像し、各感光体上の可視像を転写ベルトに重ね転写し、さらにこの転写ベルト上の重ね転写像を同一のシート状媒体（記録紙）に一括転写してカラー画像を得るタンデム型カラー画像形成装置をその全体概要について示している。
【００７１】
まず、装置内の下部側には水平方向に配設されて給紙カセット１０から給紙される転写紙（図示せず）を搬送する中間転写ベルトとしての転写ベルト２０５’が設けられている。この搬送ベルト２０５’上にはイエローＹ用の感光体１６０Ｙ’、マゼンタＭ用の感光体１６０Ｍ’、シアンＣ用の感光体１６０Ｃ’及びブラックＫ用の感光体１６０Ｋ’が上流側から順に等間隔で配設されている。
【００７２】
なお、以下、符号に対する添字Ｙ、Ｍ、Ｃ、Ｋを適宜付けて区別するものとする。これらの感光体１６０Ｙ’、１６０Ｍ’、１６０Ｃ’、１６０Ｋ’は全て同一径に形成されたもので、その周囲には、電子写真プロセスに従いプロセス部材が順に配設されている。
【００７３】
感光体１６０Ｙ’を例に採れば、帯電手段としての帯電チャージャ１６１Ｙ、走査結像光学系１６２Ｙ、現像手段としての現像装置１６３Ｙ、転写手段としての転写チャージャ１６４Ｙ、クリーニング手段としてのクリーニング装置１６５Ｙ等が順に配設されている。他の感光体１６０Ｍ’、１６０Ｃ’、１６０Ｋ’に対しても同様である（但し、符号は省略した）。即ち、本実施の形態では、感光体１６０Ｙ’、１６０Ｍ’、１６０Ｃ’、１６０Ｋ’を各色毎に設定された露光、走査される被照射面とするものであり、各々に対して走査結像光学系１６２Ｙ、１６２Ｍ、１６２Ｃ、１６２Ｋが１対１の対応関係で設けられている。
【００７４】
また、転写ベルト２０５’の周囲には、感光体１６０Ｙ’よりも矢印で示す搬送方向上流側に位置させてレジストローラ１６６と、ベルト帯電手段としてのチャージャ１６７が設けられ、感光体１６０Ｋ’よりも下流側に位置させてベルト分離チャージャ１６８、除電チャージャ１６９、クリーニング装置１７０等が順に設けられている。また、ベルト分離チャージャ１６８よりも搬送方向下流側には定着手段としての定着装置１８０が設けられ、排紙トレイ１７０に向けて排紙ローラ１７１で結ばれている。
【００７５】
このような概略構成において、例えば、フルカラーモード（複数色モード）時であれば、各感光体１６０Ｙ’、１６０Ｍ’、１６０Ｃ’、１６０Ｋ’に対してＹ、Ｍ、Ｃ、Ｋ用の各色の画像信号に基づき各々の走査結像光学系による光ビームの光走査で静電潜像が形成される。
【００７６】
これらの静電潜像は現像装置１６３Ｙなど、各々の対応する色トナーの現像装置により現像されてトナー像となり、転写ベルト２０５’上に静電的に吸着されて搬送される転写紙上に順次転写されることにより重ね合わせられ、フルカラー画像として定着された後、排紙される。
【００７７】
また、黒色モード（単色モード）時であれば、感光体１６０Ｙ’、１６０Ｍ’、１６０Ｃ’及びそのプロセス部材は非動作状態とされ、感光体１６０Ｋ’Ｋに対してのみ黒色用の画像信号に基づき走査結像光学系１６２Ｋによる光ビームの光走査で静電潜像が形成される。この静電潜像は黒色トナーで現像されてトナー像となり、転写ベルト２０５’上に静電的に吸着されて搬送される記録紙上に転写されることにより、黒色なるモノクロ画像として定着された後、排紙される。
【００７８】
なお、１２０Ｍ１、１２０Ｍ２は２枚玉のｆθレンズであり、各々ｆθレンズは光学ハウジング３１に固定されているが、その際にプレート３３Ｍ上に載置されている。プレート３３Ｍはｆθレンズ１２０Ｍ１、１２０Ｍ２の当接面側の全面又は一部と接触している。ｆθレンズ１２０Ｍ１、１２０Ｍ２の材質は非球面形状が容易かつ低コストなプラスチック材質からなり、具体的には低吸水性や高透明性、成形性に優れた合成樹脂が好適である。本例では、ポリゴンミラーは符号１３０Ｕで示す上段側ミラー、符号１３０Ｄで示す下段側ミラーからなる。
【００７９】
かかるカラー画像形成装置において、光学ハウジング３１に構成された光走査装置には、これまで説明した［１］〜［７］の説明にかかる発明が適用される。なお、図８には図示してないが、図１に示したような色ずれ検知用センサ３３０を適宜設けることもできる。
【００８０】
ここで多数枚のカラー画像を連続プリントした場合、特に光走査装置内のポリゴンモータの発熱によと定着装置により急激な温度変動を発生する。そのためファーストプリントと複数枚出力した後のカラー画像において、色調が変動することが課題となっており、偏向手段と露光走査される被走査面の間に、楔形状プリズムを具備する光走査装置を用いることで、走査位置ずれを補正し、高精度な走査位置精度を実現できると共に、特に連続プリント出力時などのように急激に温度が変動する場合にも、色ずれの少ない良好なカラー画像を得ることができる。また、各発明に対応した利点がある。
【００８１】
［１０］数値実施例
（１）前提条件
・光学系性能
ｍ＝９．４、　ｆｃ＝１５　［ｍｍ］，
・楔形状プリズム
α＝２°、ｎ＝１．５１（ＢＫ７）、
・リードスクリュー型アクチュエータ
Ｔ１＝２５×１０^−３　［Ｎｍ］、ｐ＝０．３　［ｍｍ］，　Ｒ＝１６　［ｍｍ］、Ｎ_０＝２０　［ｐｕｌｓｅ／１回転］
・中間転写体
Ｌｍ＝５００ｍｍ，　ｖｍ　＝　２５０　ｍｍ／ｓ
（２）計算例
｜　ｍ×ｆｃ×（ｎ−１）×α｝　｜＝｜９．４×１５×（１．５１−１）×（２°／１８０°×π）｜＝２．５
であり請求項２の範囲を満足する。
【００８２】
【数３】

【００８３】
Δｘ＝Ｒ×ｔａｎ（Δγ）＝１６×ｔａｎ（０．２）＝３．２　［ｍｍ］
Ｔｍ＝Ｌｍ／ｖｍ＝２　［ｓｅｃ］
であり、請求項７の範囲を満足する。
【００８４】
Ｎ＝Δｘ／ｐ×Ｎ_０／Ｔｍ　＝　３．２／０．３×２０／２＝１０６　［ｐｐｓ］
であり、請求項４の範囲を満足する。
【００８５】
【数４】

【００８６】
図６と同様の規格表からＮｍａｘを読みとり、
Ｎｍａｘ＞Ｎを満足するステッピングモータを使用。
【００８７】
【発明の効果】
請求項１記載の発明では、簡易な機構、手段により楔形状プリズムを動作させることで、走査位置ずれを補正し、高精度な走査位置精度を実現できる。
【００８８】
請求項２記載の発明では、ビームスポット径に太りを生じることなく、また、補正の感度が鈍すぎすこともなく、書き込み開始位置の調整のために大きな回転角を与えることなく、経時変化補正時などの場合に高速に応答することができる。
【００８９】
請求項３記載の発明では、ステッピングモータを駆動源としているので、楔形状プリズムの回転角をデジタルのパルス信号として駆動制御できるため容易にフィードバック制御できる。
【００９０】
請求項４記載の発明では、ビームスポット位置ずれ補正を精度よく行なうことで良好に色ずれを低減することができる。
【００９１】
請求項５記載の発明では、ビームスポット位置が急激に変動する場合においてもビームスポット位置を補正可能である。
【００９２】
請求項６記載の発明では、楔形状プリズムを具備する光走査装置を用いることで、走査位置ずれを補正し、高精度な走査位置精度を実現できると共に、特に連続プリント出力時などのように急激に温度が変動する場合にも、色ずれの少ない良好なカラー画像を得ることができる。
【００９３】
請求項７記載の発明では、振動などの外乱の影響を受けにくく、また、ビームスポット位置補正制御に対する追従性もよい。
【００９４】
請求項８記載の発明では、ブラックを基準色とすることで、かかるブラック用のレーザービームを扱う走査光学系の各光学部品を剛性高く固定することができ、外乱の影響を受けにくい走査光学系を実現することができる。
【００９５】
請求項９記載の発明では、急激な温度変動を生じた場合でも、色調変化が肉眼で確認できない程度の画質を得ることができる。
【図面の簡単な説明】
【図１】光走査装置を含むカラー画像形成装置の要部構成を示した斜視図である。
【図２】楔形状プリズムによる副走査ビームスポット位置補正原理を説明した図である。
【図３】連続プリント時の光走査装置内温度変化を例示した図である。
【図４】図４（ａ）は中間転写体上の速度変動に伴う副走査ドット位置ずれを説明した図、図４（ｂ）はビームスポット位置補正後の副走査ドット位置ずれを説明した図である。
【図５】書き込み開始位置補正手段としての、リードスクリュー型のアクチュエータを説明した概略構成図である。
【図６】ステッピングモータの駆動周波数とトルクの関係を示した図である。
【図７】ビームスポット位置検知手段としての非平行フォトダイオードセンサーによる検知原理を説明した図である。
【図８】カラー画像形成装置の一例を説明した図である。
【符号の説明】
１　楔形状プリズム
１４０　書き込み開始位置補正手段
１６０Ｙ、１６０Ｍ、１６０Ｃ、１６０Ｋ、１６０Ｙ’、１６０Ｍ’、１６０Ｃ’、１６０Ｋ’　感光体
３００ａ、３００ｂ　（位置ずれ検知手段としての機能を有する）ビームスポット位置検知手段[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical scanning device and an image forming apparatus, and more particularly to an optical scanning device of a digital color copying machine and a color laser printer, and an image forming apparatus using the same.
[0002]
[Prior art]
In recent years, with the speeding up of color image forming apparatuses, for example, four photosensitive drums are arranged in the direction of conveyance of recording paper, and latent images are simultaneously exposed by a plurality of scanning optical systems corresponding to these photosensitive drums. These latent images are visualized by a developing device using developers of different colors such as yellow, magenta, cyan, and black, and then these visible images are sequentially superimposed on the same recording paper. Digital copying machines and laser printers that obtain a color image by transferring a color image have been put to practical use (a so-called 4-drum tandem system).
[0003]
It has only one photoreceptor drum, rotates by the number of colors, performs exposure, development, and transfer each time this rotation is performed, and superimposes the visible image on the same recording paper (or temporarily transfers it to an intermediate transfer member). There is also a one-drum type in which a color image is formed by superimposing and transferring the image onto a recording paper).
[0004]
The four-drum tandem method is advantageous for high-speed printing because color and monochrome can be output at the same speed as one-drum method. On the other hand, there are four scanning optical systems corresponding to the four photoconductors, and the size of the apparatus for performing exposure tends to be large, and miniaturization is an issue. Another problem is to reduce color misregistration at the time of superimposing and transferring the toner images written and developed on the respective photoconductors onto recording paper.
Here, the following factors can be cited as causes of color misregistration particularly in the sub-scanning direction.
[0005]
(1) Uneven feed speed in the circumferential direction (sub-scanning direction) of the photoconductor
(2) Uneven feed speed in the circumferential direction (sub-scanning direction) of the intermediate transfer body
(3) Position error between photoconductors
(4) Beam spot writing position shift between scanning optical systems
(5) misalignment due to environmental fluctuations in (1) to (4) above or temperature fluctuations during continuous printing, etc.
{Circle over (6)} When writing is performed simultaneously on each photoconductor with a multi-beam, the rotation of the polygon scanner and the photoconductor feed speed are generally asynchronous, so that there is a possibility that the rotation will be shifted by the number of beams in the sub-scanning direction.
[0006]
The following method is known as a method for reducing such color shift.
A. In an image forming apparatus using a plurality of scanning units, there is disclosed an invention in which the position of each scanning unit (housing) as a whole is adjusted with respect to a photoconductor, and scanning lines on the respective photoconductors are matched. However, the mechanism for adjustment becomes complicated, and it takes an adjustment time. In addition, since a heavy housing is adjusted, it is difficult to cope with a temporal change due to a temperature change or the like, and it is difficult to accurately correct color misregistration during printing or in an environment of use (for example, see Patent Document 1). 1).
[0007]
B. As another solution to the above problem, a method of controlling a sub-scanning beam position using a galvanomirror has been proposed. However, since the galvanometer mirror is too sensitive to control the sub-scanning position, it is susceptible to external vibration, and high surface accuracy is required to secure a better beam spot diameter (about the transmission surface). (For example, see Patent Document 2).
[0008]
C. As a means for solving the problem of misalignment between multi-beams, sub-scanning is performed by switching a laser beam for writing an image on a photosensitive member first among a plurality of laser beams according to a phase relationship between an intermediate transfer reference signal and a line synchronization signal. There has been proposed a color image forming apparatus including a correction unit that corrects a color shift by adjusting an image writing start position for each color in the direction (for example, see Patent Document 3). However, even with this method, it is difficult to correct one line or less. For example, in the case of 600 dpi writing, a color shift of at least 42 μm occurs.
[0009]
On the other hand, in a tandem type full-color copying machine, four photosensitive drums corresponding to each color of cyan (C), magenta (M), yellow (Y), and black (K) are arranged along the transfer surface of the transfer belt. Arranged, a beam scanning device scans a beam provided corresponding to each photosensitive drum, forms an electrostatic latent image on the peripheral surface of the photosensitive drum, and visualizes the latent image with toner of a corresponding color, The multi-color image is formed by sequentially superimposing and transferring this on the recording paper conveyed by the transfer belt, so that if the scanning position shifts in the sub-scanning corresponding direction for each color, the image quality will be degraded. It causes deterioration and color shift.
[0010]
Here, the causes of the color misregistration particularly in the sub-scanning direction are as follows.
(1) Uneven feed speed in the circumferential direction (sub-scanning direction) of the photoconductor
(2) Uneven feed speed in the circumferential direction (sub-scanning direction) of the intermediate transfer body
(3) Position error between photoconductors
(4) Beam spot writing position shift between scanning optical systems
(5) misalignment due to environmental fluctuations in (1) to (4) above or temperature fluctuations during continuous printing, etc.
{Circle over (6)} When writing is performed simultaneously on each photoconductor with a multi-beam, the rotation of the polygon scanner and the photoconductor feeding speed are generally asynchronous, and thus may be shifted by the number of beams in the sub-scanning direction.
[0011]
[Patent Document 1]
JP 2001-133718 A
[Patent Document 2]
JP 2001-100127 A
[Patent Document 3]
JP-A-10-239939
[0012]
[Problems to be solved by the invention]
An object of the present invention is to provide an optical scanning device and an image forming apparatus capable of effectively correcting relative color misregistration between each color even during continuous printing and outputting a good color image with little color misregistration. is there.
[0013]
[Means for Solving the Problems]
The present invention has the following configuration to achieve the above object.
(1). In an optical scanning device that scans a laser beam emitted from a plurality of light source devices by a deflecting unit and exposes a plurality of image carriers, at least one wedge-shaped prism is provided in an optical path from the light source device to the deflecting unit. And a writing start position correcting means for changing the beam spot position in the sub-scanning direction by adjusting the rotation of the wedge-shaped prism about the optical axis. The beam spot position on the image carrier is controlled during writing (claim 1).
(2). In the optical scanning device described in (1), the light source device has at least a laser light source and a collimating lens, and the wedge-shaped prism satisfies the following condition (claim 2).
[0014]
1 <| m * fc * (n-1) * [alpha] | <30
Where α is the vertex angle of the wedge-shaped prism,
n: refractive index of prism glass material,
fc: focal length of collimating lens
m: Sub-scanning lateral magnification from the light source to the surface to be scanned
(3). In the optical scanning device described in (1) or (2), the rotation adjusting means of the wedge-shaped prism is a lead screw type actuator using a stepping motor as a driving source.
(4). In the optical scanning device described in (3), the driving frequency N of the stepping motor satisfies the following relationship (claim 4).
[0015]
N = Δx / p × N ₀ / Tm
10 <N <1000 [pps]
Here, Δx: maximum displacement of the lead screw,
p: Lead screw thread pitch
N ₀ : Number of pulses for one rotation of the stepping motor,
Tm: time of one rotation of the intermediate transfer member [sec]
(5). In a light scanning device for a color machine that scans laser beams emitted from a plurality of light source devices by deflecting means and exposes a plurality of image carriers, a relative displacement in a sub-scanning direction on each image carrier is determined. A displacement detecting unit for detecting, and at least one wedge-shaped prism in an optical path from the light source device to the deflecting unit, and rotating and adjusting the wedge-shaped prism about an optical axis in the sub-scanning direction. Having a write start position correcting means for making the beam spot position variable, and controlling the beam spot position on the image carrier during image data writing based on the positional shift data measured by the positional shift detecting means. (Claim 5).
(6). A plurality of image carriers are exposed by an optical scanning device to form an electrostatic latent image and then developed, and the visible images on each image carrier are overlaid and transferred onto an intermediate transfer member. In an image forming apparatus which collectively transfers a superimposed transfer image onto the same sheet-like medium to obtain a color image, the optical scanning device according to any one of claims 1 to 5 is used as the optical scanning device (claim 6).
(7). (6) In the image forming apparatus described in (6), the intermediate transfer body is configured as a rotating body, and the length of one rotation is L. _m , The linear velocity of the intermediate transfer _m In this case, the following condition is satisfied (claim 7).
[0016]
0.5 <L _m / V _m <5 [sec]
(8). (6) In the image forming apparatus described in (6), the plurality of image carriers correspond to four colors of cyan, magenta, yellow, and black, and black is used as a reference color to correct a relative color shift with respect to black. (Claim 8).
(9). (6) In the image forming apparatus described in (6), the detection time T of the displacement detection means is _S (Time from start of displacement detection to completion of detection) satisfies the following condition (claim 9).
T _S <10 × (L _p / V _p )
here,
L _p : Length of printout paper in the output direction,
v _p ： Line speed of photoconductor
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
[1] Description of Claim 1
FIG. 1 shows a main part of an optical scanning device according to an embodiment, together with a drum-shaped image carrier (hereinafter, photoconductor) and a conveyance belt which form a part of a color image forming apparatus. Around the photoreceptor, processing members such as a charging unit and a cleaning unit for removing residual toner after transfer are arranged in accordance with an electrophotographic process, and a paper cassette for storing and supplying recording paper is provided below the transfer belt 205. Paper transfer means, a transfer means at a position inside the transfer belt 205 facing the photoreceptor, a belt charging means at an upstream position in the rotation direction of the transfer belt 205 as indicated by an arrow, and a belt separation charger and a fixing means at a downstream position. However, these are well known, have a configuration according to FIG. 8 described later, and are not shown here because they are complicated.
[0018]
In this example, a plurality of photoconductors are exposed by an optical scanning device to form an electrostatic latent image, and then developed, and the visible images on each photoconductor are superimposedly transferred onto a transfer belt 205. 1 shows a main part of an image forming apparatus for obtaining a color image by collectively transferring the superimposed transfer image onto the same sheet-like medium (recording paper).
[0019]
The optical scanning device emits a light beam (also referred to as a laser beam or a light beam) for forming a toner image for color misregistration detection every predetermined number of prints. As a result, when the color misregistration detecting toner images 330 </ b> Z are formed on the transfer belt 205 in three places, color misregistration due to these toner images is detected by the color misregistration detecting sensor 330.
[0020]
The optical scanning device includes a box-shaped optical housing (not shown) and a light source device 150, a polygon mirror 130, the various lenses 209 and 120, a mirror 110M, a writing start position correcting unit 140, and a position deviation detecting unit. It is a unit that supports members including a scanning image forming optical system such as beam spot position detecting means 300a and 300b having a function, and is located above the photoconductors 160Y, 160M, 160C and 160K. The writing start position correcting means 140 includes a lead screw type actuator using a stepping motor as a driving source, as will be described later with reference to FIG. 5, as a rotation adjusting means for the wedge-shaped prism.
[0021]
The color shift detecting sensor 330 installed on the image forming apparatus main body side is also used as a position shift detecting unit, and is used for obtaining a correction amount by the writing start position correcting unit 140, and the beam spot position detecting units 300a and 300b are used. It can be used together with. As a usage method in the case of the combined use, for example, the color shift detection sensor 330 is used for coarse adjustment, and the beam spot position detection means 300a and 300b are used for fine adjustment. The writing start position correcting means 140 controls the correction of the beam spot position on the photosensitive member during the writing of the image data based on the detection result of the positional deviation.
[0022]
The four photoconductors 16Y, 16M, 16C, and 16K, which are surfaces to be scanned by a laser beam (also referred to as a light beam, a scanning beam, or the like) from the optical scanning device, are arranged along a straight line and are driven to rotate. Has become. At the time of image formation, a latent image to be developed with toner images of different colors is formed on the photoconductors 160Y, 160M, 160C, and 160K by a scanning laser beam from an optical scanning device, and after development, the intermediate image is formed as an intermediate transfer body. These color toner images are transferred onto the transfer belt 205 in a superimposed manner.
[0023]
The optical scanning device has four light source devices 150 that emit laser beams for forming latent images of different colors corresponding to the respective photoconductors 160Y, 160M, 160C, and 160K. Explaining one of them as a representative, it has at least a semiconductor laser 150a as a light source and a collimate lens 150b.
[0024]
The laser beams from the four light source devices 150 pass through the cylinder lens 209, pass through the writing start position correcting unit 140, and then go to the polygon mirror 130 as a deflecting unit.
[0025]
Further, the light is deflected and scanned by the polygon mirror 130, passes through the fθ lens 120, the return mirror 110M, and the toroidal lens 100, and is exposed and scanned on each of the photoconductors 160Y, 160M, 160C, and 160K. The position deviation in the sub-scanning direction, which is the beam spot position on each photoconductor, is determined by using the write start position correction unit 140 based on the position deviation data recorded in advance or the position deviation data measured by the position deviation detection unit. Is corrected.
[0026]
The laser beams from the four light source devices 150 pass through the scanning optical system from the cylinder lens 209 to the toroidal lens 100, respectively, and reach the corresponding photoconductors. Here, the polygon mirror 130 is directly connected to a polygon motor (not shown) and is driven to rotate.
[0027]
The longitudinal directions of the photoconductors 160Y, 160M, 160C, and 160K correspond to the main scanning direction, and the beam spot position detection units 300a and 300b face each other on both sides of the effective image area of each photoconductor. Are located. 300a is for detecting a write start position, and 300b is for detecting a write end position.
[0028]
According to the present invention, the laser beam emitted from the plurality of light source devices 150 is scanned by the polygon mirror 150 to expose the plurality of photoconductors 160Y, 160M, 160C, and 160K. The optical path to the mirror 150 has at least one wedge-shaped prism (hereinafter, a wedge-shaped prism).
[0029]
The wedge-shaped prism and the wedge-shaped prism are rotated around the optical axis of the substantially collimating lens 150b to constitute a writing start position correcting unit 140 that makes the beam spot position in the sub-scanning direction variable.
[0030]
Here, the principle of sub-scanning beam spot position correction by the wedge-shaped prism will be described with reference to FIG.
In FIG. 2, the wedge-shaped prism is denoted by reference numeral 1 and has a wedge-like (trapezoidal) shape. For example, when the reference numeral OO is the optical axis of the collimating lens 150 b, the arrow 3 is approximately around the optical axis OO. By rotating as shown, the incident beam can be deflected within the range of the maximum deflection angle φ as shown by arrow 4, and as a result, the position of the sub-scanning beam spot on the surface to be scanned can be variably adjusted. Can be.
[0031]
The use of the wedge-shaped prism 1 has the following advantages with respect to the following conventional methods.
This is a control related to a simple operation of rotating a wedge-shaped prism with a simple configuration.Beam spot position correction during image data writing compared to position correction control for each batch, such as when starting up the machine or before printing output When the temperature fluctuates abruptly, for example, between the rising section A and the time of continuous printing (area B in FIG. 3) as shown in FIG. Even in the case of occurrence of color shift, the color shift can be greatly reduced because this shift can be corrected almost in real time based on the position shift detection means and position shift data from the position shift data, which will be described later with reference to FIG. Can be reduced.
[0032]
In FIG. 2, an appropriate sensitivity can be set by setting the prism vertex angle α to an appropriate angle. Therefore, unlike the galvanomirror method, the sensitivity is not too high, and the influence of the vibration is small, and the beam spot can be positioned with high accuracy.
[0033]
Since the weight of the wedge-shaped prism 1 and the prism holder 2 is relatively light by selecting a material (for example, resin) and a shape (for example, thin shape), a comparison between a long folding mirror, a scanning lens, a roof mirror, a light source unit, and the like is made. Compared to the conventional method in which the beam spot position is corrected by tilting / shifting an optical element having a large target weight, the response speed is faster, and it is possible to correct even a high-frequency position shift.
[0034]
In addition, compared to a method of correcting a beam spot position by changing an applied voltage, such as a liquid crystal deflecting element and an electro-optical element (PLZT, etc.), the present invention can be realized at a low cost without any displacement even when the power is turned off. .
[0035]
In our experience, by keeping the relative color shift amount to 30 μm or less, it is possible to obtain a condition in which the actual color shift is inconspicuous, and the present invention makes it possible to realize such a condition.
[0036]
[2] Description of Claim 2
As shown in FIG. 2, by rotating the wedge-shaped prism 1 around the optical axis OO of the collimating lens 150b, the deflection angle is variable within the range of the maximum deflection angle φ due to refraction. The maximum deflection angle φ can be expressed by the following (Equation 1), where α is the vertex angle of the wedge-shaped prism and n is the refractive index of the wedge-shaped prism 1 (prism glass material).
φ = (n−1) × α (Equation 1)
When the focal length of the collimating lens 150b is fc, the sub-scanning lateral magnification of the entire optical system from the light source to the surface to be scanned is m, and the adjustment angle of the wedge-shaped prism 1 around the optical axis OO is Δγ. The correction amount ΔZ in the sub-scanning direction on the photoconductor surface is represented by the following (Equation 2).
[0037]
(Equation 1)

[0038]
Here, the present invention is characterized in that the deviation of the beam spot position is corrected by setting the vertex angle α [deg] of the prism so as to satisfy the following expression (3).
[0039]
1 <| m * fc * (n-1) * [alpha] | <30 (Equation 3)
In Equation (3), if the upper limit is exceeded, a wavefront aberration is generated in the light beam, and the beam spot shape is disturbed (side lobes are generated) and the beam spot diameter is increased. If the lower limit is exceeded, the sensitivity is too low, and it is necessary to give a large rotation angle to adjust the writing start position, and it is not possible to respond at high speed in the case of correction for a change over time.
[0040]
In other words, by satisfying the condition of (Equation 3), a large rotation angle is provided for adjusting the writing start position without increasing the beam spot diameter and without making the correction sensitivity too low. Therefore, it is possible to respond at a high speed in the case of time-dependent change correction. The rotation of the wedge-shaped prism 1 can be easily controlled by using a stepping motor, an ultrasonic motor, or the like as a driving source.
[0041]
[3] Description of Claim 7
The transfer belt 205 (intermediate transfer body) is configured as the transfer belt 205 in the example of FIG. 1, but may be configured in a drum shape. When the intermediate transfer member is configured as a belt-shaped or drum-shaped rotating member, an image diagram of the sub-scanning dot position variation in the intermediate transfer member is shown in FIG.
[0042]
In this way, the intermediate transfer member periodically generates a dot position shift of ΔZ (which is the same as the correction amount in the sub-scanning direction as described above) in the sub-scanning direction. One cycle is the time T required for one rotation of the intermediate transfer body composed of a rotating body. _m Is equivalent to The intermediate transfer member is formed of a drum-shaped or belt-shaped rotary member, and the length of one rotation of the intermediate transfer member (rotary member) is L. _m , Linear velocity v _m And when
T _m = L _m / V _m … (Expression 4)
It is desirable to satisfy the relationship shown in the following (Equation 5).
0.5 <L _m / V _m <5 [sec] (Equation 5)
In the above equation (5), when the value exceeds the upper limit, one cycle is too long, which is susceptible to disturbance such as vibration. When the value exceeds the lower limit, a high response speed to the beam spot position correction control is required. The rotation speed of the prism cannot follow. Therefore, by satisfying the above (Equation 5), it is less likely to be affected by disturbance such as vibration, and the follow-up property to the beam spot position correction control is good.
[0043]
FIG. 4B shows the positional deviation of the sub-scanning dots in the sub-scanning direction after the beam spot position correction according to the embodiment of the present invention. By correcting the dot position shift on the intermediate transfer member with the beam spot position of the writing optical system, a large low-frequency position shift component can be satisfactorily corrected (however, it cannot be corrected up to a position shift having a very high cycle). ).
[0044]
[4] Description of Claims 3 and 4
FIG. 5 shows an embodiment of the present invention. The rotation adjusting means of the wedge-shaped prism is a lead screw type actuator driven by a stepping motor.
[0045]
Explaining the configuration of the lead screw type actuator, the wedge-shaped prism 1 is mounted on a prism holder 5. Here, the prism holder 5 is rotatably supported around the optical axis OO of the collimating lens as a rotation axis, and an arm 5a is formed in a part thereof.
[0046]
The lower end of an extensible spring 6 interposed between the immovable members is in contact with the upper surface on the free end side of the arm 5a, and presses the arm 5a. For this reason, the wedge-shaped prism 1 and the prism holder 5 are given a clockwise moment about the optical axis OO as a rotation axis (center of rotation) in FIG.
[0047]
The rotation of the prism holder 5 of the wedge-shaped prism 1 due to this moment is prevented by the receiving member 7 which is in contact with the lower surface on the free end side of the arm 5a. The receiving member is a cylindrical shaft having a conical top end in the figure in the axial direction, and the apex of the cone is in contact with the lower surface of the arm 5a on the free end side. This contact portion is referred to as an actuator action point P.
[0048]
On the other hand, a nut 8 is fixed (or a female screw is formed) to an end opposite to the apex of the receiving member 7 and at the lower side in the drawing, and the nut 8 is integrated with a rotating shaft of the stepping motor 9. The external thread is screwed. This screw is called a lead screw. The stepping motor 9 is fixed to a stationary member.
[0049]
The rotation means having such a configuration is referred to as a lead screw type actuator, and by driving a stepping motor 9, the wedge-shaped prism 1 rotates together with the prism holder 5 around the optical axis OO as a rotation axis (center of rotation).
[0050]
Since such a lead screw type actuator uses the stepping motor 9 as a drive source, the rotation angle of the wedge-shaped prism 1 can be drive-controlled as a digital pulse signal. Easy feedback control.
[0051]
Here, the variation amount of the linear actuator (= the displacement amount of the nut = the displacement amount of the action point P) Δx is given by the following equation (Equation 6).
Δx = R × tan (Δγ) (Equation 6)
(Here, R: distance from the rotation center to the actuator action point, Δγ: wedge-shaped prism rotation angle.)
On the other hand, as described above, the time T required for one rotation of the intermediate transfer member T _m Is represented by the following equation (Equation 4).
T _m = L _m / V _m [Sec] (Equation 4)
Accordingly, the driving frequency N of the stepping motor per unit time required to control the sub-scanning direction correction amount ΔZ on the photoconductor is given by the following equation (Equation 7).
[0052]
N = Δx / p × N ₀ / Tm (Equation 7)
(Where, p: screw pitch of the lead screw, N ₀ : Number of pulses per rotation of the stepping motor. )
Therefore, according to the fourth aspect of the invention, by satisfying the following expression, color shift can be satisfactorily reduced.
10 <N <2000 [pps] (Equation 8)
If the upper limit is exceeded (2000 pps, preferably 1000 pps), the stepping motor becomes unable to respond, so that the beam spot position deviation correction cannot follow. If the lower limit is exceeded, the resolution is too coarse and the beam spot position correction accuracy becomes insufficient.
[0053]
On the other hand, the torque T of the stepping motor is given by the following equation.
[0054]
(Equation 2)

[0055]
(Here, p: screw pitch of the lead screw, T1: torque generated by the tension of the spring (6), R: distance between the rotation center and the point of action.)
The stepping motor maximum response pulse number Nmax can be read from the pull-in driving frequency with respect to the torque T using a standard table (motor characteristic diagram) as shown in FIG.
[0056]
Therefore, the number of pulses N per unit time required to control the sub-scanning direction correction amount ΔZ on the photoconductor in Expression 7 is:
N <Nmax (Equation 10)
Condition must be satisfied.
[0057]
[5] Description of Claim 8
In the main part of the color image forming apparatus shown in FIG. 1 and in the color image forming apparatus shown in FIG. 8 described later, in order to correct the color shift in the sub-scanning direction, the scanning of each scanning optical system corresponding to each color is performed. It is also conceivable to correct the position of the line so that the deviation of each line when superimposed on the intermediate transfer member is close to zero.
[0058]
However, when adjusting the writing start positions (scanning positions) of the laser beams corresponding to the four colors, if all four are to be adjusted to the respective predetermined reference positions, a large rotational eccentricity is erroneously given, and the optical performance is reduced. (Beam spot diameter) may be degraded. In addition, the number of components of the write start position correction means increases, which causes an increase in cost.
[0059]
Therefore, according to the present invention, one of the four laser beams is set as a reference, and the other three are matched with each other, and the reference color is specified. That is, the reference color is black (black) among the four colors Y (yellow), M (magenta), C (cyan), and K (black).
[0060]
The laser beam corresponding to the black image in the scanning optical system is used as a reference, and the scanning position of the laser beam from the scanning optical system other than the reference color is corrected so as to substantially match the scanning position of the laser beam of the reference color. It is characterized by:
[0061]
As a result, since only three of the four colors need to be adjusted, it is sufficient to use three wedge-shaped prisms 1. That is, by performing the correction of the “relative color shift”, it is possible to obtain an image with high color reproducibility in which a change in color tone is sufficiently suppressed. Therefore, for example, as shown in FIG. 1, only three write start position correcting means 140 are required.
[0062]
According to the present invention, since the reference color is black, the following advantages can be obtained.
Since black has a higher contrast than other colors, the influence of fluctuations in beam spot diameter and beam spot position due to disturbances such as vibration and temperature fluctuation is likely to appear in the image. Therefore, by using black as the reference color, each optical component of the scanning optical system that handles the black laser beam can be fixed with high rigidity, and a scanning optical system that is not easily affected by disturbance can be realized. .
[0063]
[6] Description of Claim 5
As shown in FIG. 3, in the case where a plurality of images are continuously printed out, heat is generated from a polygon motor (not shown) for driving the polygon mirror 130 and a semiconductor laser as the light source 150a inside the optical scanning device. Outside the optical scanning device, the temperature inside the image forming apparatus rapidly changes due to the influence of heater heat at the time of fixing the toner in the fixing unit. In this case, the beam spot position on the photoreceptor also fluctuates rapidly, and it is a major problem that the color tone of the output color image gradually changes to the first, several, and tens of sheets.
[0064]
Therefore, in this example, in FIG. 1, a displacement detecting unit (also used as a beam spot position detecting unit) for detecting a relative displacement of the scanning beam on each of the photoconductors 160Y, 160M, 160C, and 160K in the sub-scanning direction. Writing at least one wedge-shaped prism in the optical path from the light source to the deflecting means, and adjusting the rotation of the prism about the optical axis so as to change the beam spot position in the sub-scanning direction. The optical scanning device includes a position correcting unit 140 and controls a beam spot position on a photoconductor during image data writing based on the position shift data measured by the position shift detecting unit.
[0065]
In this example, the position shift detecting means detects the beam spot position by a non-parallel photodiode sensor provided outside the writing area in the main scanning direction of the optical scanning device. This element (non-parallel photodiode (PD)) should be provided outside the effective writing area of the scanning beam for each of the photoconductors 160Y, 160M, 160C and 160K (beam spot position detecting means 300a and 300b in FIG. 1). Desirably, at this time, it may also have a function of detecting a synchronization signal for determining a writing start position in the main scanning direction. In this example, it is also used.
[0066]
In FIG. 7, the light receiving surface of the photodiode PD1 (PD1 ') is orthogonal to the scanning beam, and the light receiving surface of the photodiode PD2 (PD2') is inclined with respect to the light receiving surface of the photodiode PD1 (PD1 '). This inclination angle is set to α1. Further, when the scanning beam before the temperature change due to the heater heat is L1 and the scanning beam after the temperature change is L2, it is assumed that the scanning beam is shifted by ΔZ (unknown) in the sub-scanning direction. In this case, the time when the scanning beams L1 and L2 pass between a pair of non-parallel photodiodes, for example, between the non-parallel photodiodes PD1 and PD2 (or between the non-parallel photodiodes PD1 ′ and PD2 ′). By measuring T1 and T2 and calculating the time difference between T2 and T1, the scanning position (writing start position) in the sub-scanning direction is monitored and detected.
[0067]
The relative dot displacement in the sub-scanning direction (= sub-scanning direction correction amount ΔZ) can be easily calculated by calculating the angle α1 between the light receiving surfaces of PD1 and PD2 and the time difference T2−T1. You can ask. This correction amount is corrected by the write start position correction means 140.
[0068]
According to the present invention, even when the beam spot position on the photoreceptor fluctuates rapidly due to a temperature change or the like, for example, when a plurality of images are continuously printed, the image on the photoreceptor is written even during image data writing. The beam spot position can be corrected.
[0069]
Note that it is also possible to monitor a change in magnification in the main scanning direction by detecting a change in the time T0 required for the scanning beam to pass between the photodiodes PD1 'and PD1.
[7] Description of Claim 9
[6] In the description of [5], in the example of FIG. 1, positional deviation detection for detecting a relative positional deviation of the scanning beam in the sub-scanning direction on each of the photoconductors 160Y, 160M, 160C, and 160K. Means (beam spot position detecting means 300a, 300b, etc.), specifically, for example, detection time T of scanning beams L1 and L2 by non-parallel photodiodes PD1, PD2. _S (Time from the start of position shift detection to the completion of detection) satisfies the following.
T _S <10 × (L _p / V _p (Equation 11)
(Here, Lp is the length of the output direction of the print output paper (recording paper) (the moving direction of the transfer belt 205 indicated by the arrow in FIG. 1), and Vp is the linear velocity of the photoconductor.)
This (Equation 11) indicates that, by detecting a scan line displacement during output of at least five prints, even when a sudden temperature change occurs, a color tone change cannot be confirmed with the naked eye. Detection time T _S Is out of the range of the above expression, it indicates that the color tone may be a problem.
[0070]
Detection time T _S Also includes the time for calculating the displacement correction amount (the calculation is to improve the displacement detection accuracy such as averaging and abnormal value processing for noise reduction, and to calculate the correction amount to be fed back to the scanning line correction means). It represents time.)
[8] Description of Claim 6
As an example of a color image forming apparatus having the optical scanning device of the present invention, the following example can be given in addition to the one shown in FIG. In FIG. 7, a plurality of photoconductors (image carriers) are exposed by an optical scanning device to form an electrostatic latent image, which is then developed, and the visible images on each photoconductor are overlaid and transferred onto a transfer belt. Further, a general outline of a tandem type color image forming apparatus which collectively transfers a superimposed transfer image on the transfer belt to the same sheet-like medium (recording paper) to obtain a color image is shown.
[0071]
First, a transfer belt 205 'as an intermediate transfer belt, which is arranged in a horizontal direction and conveys transfer paper (not shown) supplied from the paper supply cassette 10, is provided at a lower side in the apparatus. On the transport belt 205 ', a photosensitive member 160Y' for yellow Y, a photosensitive member 160M 'for magenta M, a photosensitive member 160C' for cyan C, and a photosensitive member 160K 'for black K are arranged at regular intervals in order from the upstream side. It is arranged in.
[0072]
Note that, hereinafter, the reference characters Y, M, C, and K are appropriately added to distinguish them. These photoconductors 160Y ', 160M', 160C ', and 160K' are all formed to have the same diameter, and process members are sequentially arranged around the photoconductors in accordance with an electrophotographic process.
[0073]
Taking the photoconductor 160Y 'as an example, a charging charger 161Y as a charging unit, a scanning image forming optical system 162Y, a developing device 163Y as a developing unit, a transfer charger 164Y as a transfer unit, a cleaning device 165Y as a cleaning unit, and the like are included. They are arranged in order. The same applies to the other photoconductors 160M ', 160C', and 160K '(however, reference numerals are omitted). That is, in this embodiment, the photoconductors 160Y ′, 160M ′, 160C ′, and 160K ′ are used as exposure surfaces that are set for each color and that are to be scanned. Systems 162Y, 162M, 162C, 162K are provided in a one-to-one correspondence.
[0074]
Around the transfer belt 205 ′, a registration roller 166 and a charger 167 as a belt charging unit are provided at a position upstream of the photoconductor 160Y ′ in the conveyance direction indicated by an arrow, and are provided with a charger 167 as a belt charging unit. A belt separation charger 168, a charge removing charger 169, a cleaning device 170, and the like are provided in this order on the downstream side. Further, a fixing device 180 as a fixing unit is provided downstream of the belt separation charger 168 in the transport direction, and is connected to a paper discharge tray 170 by a paper discharge roller 171.
[0075]
In such a schematic configuration, for example, in the full-color mode (multi-color mode), the image of each color for Y, M, C, and K is applied to each of the photoconductors 160Y ', 160M', 160C ', and 160K'. An electrostatic latent image is formed by optical scanning of a light beam by each scanning imaging optical system based on the signal.
[0076]
These electrostatic latent images are developed into toner images by a developing device for each corresponding color toner, such as a developing device 163Y, and are sequentially transferred onto transfer paper that is electrostatically attracted onto a transfer belt 205 ′ and conveyed. Then, the sheets are superposed, fixed as a full-color image, and then discharged.
[0077]
In the black mode (monochrome mode), the photoconductors 160Y ′, 160M ′, 160C ′ and their process members are set to a non-operating state, and only the photoconductor 160K′K is set based on a black image signal. An electrostatic latent image is formed by optical scanning of the light beam by the scanning imaging optical system 162K. This electrostatic latent image is developed with a black toner to become a toner image, and is transferred onto a recording sheet that is electrostatically attracted onto a transfer belt 205 ′ and conveyed, thereby being fixed as a black monochrome image. Is discharged.
[0078]
In addition, 120M1 and 120M2 are two-piece fθ lenses, each of which is fixed to the optical housing 31 and is placed on the plate 33M at that time. The plate 33M is in contact with the entire or a part of the contact surface side of the fθ lenses 120M1 and 120M2. The fθ lenses 120M1 and 120M2 are made of a plastic material having an easy aspherical shape and low cost, and specifically, a synthetic resin excellent in low water absorption, high transparency, and excellent moldability is preferable. In this example, the polygon mirror includes an upper mirror 130U and a lower mirror 130D.
[0079]
In such a color image forming apparatus, the invention described in the above-described [1] to [7] is applied to the optical scanning device configured in the optical housing 31. Although not shown in FIG. 8, a color misregistration detection sensor 330 as shown in FIG. 1 may be provided as appropriate.
[0080]
In this case, when a large number of color images are continuously printed, the fixing device causes a sharp temperature change, particularly due to the heat generated by the polygon motor in the optical scanning device. For this reason, it is a problem that the color tone fluctuates in the color image after the first print and a plurality of sheets are output, and an optical scanning device having a wedge-shaped prism between the deflecting unit and the surface to be scanned for exposure is used. By using this, it is possible to correct the scanning position deviation and realize high-precision scanning position accuracy, and to obtain a good color image with little color deviation even when the temperature fluctuates abruptly, especially during continuous printing. Obtainable. There are also advantages corresponding to each invention.
[0081]
[10] Numerical example
(1) Preconditions
・ Optical system performance
m = 9.4, fc = 15 [mm],
・ Wedge-shaped prism
α = 2 °, n = 1.51 (BK7),
・ Lead screw type actuator
T1 = 25 × 10 ^-3 [Nm], p = 0.3 [mm], R = 16 [mm], N ₀ = 20 [pulse / 1 rotation]
・ Intermediate transfer member
Lm = 500 mm, vm = 250 mm / s
(2) Calculation example
| M × fc × (n−1) × α} | = | 9.4 × 15 × (1.51-1) × (2 ° / 180 ° × π) | = 2.5
And satisfies the scope of claim 2.
[0082]
[Equation 3]

[0083]
Δx = R × tan (Δγ) = 16 × tan (0.2) = 3.2 [mm]
Tm = Lm / vm = 2 [sec]
Which satisfies the scope of claim 7.
[0084]
N = Δx / p × N ₀ /Tm=3.2/0.3×20/2=106 [pps]
Which satisfies the scope of claim 4.
[0085]
(Equation 4)

[0086]
Read Nmax from the same standard table as FIG.
Uses a stepping motor that satisfies Nmax> N.
[0087]
【The invention's effect】
According to the first aspect of the present invention, by operating the wedge-shaped prism with a simple mechanism and means, it is possible to correct the scanning position shift and realize high-accuracy scanning position accuracy.
[0088]
According to the invention described in claim 2, the beam spot diameter does not increase, the sensitivity of the correction is not too slow, and a large rotation angle is not provided for adjusting the writing start position, and the change with time can be corrected. It is possible to respond quickly in the case of time.
[0089]
According to the third aspect of the invention, since the stepping motor is used as a drive source, the rotation angle of the wedge-shaped prism can be drive-controlled as a digital pulse signal, so that feedback control can be easily performed.
[0090]
According to the fourth aspect of the present invention, it is possible to satisfactorily reduce the color shift by accurately correcting the beam spot position shift.
[0091]
According to the fifth aspect of the present invention, the beam spot position can be corrected even when the beam spot position fluctuates rapidly.
[0092]
According to the sixth aspect of the present invention, by using the optical scanning device having the wedge-shaped prism, it is possible to correct the scanning position deviation and to realize a high accuracy of the scanning position, and particularly, to suddenly reduce the scanning position as in the case of continuous printing output. Even when the temperature fluctuates, a good color image with little color shift can be obtained.
[0093]
According to the seventh aspect of the present invention, the apparatus is less susceptible to disturbances such as vibrations, and has good follow-up to beam spot position correction control.
[0094]
According to the invention described in claim 8, by using black as the reference color, each optical component of the scanning optical system that handles the laser beam for black can be fixed with high rigidity, and the scanning optical system that is not easily affected by disturbance. Can be realized.
[0095]
According to the ninth aspect of the present invention, it is possible to obtain an image quality such that a change in color tone cannot be confirmed with the naked eye even when a rapid temperature change occurs.
[Brief description of the drawings]
FIG. 1 is a perspective view illustrating a main configuration of a color image forming apparatus including an optical scanning device.
FIG. 2 is a diagram illustrating a principle of correcting a sub-scanning beam spot position by a wedge-shaped prism.
FIG. 3 is a diagram illustrating a temperature change in the optical scanning device during continuous printing.
FIG. 4A is a diagram illustrating a sub-scanning dot position shift due to a speed variation on an intermediate transfer member, and FIG. 4B is a diagram illustrating a sub-scanning dot position shift after beam spot position correction; It is.
FIG. 5 is a schematic configuration diagram illustrating a lead screw type actuator as a write start position correction unit.
FIG. 6 is a diagram showing a relationship between a driving frequency and a torque of a stepping motor.
FIG. 7 is a diagram illustrating a principle of detection by a non-parallel photodiode sensor as a beam spot position detecting unit.
FIG. 8 is a diagram illustrating an example of a color image forming apparatus.
[Explanation of symbols]
1 Wedge-shaped prism
140 write start position correction means
160Y, 160M, 160C, 160K, 160Y ', 160M', 160C ', 160K'
300a, 300b Beam spot position detecting means (having a function as position deviation detecting means)

Claims (9)

In an optical scanning device that scans laser beams emitted from a plurality of light source devices by deflecting means and exposes a plurality of image carriers,
A writing method in which at least one wedge-shaped prism is provided in an optical path from the light source device to the deflecting unit, and the beam spot position in the sub-scanning direction is made variable by rotating and adjusting the wedge-shaped prism substantially about the optical axis. Having a start position correcting means,
An optical scanning device, wherein a beam spot position on an image carrier is controlled during writing of image data using the writing start position correcting means. The optical scanning device according to claim 1, wherein the light source device has at least a laser light source and a collimating lens,
The wedge-shaped prism satisfies the following condition.
1 <| m * fc * (n-1) * [alpha] | <30
Where α is the vertex angle of the wedge-shaped prism,
n: refractive index of prism glass material,
fc: focal length of the collimating lens m: lateral scanning magnification from the light source to the surface to be scanned 3. The optical scanning device according to claim 1, wherein the rotation adjusting means of the wedge-shaped prism is a lead screw type actuator driven by a stepping motor. 4. The optical scanning device according to claim 3, wherein the driving frequency N of the stepping motor satisfies the following relationship.
N = Δx / p × N ₀ / Tm
10 <N <1000 [pps]
Here, Δx: maximum displacement of the lead screw,
p: thread pitch of the lead screw N ₀ : pulse number of one rotation of the stepping motor,
Tm: time of one rotation of the intermediate transfer member [sec] In the optical scanning device for a color machine that scans laser beams emitted from a plurality of light source devices by deflecting means and exposes a plurality of image carriers,
Position shift detecting means for detecting a relative position shift in the sub-scanning direction on each image carrier,
In the optical path from the light source device to the deflecting means, at least one wedge-shaped prism is provided, and by rotating and adjusting the wedge-shaped prism substantially about the optical axis, the beam spot position in the sub-scanning direction is made variable. Having a writing start position correcting means,
Based on the displacement data measured by the displacement detecting means,
An optical scanning device, wherein a beam spot position on the image carrier is controlled during writing of image data. A plurality of image carriers are exposed by an optical scanning device to form an electrostatic latent image and then developed, and the visible images on each image carrier are overlaid and transferred onto an intermediate transfer member. In an image forming apparatus that collectively transfers a superimposed transfer image onto the same sheet-like medium to obtain a color image,
An image forming apparatus using the optical scanning device according to claim 1 as the optical scanning device. The image forming apparatus according to claim 6, wherein the intermediate transfer member is configured as a rotary member, rotation of the lengths L _m, when the linear velocity of the intermediate transfer member was v _m, satisfies the following conditions An image forming apparatus, comprising:
_{0.5 <L m / v m <} 5 [sec] 7. The image forming apparatus according to claim 6, wherein the plurality of image carriers correspond to four colors of cyan, magenta, yellow, and black, and black is used as a reference color to correct a relative color shift with respect to black. An image forming apparatus comprising: 7. The image forming apparatus according to claim 6, wherein a detection time T _S (time from the start of the position shift detection to the completion of the detection) of the position shift detecting means satisfies the following condition. .
T _S <10 × (L _p / V _p )
here,
L _p : length of print output paper in the output direction,
v _p : linear velocity of photoconductor

Images