WO2012007832A1

WO2012007832A1 - Depth control of laser cutter

Info

Publication number: WO2012007832A1
Application number: PCT/IB2011/001640
Authority: WO
Inventors: Samuel Isaac Ginsberg
Original assignee: University Of Cape Town
Priority date: 2010-07-14
Filing date: 2011-07-14
Publication date: 2012-01-19

Abstract

The invention provides a laser cutter producing a pulsed light beam and which is characterised in that it includes a first detector for measuring the waveform of light reflected from an object to be cut. A processor is provided which is configured to compare the measurements of the waveform over a period of time and to determine depth of cut as a function of a difference between the measurements of the waveform. Phase difference is preferably measured. A second detector can be provided for measuring the waveform of light directed towards the object, with the processor configured to compare the measurements of the waveform from the first detector and the second detector over a period of time and to determine depth of cut as a function of the difference between the measurements of the waveform.

Description

A LASER CUTTER

FIELD OF THE INVENTION

This invention relates to a laser cutter, more particularly to a laser cutter capable of determining the depth of a cut, and preferably also controlling the depth of cut.

BACKGROUND TO THE INVENTION Laser cutting is very widely used. Many materials can be cut cheaply, speedily and very accurately by laser and numerous companies manufacture laser cutters.

Laser cutters are generally 2 axis computer controlled machines. By moving the light from the laser over the material in the X-Y plane a cut may be made. Some laser cutters offer rudimentary 3-dimensional control by allowing the laser power, pulse rate and speed of beam travel to be varied by the operator. However, the operator must test each material and decide on the laser settings for each different depth of cut to be made. There is no feedback mechanism, so the laser cutter cannot measure the cut depth in real time and thus accurate control of cut depth is not guaranteed. This is particularly problematic where the material being cut is non-homogenous, such as timber and other natural materials. Laser cutters with this feature are considered "2.5D" machines, implying full X-Y control and partial Z axis control.

Numerous prior art laser cutters have means for measuring the height of the laser cutter above the surface of the work piece. While this is quite useful for work pieces which do not have a flat surface, it does not overcome the problem of determining cut depth during operation of the laser cutter.

OBJECT OF THE INVENTION It is an object of this invention to provide a laser cutter which will, at least partially, alleviate some of the abovementioned problems.

SUMMARY OF THE INVENTION According to this invention there is provided a laser cutter producing a pulsed light beam characterised in that it includes a first detector for measuring the waveform of light reflected from an object to be cut and a processor configured to compare the measurements of the waveform over a period of time and to determine depth of cut as a function of a difference between the measurements of the waveform.

Further features of the invention provide for the processor to determine depth of cut as a function of the phase or time difference between the measurements of the waveform, preferably the phase difference; for the processor to further control operation of the pulsed light beam based on a set of user defined cut depths and measured cut depths; and for the processor to control movement of the beam relative to the object.

According to an aspect of the invention there is provided for there to be a second detector for measuring the waveform of light directed towards the object and for the processor to compare the measurements of the waveform from the first detector and the second detector over a period of time and to determine depth of cut as a function of the difference between the measurements of the waveform.

Further features of the invention provide for the waveform of light reflected from the surface of the object to first be measured and the phase difference between it and the directed light to be determined, and for the depth of the cut to be determined from subsequent phase differences; and for the measurements to be made during cutting. Still further features of the invention provide for the beam to be reflected onto the object through a partially reflective surface, and for directed and reflected light to be measured through the partially reflective surface.

The invention also provides a method of measuring the depth of a cut in an object made by a laser during operation thereof which includes directing a pulsed light beam produced by the laser onto the object, measuring the waveform of light reflected from the object over a period of time, and calculating the depth of the cut in the object as a function of a difference between the measurements of the waveform.

A further feature of the invention provides for the depth of the cut in the object to be calculated as a function of the phase or time difference between the measurements of the waveform, preferably the phase difference. According to one aspect of the invention there is provided for the waveform of the light directed towards the object to be measured and to be compared to the waveform of the reflected light and for the depth of the cut in the object to be calculated as a function of the difference between the measurements of the waveform of the directed light and reflected light.

Further features of the invention provide for the waveform of light reflected from the surface of the object to first be measured and the difference between it and the directed light to be determined, and for the depth of the cut to be determined from subsequent differences between the reflected light and directed light; and for the measurements to be used to control the operation of the laser. Still further features of the invention provide for the beam to be reflected onto the object through a partially reflective surface, and for directed light and reflected light to be measured through the partially reflective surface. BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:-

Figure 1 is diagrammatic illustration of one embodiment of a laser cutter;

Figure 2 is a diagrammatic illustration of a second embodiment of a laser cutter; and

Figure 3 is a diagrammatic illustration of a third embodiment of a laser cutter.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

A laser cutter (1) is shown in Figure 1 and includes a laser (2) which generates a pulsed light beam (4) which is directed through four movable mirrors (6, 7, 8, 9) and a focusing lens (10) onto an object (12) to be cut. The laser cutter thus far described is of conventional configuration.

In accordance with the invention, the mirror (9) which directs the light beam (4) directly onto the object (12) is only partially reflective and allows a small amount of light to be transmitted through it. A first sensor (14) is located directly above the point of incidence of the beam (4) on the object (12), on the opposite side of the mirror (9). A second sensor (16) is located in line with the incident, or directed, beam (4) on the mirror (9) on the opposite side to the beam (4). Both the sensors (14, 16) are high speed photodetectors, in this embodiment photodiodes, and are linked to a processor (20).

In use, the laser (2) is operated to produce a high frequency, in this embodiment, 100MHz, pulsed beam (4). The beam (4) is directed onto the surface of the object (12) by the mirrors (6, 7, 8, 9). However, part of the beam (4) passes through the mirror (9) and into the second sensor (16). Some of the light from the beam (4) is reflected from the object back towards the mirror (9) and is in turn reflected towards the mirror (8). However, part of the reflected light passes through the mirror (9) and into the first sensor (14). Each of the sensors (14, 16) produces a waveform which has pulses at the same frequency as the laser's pulses. These form inputs to the processor (20) which measures the phase difference between the two waveforms.

As the beam (4) penetrates beyond the surface of the object (12) the total distance travelled by the light increases and thus the phase difference between the waveforms produced by each sensor (14, 16) increases. The processor (20) is configured, in this embodiment through software, to calculate the depth of the cut (22) as a function of the measured phase differences. The following example illustrates part of the calculation performed by the processor (20):

Modulation frequency = 100x10⁶ Hz

Distance between first sensor (14) and surface of object (12) = 70mm

Speed of light = 3x10⁸ m/s

Time of flight = (2x0.07m) / 3x10⁸ m/s = 466x10^'12s

Period of modulation = 1/100x10⁶ = 10x10^"9s

Reference phase difference = 360x(466x10^"12/10x10^"9) = 16.7999 degrees

Additional phase difference per millimetre of cut depth:

Additional time of flight = (2x0.001 m) / 3x10⁸ m/s = 6.667x10-¹²s

Period of modulation = 1/100x10⁶ = 10x10^"9s

Additional phase difference = 360x(6.667x10^"12/10x10^"9) = 0.24 degrees

Thus each additional phase difference of 0.24 degrees indicates an increase in cut depth of 1 mm. Clearly, higher modulation frequencies will result in better depth resolution.

Also, the time taken for the beam to penetrate the surface of the object allows the distance to the surface to be measured and this permits working on objects which do not have flat surfaces.

The processor (20) is user programmable and controls the operation of the laser (2). Thus, a user can programme into the processor the full 3D shape of an object to be cut and allow the laser cutter to continue automatically. The desired depth of cut can be set without knowing the precise parameters needed to penetrate the material being cut and uniform cut depths can be made in non-homogenous materials. This allows the laser cutter to perform functions reserved for traditional 3D techniques, such as milling, and brings the cost and speed benefits of laser cutting to this area.

The following is a simple algorithm which could be employed to enable cutting to an accurate depth:

• Point laser at the spot to be cut.

• Measure the phase difference between the sensors and use this as the "surface reference".

• Until correct depth is reached continue to:

o emit laser pulses while measuring phase difference; o subtract the phase differences of the senses from the surface reference phase difference measurement.

· Move laser beam to the next point to be cut.

It will be appreciated that many other embodiments of a laser cutter exist which fall within the scope of the invention. For example, as shown in Figure 2, the laser cutter (30) need not make use of mirrors and the laser (32), in this embodiment a laser diode, could direct the beam (34) directly onto the object (36) to be cut through a focusing lens (38). A first sensor (40) is positioned to receive light (42) reflected from the object (36) while a second sensor (44) is positioned to receive light (46) reflected or dispersed from the lens (38). Both sensors (40, 44) are linked to a processor (48) which operates substantially as described above with reference to Figure 1.

As shown in Figure 3, a laser cutter (50) can even have a single sensor (52). In this embodiment, the laser (54) directs the beam (56) directly onto the object (58) to be cut. The sensor (52) is positioned to receive light (60) reflected from the object (58) and is linked to a processor (62) as with the other embodiments described above. In this embodiment, instead of determining the phase shift of the reflected light (60) with reference to the incident beam (56), the phase of the reflected beam is compared with the phase of the pulsed current used to control the laser. The first measurement is made when the beam reflects off the surface of the object and this measurement is used as the reference for further measurements as cutting progresses.

An inaccuracy associated with this configuration is the laser's delay in reaching full power, often referred to as the "lasing delay", which results in an unstable waveform until full power is reached. The longer the delay the more inaccurate will be the initial measurement from the surface of the object. For this reason two sensors are preferred.

It will further be appreciated that any suitable laser could be used provided that it can be operated to provide a pulsed or modulated output. Also, any suitable signal processing could be used to effect measurements. For example, it is possible to heterodyne the signals of the incident and reflected beams in order to reduce the frequency of the signals being processed whilst preserving the phase information contained in the signals. Importantly, it is not necessary to calculate the depth of the cut in the object as a function of the phase difference between the measurements of the waveform. Any suitable difference can be measured, including a time difference.

Claims

1. A laser cutter producing a pulsed light beam characterised in that it includes a first detector for measuring the waveform of light reflected from an object to be cut and a processor configured to compare the measurements of the waveform over a period of time and to determine depth of cut as a function of a difference between the measurements of the waveform.

2. A laser cutter as claimed in claim 1 wherein the processor determines depth of cut as a function of the phase difference or time difference between the measurements of the waveform.

3. A laser cutter as claimed in claim 1 wherein the processor determines depth of cut as a function of the phase difference.

4. A laser cutter as claimed in claim 1 or claim 2 wherein the processor further controls operation of the pulsed light beam based on a set of user defined cut depths and measured cut depths.

5. A laser cutter as claimed in any one of the preceding claims wherein the processor controls movement of the beam relative to the object.

6. A laser cutter as claimed in any one of the preceding claims having a second detector for measuring the waveform of light directed towards the object and wherein the processor compares the measurements of the waveform from the first detector and the second detector over a period of time and determines depth of cut as a function of the difference between the measurements of the waveform.

7. A laser cutter as claimed in claim 6 wherein the waveform of light reflected from the surface of the object is measured and the phase difference between the reflected light and the light directed at the surface is determined, and the depth of the cut determined from subsequent phase differences.

8. A laser cutter as claimed in claim 6 or claim 7 wherein the beam is reflected onto the object through a partially reflective surface, and the directed light and reflected light are measured through the partially reflective surface.

9. A laser cutter as claimed in any one of the preceding claims wherein the measurements are made during cutting of the object.

10. A method of measuring the depth of a cut in an object made by a laser during operation thereof which includes directing a pulsed light beam produced by the laser onto the object, measuring the waveform of light reflected from the object over a period of time, and calculating the depth of the cut in the object as a function of a difference between the measurements of the waveform.

11. A method as claimed in claim 10 wherein the depth of the cut in the object is calculated as a function of the phase difference or time difference between the measurements of the waveform.

12. A method as claimed in claim 10 or claim 11 wherein the waveform of the light directed towards the object is measured and compared to the waveform of the reflected light and the depth of the cut in the object calculated as a function of the difference between the measurements of the waveform of the light directed onto the object and the light reflected from the object.

13. A method as claimed in claim 12 wherein the waveform of light reflected from the surface of the object is measured and the difference between it and the directed light is determined, and for the depth of the cut to be determined from subsequent differences between the reflected light and directed light.

14. A method as claimed in claim 12 or claim 13 wherein the beam is reflected onto the object through a partially reflective surface, and for directed light and reflected light to be measured through the partially reflective surface.

15. A method as claimed in any one of claims 10 to 14 wherein for the measurements are used to control the operation of the laser.