NL2013776B1

NL2013776B1 - Controllable electromagnetic brake system.

Info

Publication number: NL2013776B1
Application number: NL2013776A
Authority: NL
Inventors: Adrianus Antonius Theodorus Dams Johannes; Hendrikus Marinus Engelen Dirk; Johannes Petrus Van Dijk Paulus
Original assignee: Magnetic Innovations B V
Priority date: 2014-11-11
Filing date: 2014-11-11
Publication date: 2016-10-06
Also published as: WO2016076708A2; WO2016076708A3

Abstract

An electromagnetic brake system is described, the system comprising: - an AC generator having an AC output terminal, the AC generator being configured to be engaged with a mechanical driving unit and configured to generate an output voltage at the AC output terminal in response to a rotation of the mechanical driving unit; - an external power resistor assembly having an input terminal and an output terminal, the input terminal of the external power resistor assembly being connected to the AC output terminal; - a power electronics circuit that is configured to control a current supplied by the AC generator to the external power resistor assembly, the power electronics circuit having an input terminal connected to the output terminal of the external power resistor assembly, the power electronics circuit comprising a plurality of controllable electronic switches; - a control unit having an input terminal configured to receive a load signal, the control unit further being configured to: • determine, based on the load signal, a required amplitude and phase angle of the current supplied by the AC generator, and • generate a control signal for controlling the plurality of controllable electronic switches of the electronic circuit, based on the required amplitude and phase angle, and provide the control signal to the plurality of electronic switches, so as to obtain the current having the required amplitude and phase angle.

Description

Title: controllable electromagnetic brake system FIELD OF THE INVENTION

The present invention relates to the field of electromagnetic brakes, in particular to controllable electromagnetic brakes as can e.g. be applied as a load for an exercise device such as a bicycle trainer.

BACKGROUND OF THE INVENTION

The present invention relates to electromagnetic brakes or electromagnetic resistanceproviding arrangements which can e.g. be applied in exercise equipment such as bicycle trainers or which can be applied as a controllable load to test rotary equipment such as electric motors or combustion engines.

Typically, such an electromagnetic brake includes an electromagnetic generator (e.g. a three-phase AC generator) that is configured to be driven by a mechanical driving unit that may be connected to an electromotor to be tested or to a bicycle that is powered by a person.

In the latter case, particular load characteristics may be desired including a comparatively large power at a comparatively large velocity (during a comparatively short period of time) and a comparatively large torque at a comparatively small velocity (during a comparatively long period of time).

In all cases, the mechanical power as generated, either by the human person or the driving electromotor, is converted by the electromagnetic generator into electrical energy which typically has to be dissipated.

In known arrangements, the electromagnetic brake may include a power stage configured to convert the AC output power of the generator to a DC power, using a DC link, which may then be dissipated by a controllable DC load, e.g. including a chopper circuit. Alternatively, the AC output power of the generator may be synchronized with an AC mains supply and thus fed to the mains grid.

In such arrangement, the generated AC power is thus primarily dissipated by the power electronic components of the power stage and the controllable DC load. Given the fact that there may be a large spread in the operating conditions of the driving unit and thus of the characteristics of the power to be dissipated, it may be difficult to realize the dissipation in an effective manner using primarily the power electronics. As an example, in case the electromagnetic brake needs to provide in a comparatively large opposing torque to the drive unit, at a comparatively low speed, this would result in comparatively high currents in the power stage and DC load. As in general, the overload capacity of the power electronic components are rather small (since such components do not have a large heat capacity), this would thus require an over-dimensioning of the power electronic components.

The use of both a power stage, DC link and controllable DC load result in comparatively complex system including a large number of components to be controlled using control circuitry and software. Such an arrangement in particular poses a particular control problem in that it requires to continuously balance the required load (e.g. the load as required by a user) and the required dissipation, in order to avoid an excessive voltage drop or rise.

It would therefor be desirable to provide in an alternative way of controlling an electromagnetic resistance unit or brake.

SUMMARY OF THE INVENTION

It would be desirable to provide an alternative way of controlling an electromagnetic resistance unit, in particular, it would be desirable to provide in a less complex/costly resistance unit.

To address this, the present invention provides in an electromagnetic brake system comprising: an AC generator having an AC output terminal, the AC generator being configured to be engaged with a mechanical driving unit and configured to generate an output voltage at the AC output terminal in response to a rotation of the mechanical driving unit; an external power resistor assembly having an input terminal and an output terminal, the input terminal of the external power resistor assembly being connected to the AC output terminal; a power electronics circuit that is configured to control a current supplied by the AC generator to the external power resistor assembly, the power electronics circuit having an input terminal connected to the output terminal of the external power resistor assembly, the power electronics circuit comprising a plurality of controllable electronic switches; a control unit having an input terminal configured to receive a load signal, the control unit further being configured to: determine, based on the load signal, a required amplitude and phase angle of the current I supplied by the AC generator, and generate a control signal for controlling the plurality of controllable electronic switches of the electronic circuit, based on the required amplitude and phase angle, and provide the control signal to the plurality of electronic switches, so as to obtain the current having the required amplitude and phase angle.

The electromagnetic brake system according to the present invention comprises an AC generator, e.g. a 3-phase generator, which is configured to be engaged with a mechanical drive unit. The AC generator as applied in the electromagnetic brake system according to the present invention may e.g. have a shaft to which a drive unit can be coupled for exerting a torque. As an example, the shaft may be driven by a bicycle wheel of a bicycle trainer or any other type of exercise equipment. This may be realized by a direct coupling of the bicycle wheel, resulting in the shaft of the generator rotating at the same velocity as the bicycle wheel, or by means of a frictional coupling, whereby the bicycle wheel drives, by means of friction, a wheel mounted to the generator shaft. Alternatively or in addition, a gearbox may be provided to connect the drive unit and the AC generator.

When the generator shaft is driven, an AC voltage is generated at an AC output terminal of the generator. The AC generator may e.g. comprises a rotor equipped with permanent magnets arranged to generate a magnetic field distribution and a stator equipped with a multiphase stator winding (e.g. a three-phase winding) that is magnetically coupled with the magnetic field distribution generated by the permanent magnets. As a result, when the rotor is rotated, i.e. driven by the mechanical drive unit, an AC voltage is generated in the stator winding, said voltage being available at the AC output terminal.

In accordance with the present invention, an external power resistance assembly, having an input terminal and an output terminal, is connected, via the input terminal, to the AC output terminal of the generator. In an embodiment, the AC output terminal is a three-phase output terminal and the external power resistance assembly comprises three power resistances connected at respective three terminals of the three-phase output terminal.

Within the meaning of the present invention, the feature ‘power resistor’ is used to denote a resistor that is configured to dissipate a comparatively large power, e.g. ranging from 10 W to 100 kW. In an embodiment of the present invention, use is made of power resistors in the range of 10 Watt to 500 Watt. Typical resistance values of such power resistors as can be applied in the present invention are in the range of 0.01 to 3 Ω.

Such power resistors provide the advantage of allowing a comparatively large peak load and may withstand comparatively high temperatures, e.g. > 200 °C

In an embodiment, the external power resistance assembly comprises a cooling assembly, also referred to as a heat sink. Such assembly may e.g. be an Aluminum structure provided with cooling fins. The cooling assembly may further comprise a fan for improving the heat transfer from the Aluminum structure to the environment. Alternative ways of cooling the power resistors, including e.g. liquid cooling, evaporative cooling or using heat pipe type of coolers may be considered as well. In case of an evaporative cooling, ammonium, C02 or Freon may be applied as cooling means.

In an embodiment, similar cooling means may be applied for cooling the AC generator as well.

In accordance with the present invention, the electromagnetic resistance unit further comprises a power electronics circuit and a control unit. In accordance with the present invention, the power electronics circuit is configured to control the current as retrieved from the AC generator and supplied to the power resistance assembly.

Compared to known electromagnetic brakes, which apply a controllable DC load for dissipating the generated power, the present invention provides in an alternative by ensuring that a substantial amount of power is dissipated by the external power resistor assembly and by the AC generator. In accordance with the present invention, the power electronics circuit as applied need not be designed to dissipate a comparatively large portion of the power. Rather, the main objective or purpose of the power electronics circuit is to ensure that the appropriate current is supplied by the AC generator to the power resistor assembly, so as to obtain the desired load characteristic.

In order to realize this, the power electronics circuit as applied in the electromagnetic brake system according to the present invention has an input terminal connected to the output terminal of the external power resistor assembly and comprises a plurality of controllable electronic switches. The controllable electronic switches are controlled by a control signal provided by a control unit (which can e.g. be a microprocessor based control unit) of the electromagnetic brake system.

Examples of such controllable electronic switches may e.g. include FETs or MOSFETs. In an embodiment, the power electronics circuit may e.g. include a full-bridge or half bridge rectifier circuit.

In an embodiment, the power electronics circuit may include one or more analogue amplifiers. In such arrangement, operational amplifiers (op-amps) may be used as switches to control the current as supplied by the AC generator.

The control unit as applied in the present invention has an input terminal configured to receive a load signal.

In an embodiment, the load signal comprises a desired load power.

Alternatively, the load signal may comprise a desired load torque and a velocity signal, representative of a rotational speed of the mechanical driving unit.

The load signal as provided to the control unit may e.g. originate from a user interface which allows a user to select a desired torque T (i.e. an opposing torque as provided by the AC generator) or a desired power level P. The relationship between the desired torque T and the desired power P is given by P = Τ*ω, ω being the rotational speed of the mechanical driving unit.

In accordance with the present invention, the control unit is configured to control both the amplitude and the phase angle of the current as supplied to the power resistor assembly.

It has been devised by the inventor that, by doing so, the desired power (and associated torque) may be generated in an effective manner using a less complex topology, compared to known arrangements.

The control unit as applied in the electromagnetic resistance unit according to the invention is thus configured to determine, based on the load signal, the required amplitude and phase angle of the current. Using this required amplitude and phase angle information, the control unit may determine a control signal for controlling the plurality of electronic switches of the power converter.

In an embodiment, the phase angle of the current is defined relative to the EMF of the AC generator. In particular, the phase angle of the current may be defined relative to the phase angle of the EMF of the AC generator. The EMF of the AC generator is the voltage as generated in one or more windings of the AC generator. In an unloaded state, i.e. when the generator is not supplying a current, this EMF can be perceived or observed at the AC output terminal of the generator.

In such embodiment, the control unit may e.g. be configured to determine a phase angle of an EMF of the AC generator, and wherein the phase angle of the current is defined relative to the phase angle of the EMF.

In an embodiment, the control unit is configured to apply a field orientation algorithm to determine the required phase angle and amplitude of the current supplied by the generator.

In such embodiment, the phase angle of the current is typically defined relative to the EMF of the AC generator, i.e. the voltage as generated by the generator in an unloaded state, i.e. when the generator is not supplying a current.

Field orientation algorithms (also known as vector control algorithms) are known in the art and are typically applied to control the operation of induction motors or synchronous motors. In an embodiment of the present invention, a field orientation algorithm is used to determine, based on a desired load characteristic, so-called d and q current components, the d-current component being a current component in phase with the EMF generated by the AC generator, whereas the q-current component is 90 degrees out of phase with the d-current component.

In order to subsequently control the electronic circuit of the power converter such that the appropriate current is supplied by the generator, the field orientation algorithms rely on knowledge about the phase angle of the generated EMF. The EMF as generated by an AC-generator corresponds to the voltage at the generator terminals when the generator is not supplying any current. In case of a synchronous generator, the phase angle of the EMF may easily be derived from the position of the rotor of the generator. When the generator is supplying a current, the phase angle of the voltage as noticed at the generator terminals, i.e. the phase angle of the output voltage U, will in general be different from the phase angle of the EMF, due to the impedance of the generator windings.

In order to retrieve the EMF phase angle when the generator is supplying a current, various options exist.

In an embodiment, the control unit is configured to receive a rotor position signal representative of a rotor position of the AC generator and to determine a d-current component and a q-current component based on the load signal and the rotor position signal. Within the meaning of the present invention, the rotor position refers to the position of the rotor of the AC generator with respect to the stator of the AC generator.

In such embodiment, the AC generator may be provided with a position measurement system (e.g. a rotary encoder system) for providing the rotor position signal. As an alternative, one or more Hall sensors may be applied for generating a signal representative of the rotor position, i.e. the position of the rotor relative to the stator. Based on this signal representing the rotor position, the phase angle of the EMF can be determined. This EMF phase angle can be used as a reference to implement the desired phase angle of the current as provided by the generator.

In an embodiment, a sensor less approach is adopted whereby the rotor position is e.g. estimated based on the output voltage U of the AC generator and known characteristics of the generator, i.e. including impedance characteristics of the AC generator.

In accordance with the present invention, the electromagnetic brake unit does not comprise or requires a DC load circuit or a variable DC load circuit. Instead, all the required power (generated by the AC generator and required to provide in an opposing torque to the mechanical input torque which, during use, drives the generator) is dissipated in the generator windings and the external power resistance assembly.

In an embodiment, the external power resistance assembly comprises a power resistor for each phase of the AC generator, a resistance value of the power resistor substantially corresponding to a resistance value of a phase of the AC generator. In such embodiment, the AC generator and the power resistor assembly each take up half the required power.

It may be noted that the AC generator and/or the external power resistance assembly as applied in the present invention, may be equipped with cooling means in order to avoid excessive heating of the generator and/or power resistor(s).

It may further be advantageous to provide, to the control unit, as a feedback signal, a current signal representative of the current I supplied by the AC generator. The current signal may then be used, in a feedback loop, to monitor the phase angle of the current I, thus enabling the control unit to control, e.g. adjust, the control signal for controlling the electronic switches when the phase angle deviates from the desired value. In an embodiment, the current signal is obtained by a voltage measurement on the external power resistance assembly.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a first electromagnetic brake system as known in the art.

Figure 2 depicts a second electromagnetic brake system as known in the art.

Figure 3 depicts an embodiment of an electromagnetic brake system according to the present invention.

Figure 4 depicts a phase diagram of an AC generator as can be applied in an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 depicts an electromagnetic brake system 100 as known in the art, the system including an AC generator 110 and an electric load 120. In the arrangement as shown, the AC generator 110 is a three phase generator, the electric load 120 comprises a set of three sliding power resistors 120.1 which can be controlled, i.e. varied.

Figure 1 further schematically shows a shaft 130 of the AC generator 110, the shaft 130 being configured to be connected to a mechanical driving unit, e.g. a motor or an exercise equipment. In the system as schematically shown, the AC generator 110 generates an opposing torque Te, acting against the torque T generated by the mechanical driving unit, whereby the mechanical input power T x ω substantially equals the dissipated power P. (it can be noted that, apart from the dissipated power P, other losses may occur in the generator as well, e.g. iron losses, and frictional losses such as bearing losses or air resistance losses) The dissipated power P has a component Pe representing the power dissipated in the windings of the AC generator 110 and a component Pr representing the power dissipated in the electric load 120. By varying the resistance value of the electric load 120, e.g. controlling the sliding power resistor, the dissipated power Pr can be adjusted, thereby controlling the opposing torque Te generated by the generator.

As a drawback of the system 100, it can be mentioned that an accurate control of the sliding power resistors may be challenging. Further, the sliding power resistors may be subject to mechanical wear, adversely affecting the proper operation of the variable load.

Figure 2 depicts a more advanced electromagnetic brake system 200 as known in the art, the system including an AC generator 210, which is connected to a power converter 240 for converting the AC output power of the generator to a DC power, supplied to a DC bus 250. In order to maintain a DC bus voltage, a capacitance 260 is provided between the DC bus and ground 270. In the arrangement as shown, a variable DC load 280. As an example of such a variable DC load, a chopper circuit can be mentioned. Such a circuit may comprise a plurality of parallel branches, each branch comprising a series connection of a power resistor and an electronic switch for controlling a current through the power resistor.

By means of a control signal, provided by a control unit 290, the switches can be operated at a desired duty cycle between 0% and 100%, in order to obtain a desired load. In the arrangement as shown, the control unit 290 may further control the power converter 240, which may e.g. be a full bridge rectifier. In the embodiment as shown, the control unit 290 is configured to receive an input signal 292 (e.g. from a user interface 300), representing a desired power or torque.

With respect to the system 200 shown in Figure 2, it can be noted that the system requires a large amount of power electronics in both the power converter 240 and the variable DC load 280 that need to be controlled. In the arrangement as shown, the power is primarily dissipated in the AC generator 210 and the variable DC load 280.

In accordance with the present invention, an alternative electromagnetic braking system is proposed.

Figure 3 schematically shows an embodiment of an electromagnetic braking system 300 according to the present invention. The embodiment of the electromagnetic braking system as shown comprises an AC generator 310, e.g. a three phase AC generator having an output terminal 320 (in general an n-phase output terminal). In accordance with the present invention, the AC generator is configured to be engaged with a mechanical driving unit, such as a motor or an exercise device, via a shaft 330.

The AC generator 310 is further configured to generate an output voltage at the AC output terminal 320 in response to a rotation of the mechanical driving unit;

The electromagnetic braking system according to the present invention further comprises an external power resistor assembly 325 having an input terminal 325.1 and an output terminal 325.2, the input terminal 325.1 of the external power resistor assembly 325 being connected to the AC output terminal 320.

The electromagnetic braking system according to the present invention further comprises a power electronics circuit 340 that is configured to control a current supplied by the AC generator 310 to the external power resistor assembly 325. In accordance with the present invention, the power electronics circuit 340 has an input terminal 340.1 connected to the output terminal 325.2 of the external power resistor assembly 325. In accordance with the present invention, the power electronics circuit 340 comprising a plurality of controllable electronic switches. In an embodiment, the power electronics circuit comprises a half-bridge or full bridge rectifier.

In the embodiment as shown, the power electronics circuit 340 has a (DC) output terminal 350,370 providing a DC bus voltage between a terminal 350 and ground 370. In the embodiment as shown, a capacitance 360 is connected to the DC bus voltage.

The electromagnetic braking system according to the present invention further comprises a control unit 390 having an input terminal 390.1 configured to receive a load signal 392 and, optionally, a velocity signal 393. The load signal 392 may e.g. represent a desired load power P or load torque T and may e.g. be received from a user interface 400. In the latter case, the control unit 390 may further receive the velocity signal 393 representing a rotational speed ω of the mechanical driving unit or the shaft 330, whereby the control unit 390 may then determine the desired load power P as = T x ω. In accordance with the present invention, the control unit 390 is configured to: determine, based on the load signal 392 and optionally the velocity signal 393, a required amplitude and phase angle of the current supplied by the AC generator 310, and generate a control signal 395 for controlling the plurality of controllable electronic switches of the power electronics circuit 340, based on the required amplitude and phase angle, and provide the control signal to the plurality of electronic switches of the power electronics circuit 340, so as to obtain the current having the required amplitude and phase angle.

In the electromagnetic braking system according to the present invention, a power resistor assembly is arranged between the AC generator 310 and the power converter 340. Rather than applying a set of sliding power resistors as a variable load, the power resistor assembly can comprises a set of power resistors having a fixed value. In an embodiment, a power resistor per phase of the AC generator may be provided. Each of such power resistors may e.g. be configured to dissipate a comparatively large power, typically in the range of 10 Watt to 500 Watt.

In an embodiment of the present invention, the per phase external power resistance Re of the power resistance assembly and the resistance of phase winding of the generator Rg are selected to fulfil the following:

Typical resistance values of such power resistors as applied in the present invention are in the range of 0.01 to 3 Ω. In an embodiment, the external power resistance assembly 325 further comprises a cooling assembly, also referred to as a heat sink. Such assembly may e.g. be an Aluminum structure provided with cooling fins. The cooling assembly may further comprise a fan for improving the heat transfer from the Aluminum structure to the environment. Alternative ways of cooling the power resistors, including e.g. liquid cooling or including heat pipes, may be considered as well.

Rather than relying on a variable resistance value to modify the dissipated power, the dissipated power (i.e. the sum of the power dissipated by the generator and the power dissipated by the power resistance assembly) is controlled by controlling the amplitude and the phase angle of the current as provided by the generator. In accordance with the present invention, this is realized by controlling the electronic switches of the power electronics circuit of the power converter 340, thereby controlling the amplitude and phase angle of the current of the AC generator 310.

In such embodiment, the control unit may e.g. be configured to determine the phase angle of the EMF of the AC generator (e.g. based on a rotor position signal), and wherein the phase angle of the current is defined relative to the phase angle of the EMF.

In an embodiment of the present invention, the phase angle of the EMF may be used to apply field orientation control, also know as vector control, to obtain the desired load.

Field oriented control or vector control is generally known and is typically used to control a synchronous or asynchronous machine. In field oriented control, a synchronous reference frame (known as a d-q reference frame) is used whereby currents in this reference frame can be considered to be composed of a flux-forming component (referred to as the d-current) and a torque-forming component (referred to as the q-current). The d-q reference frame can be considered a (d,q) coordinate system with orthogonal components along d (direct) and q (quadrature) axes whereby flux forming components of the current are aligned along the d axis and torque-forming components of the current are aligned along the q axis. In field orientation control, the d,q reference frame is defined by the rotor flux, whereby the rotor flux is considered to be aligned with the d-axis. In case of a synchronous AC generator (e.g. provided with a permanent magnet rotor), the position of the d-axis may thus be determined based on the rotor position.

Alternatively, the position of the d-axis of the d,q reference frame may be based on the EMF of the AC generator (i.e. the voltage induced in the stator windings of the AC generator due to the rotor flux, generated by permanent magnets or a field winding).

In general, when a generator is supplying a current, the phase angle of the voltage as noticed at the generator output terminal, i.e. the phase angle of the output voltage U, will be different from the phase angle of the EMF, due to the impedance of the generator windings.

This is schematically illustrated in Figure 4.

Figure 4 schematically shows a phase diagram of an AC generator showing a vector U representing the output voltage at the AC generator output terminal, a vector I representing the current supplied by the generator, a resistive voltage drop l*Rg of the generator winding and and an inductive voltage drop jl*X of the generator winding, and the EMF as generated by the rotor of the generator.

The current I can be considered as having a first component Iq (a torque forming component in phase with the EMF and a second component Id (affecting the flux), 90° out of phase with the EMF.

In order to determine the required current components, the following formulas can be relied on:

(1) (2)

(3) wherein: P = the desired load power that is to be dissipated by the generator and the power resistance assembly; n = the number of phases of the AC generator; I = the effective value of the current supplied by the AC generator; Iq and Id are the q-current and d-current components of the current I. R = the combined resistance of the external power resistance (per phase) and a resistance of a phase winding of the AC generator; a = the phase angle of the current I. K = is the generator constant indicating the amount of Nm generated per Ampere.

Assuming the following situation: R = 1 Ohm T= 10 Nm ω = 100 rad/sec P = T x co = 1000 Watt.

K = 1 Nm/A

Based on equation (1), one can derive that, for the given load situation, a q-current = 10 A is required. Given that the overall dissipation P needs to be 1000 W, Id can be derived from equation (2). When the d- and q-currents are known, the phase angle between the EMF and the current I can easily be determined using equation (3).

As can be seen from Figure 4, the phase angle (a1) between the EMF and the current I is different from the phase angle (a2) between the output voltage U of the generator and the current I.

In the embodiment of the present invention which applies field oriented control, the phase angle of the EMF should be derived, in order to appropriately control the power electronics circuit 340 as shown in Figure 4, so as to obtain the desired current I (consisting of the d-and q-current components) and the appropriate phase angle a.

In order to determine the EMF phase angle, various options exist:

As a first option, use can be made of a rotor position signal representative of a rotor position of the AC generator. Such rotor position signal may e.g. be obtained from an encoder based position measurement system or a measurement system including one or more Hall sensors. It can further be noted that such a rotor position signal may, in an embodiment, also be used to derive the angular velocity ω as e.g. applied in equation (3).

When the phase angle of the EMF is know, the phase angle as derived using equation (5) can be applied as the phase angle between the EMF and the current I to be supplied by the generator. Based on this phase angle, and the required amplitude of the current I, the control unit may then determine a control signal for controlling the electronic switches of the power electronics circuit 340, in order to arrive at the desired current I.

Such control may e.g. include the use of a PWM based switching sequence. In such arrangement, the electronic switches may e.g. be controlled by PWM pulses at a comparatively high frequency (e.g. ranging from 5000 Hz - 20000 Hz), whereby the pulse width is varied, thereby varying the amplitude of the current I. Note that the pulse width of the pulses may also be expressed as a duty cycle (DC), e.g. expressing the percentage of opening or closing of the switches during a switching period (e.g. 1/5000 sec).

As an alternative to determining the d- (or q-) axis position using a signal representing the rotor position, a rotor position estimate can be determined. In such an alternative approach, there is thus no need to a rotor position sensor such as an encoder based sensor or Hall sensor.

Rather, in an embodiment, the control unit is configured to receive a voltage signal representative of the output voltage U of the AC generator and to determine a rotor position estimate based on the voltage signal and impedance data of the generator. Referring to Figure 4, one can derive that, when U, I and the impedance (R, jX) of the generator are known, the phase angle of the EMF can be derived.

It should be noted that several methods are described in literature which enable to arrive at a rotor position estimate, substantially without making use of a rotor position sensor.

Known methods e.g. include the use of extended Kalman filters or sliding-mode observers. The following reference provides in a comprehensive overview of both sensor based and sensor less methods which can be applied in field oriented control: Position and Speed Control of Brushless DC Motors Using Sensorless Techniques and Application Trends, Sensors (Basel). 2010; 10(7): p. 6901-6947.

In an embodiment, the external power resistance assembly comprises a power resistor for each phase of the AC generator.

In an embodiment, a resistance value of the power resistor substantially corresponds to a resistance value of a phase winding of the AC generator. In such embodiment, the AC generator and the power resistor assembly may each take up half the required power. In case the generator takes up a substantial part of the dissipation, it may be advantageous to provide in cooling means for cooling the AC generator, e.g. by means of a fan or a water cooling circuit. Alternatively, the resistors of the power resistor assembly may be selected in such manner that a comparatively large part of the dissipation takes place in the power resistor assembly.

As such, the electromagnetic brake system according to the present invention is less complex than the system as described in Figure 2, while maintaining a similar flexibility with respect to the selection of desired load.

Compared to the arrangement of Figure 2, it is worth mentioning that, in general, power resistors have a substantial overload capacity compared to power electronic components. Therefore, in order to accommodate for peak loads during a short period of time (e.g. <10 sec.) no overdimensioning of the power resistors is required.

It can further be mentioned that the use of power resistors enables an accurate power control because of the comparatively low temperature coefficient.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. A single processor or other unit may fulfil the functions of several items recited in the claims.

Claims

An electromagnetic braking system comprising: - an alternating current generator with an AC output terminal, wherein the alternating current generator is adapted to be coupled to a mechanical drive unit and is adapted to generate an output voltage at the AC output terminal in response to a rotation of the mechanical drive unit; - an external power resistor assembly! having an input terminal and an output terminal, the input terminal of the external power resistor assembly being connected to the AC output terminal; - a power electronics circuit adapted to control a current supplied from the alternator to the external power resistor assembly, the power electronics circuit having an input connected to the output terminal of the external power resistor assembly, the power electronics circuit comprising a plurality of controllable electronic switches; - a control unit with an input terminal adapted to receive a load signal, the control unit being further adapted to: determine, on the basis of the load signal, a required amplitude and phase angle of the current supplied by the erasure current generator, and a control signal for controlling of the plurality of controllable electronic switches of the power electronics circuit based on the required amplitude and phase angle and to provide a control signal to the plurality of electronic switches in order to obtain the current with the required amplitude and phase angle.

The electromagnetic braking system according to claim 1, wherein the load signal comprises a desired load power P or a desired load torque T and a speed signal co, representative of a rotational speed of the mechanical drive unit.

The electromagnetic braking system according to claim 1 or claim 2, wherein the power electronics circuit is a full bridge or half bridge rectifier with an output terminal connected to a capacitance.

The electromagnetic braking system according to any of the preceding claims, wherein the control unit is further adapted to determine a phase angle of the alternating current generator EMK, and wherein the phase angle of the current is defined with respect to the phase angle of the EMK.

The electromagnetic braking system according to claim 4, wherein the control unit is further adapted to receive a rotor position signal representative of a rotor position of the alternating current generator relative to a stator position of the alternating current generator, and to determine the phase angle of the alternating current EMK of the alternating current generator based on the rotor position signal.

The electromagnetic braking system of claim 5, wherein the alternating current generator comprises one or more Hall sensors for generating the rotor position signal.

The electromagnetic braking system according to claim 5 or 6, wherein the control unit is adapted to determine a speed co representative of a rotational speed of the mechanical drive unit on the basis of the rotor position signal.

The electromagnetic braking system according to claim 4, wherein the control unit is adapted to receive a voltage signal representative of the output voltage U of the alternating current generator and to determine the phase angle of the alternating current generator EMK based on the voltage signal and impedance data of the generator .

The electromagnetic braking system according to any of the preceding claims, wherein the control unit is adapted to receive, as a feedback signal, a current signal representative of the current supplied by the alternating current generator.

The electromagnetic braking system of claim 9, wherein the current signal is obtained by a voltage measurement across the external power resistor assembly.

The electromagnetic braking system according to any of claims 4 to 10, wherein the control unit is adapted to apply a field orientation control algorithm to determine the phase angle of the current.

The electromagnetic braking system according to claim 11, wherein the control unit is adapted to determine a d-current component 1d and a Q-current component Iq on the basis of the load signal and the phase angle of the EMF.

The electromagnetic braking system according to any of claims 11 to 12, wherein the control unit is adapted to determine the amplitude and phase angle of the current I relative to the phase angle of the EMK based on:

where η = the number of phases of the alternator; K = a generator constant that indicates the amount of Nm generated per ampere. P = the desired load capacity; T = the desired load torque. α - the phase angle of the current I relative to the EMC of the alternating current generator and Iq and ld are q-axis and d-axis components of the current I.

The electromagnetic braking system according to any of the preceding claims, wherein the alternating current generator is a three-phase generator.

The electromagnetic braking system according to any of the preceding claims, wherein the external power resistor assembly comprises a power resistor for each phase of the alternating current generator, and wherein a resistor value Re of the power resistor and a resistor value Rg of a phase of the AC generator satisfy:

The electromagnetic braking system of claim 15, wherein Re is substantially equal to Rg.

A training device, such as an exercise bike, comprising an electromagnetic braking system according to any of the preceding claims.