Circuit and method for speed monitoring of an electric motor

A circuit for speed monitoring of an electric motor comprises a circuit for generating a time-frame signal, a circuit for receiving a first signal from a chopper driver circuit designed to drive the electric motor, a circuit for detecting chopper pulses in the first signal, a pulse counter, and a circuit for at least one of outputting and evaluating a state of the pulse counter, after the inactive state of the time-frame has been indicated. The time-frame signal indicates when a time-frame of predefined length changes from an inactive state to an active state and indicates when the time-frame changes back from the active state to the inactive state. The pulse counter is designed to count the detected chopper pulses while the active state is indicated by the circuit for generating the time-frame signal.

FIELD OF THE INVENTION

This invention in general relates to two circuits for speed monitoring of an electric motor. Further, the invention relates to two methods for speed monitoring of an electric motor.

BACKGROUND OF THE INVENTION

Electric motors and in particular stepper motors can be driven by a chopper control. A chopper control can be recommendable for high power motors, because of its high efficiency. Velocity measurement can be used to detect a stalled or blocked motor.

SUMMARY OF THE INVENTION

The present invention provides circuits and methods for speed monitoring of an electric motor as described in the accompanying claims. Specific embodiments of the invention are set forth in the dependent claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows schematically a circuit arrangement10comprising a winding12of an electric motor14, a chopper control circuit16to drive the electric motor14, and an example embodiment of a speed monitoring circuit18. The electric behaviour of the winding12of the electric motor14may be described by an equivalent circuit20consisting of a resistance RL, an inductor L, and a voltage source UBconnected in series. The current Iwthrough the motor winding12is supplied via a chopper switch30from one pole32of a DC voltage supply34. The DC voltage supply34provides a power supply voltage Up. Within the embodiment shown in the figure the chopper switch30is a MOSFET (metal-oxide-semiconductor field-effect transistor). The opened-state respectively closed-state of the MOSFET30is controlled by an output38of a Schmitt-Trigger40, wherein the output38is connected to a gate42of the MOSFET30. Current Iwled through the winding12is guided via a measuring resistor44to a mass46. The mass46is connected to a second pole48of the DC voltage supply34. A tap50between the motor winding12and the measuring resistor44is connected to an inverting input52of the Schmitt-Trigger40. Thus, the voltage drop Viat the measuring resistor44is applied to the inverting input52of the Schmitt-Trigger40. A speed controller53applies a control voltage Vsetto the non-inverting input56of the Schmitt-Trigger40, wherein the height of the control voltage Vsetis a value for controlling a desired current of the motor14.

At start-up no current Iwis flowing through the motor winding12and thus neither through the measuring resistor44. Therefore, at start-up the inverting input52of the Schmitt-Trigger40has a potential of 0 Volt, while the control voltage Vsetat the non-inverting input56of the Schmitt-Trigger40is higher. Thus, the output38of the Schmitt-Trigger40provides a positive voltage Vgto the gate42of the MOSFET30. Then, the MOSFET30activates its source-drain channel and causes current Iwto flow through the motor winding12and through the measuring resistor44. In the following, this mode of operation is called “boost mode”. Because of the inductive behaviour of the motor winding12(i.e. the inductor L within the equivalent circuit20), the current Iwthrough the motor winding12does not increase suddenly, but ramp-like. The ohmic resistances of the motor winding12, of the measuring resistor44, of the chopper switch30, and of the electric lines58,60,62,64are disregarded for following rough estimations. The slew rate dIw/dt of the winding current Iwcan be calculated by dIw/dt=(Up−UB−Iw*RL)/L (Equation 1). UBdesignates the back electromotive force BEMF and RLrepresents a series winding resistance. The BEMF is a voltage VBwhich is proportional to a velocity ω of a motor rotation. When the motor14is stalled the BEMF VBis 0 Volt. In a rotating motor14the BEMF VBis opposing the driving voltage UW. In the boost mode the BEMF VBis reducing the slew rate dIw/dt. Consequently, same applies to the velocity ω of the motor rotation. During the boost mode, the highest slew rate dIw/dt is provided when the motor14is stalled. With higher velocity ω of the motor rotation the slew rate dIw/dt is decreased more and more by the increased BEMF VB. As Equation 1 shows, this dependency between slew rate dIw/dt and velocity ω of the motor rotation is linear but not proportional. The current Iwthrough the motor winding12causes a voltage drop Viat the measuring resistor44and simultaneously increases the potential at the inverting input52of the Schmitt-Trigger40. Finally, the voltage drop68applied to the inverting input52gets higher than the control voltage Vsetat the non-inverting input56plus a hysteresis amount of the Schmitt-Trigger40. The value Viof current Iwthrough the motor winding12reached at this time is called IMAX. When the potential at the inverting input52gets higher than the control voltage Vsetplus the hysteresis amount of the Schmitt-Trigger40, the Schmitt-Trigger40changes its state and outputs a low voltage Vgto the gate42of the MOSFET30. Then, the MOSFET30deactivates its source-drain channel and then no current Iwis flowing any longer through the MOSFET30. Following equation 1, the length of the boost period74depends on the hysteresis amount of the hysteresis curve of the Schmitt-Trigger40, on the value of the inductor L of the equivalent circuit20of the motor winding12, on the velocity ω of the motor rotation, and on the value of the voltage Upof the power supply34. In practice, the length of the boost period74may be influenced in addition by the value of an internal resistance of the chopper switch30and electric lines58,60,62, and64.

In the following, the operation with the deactivated MOSFET30is called “free-wheeling mode”. As the equivalent circuit20of the motor winding12comprises an inductor L, the energy of the magnetic field built-up in the inductor L causes the current Iwto continue, which has been flowing through the winding12. To facilitate a well-organized continuation of the current flow Iwthrough the motor winding12and other parts58,60,62,64of the circuit arrangement10, a free-wheeling diode84is provided. The free-wheeling current circle82through the motor winding12, the measuring resistor44, the free-wheeling diode84, and back to the motor winding12has no external power supply, because of the deactivated MOSFET30. Therefore, in the free-wheeling mode the slew rate dIw/dt of the winding current Iwmay be calculated by dIw/dt=−(UB+RL*Iw)/L (Equation 2). When the motor14is stalled the BEMF VBis 0 Volt, and the slew rate dIw/dt of the winding current Iwis determined by the time constant of the free-wheeling current circle82. If there was no ohmic loss in the in the free-wheeling current circle82, the slew rate dIw/dt of the winding current Iwwould be Zero. In the free-wheeling mode the energy from the inductor L is being dissipated with passing time in the resistance RLof the motor winding12, of the measuring resistor44, and other components58,60,62,64of the free-wheeling current circle82. In the free-wheeling mode the BEMF VBis increasing the negative slew rate dIw/dt. During the free-wheeling mode, the lowest absolute value of the negative slew rate dIw/dt is provided when the motor14is stalled. With higher velocity ω of the motor rotation the slew rate dIw/dt of the free-wheeling mode is increased more and more by the increased BEMF VB. As above Equation 2 shows, this dependency between slew rate dIw/dt and velocity ω of the motor rotation is proportional. From the decrease of the free-wheeling current Iwresults a decrease of the voltage drop Viat the measuring resistor44and a decrease of the potential of the inverting input52of the Schmitt-Trigger40. As soon as the voltage VSat the inverting input52is lower than the control voltage Vsetminus the hysteresis amount of the Schmitt-Trigger40, the Schmitt-Trigger40switches back to the boost mode. The value of the current Iwthrough the motor winding12reached at this time is called IMIN. When the potential Viof the inverting input52gets lower than the control voltage Vsetplus the hysteresis amount of the Schmitt-Trigger40, the output38of the Schmitt-Trigger40activates the MOSFET30. From thereon, the described procedure is repeated. In the chopper-controlled motor14, the current Iwis alternating between the two levels Imaxand Imin. The length of the free-wheeling period90depends on the hysteresis amount of the hysteresis curve of the Schmitt-Trigger40, on the value of the inductor L of the equivalent circuit20of the motor winding12, on the velocity ω of the motor rotation, and on the value of the sum of the ohmic resistances in the free-wheeling current circle82. Summarized, in a rotating motor the BEMF VBis opposing the driving voltage UWand increases the current rise time74and reduced the current fall time90of the current Iw. This impacts the rise slew rate dIw/dt, the fall slew rate dIw/dt, a chopper frequency f, a length 1/f of the chopper duty cycle102, and the chopper duty rate g. In principle, each of these values can be used in a speed monitoring circuit18to measure the velocity ω and hence a stalling of the motor14. In practice it is most suitable to measure the value of one of the chopping frequency f, the length 1/f of the chopper duty cycle, or the chopper duty rate g=(current rise time74)/((current rise time74)+(current fall time90)). Following equations 1 and 2, with Up>UBthe chopper duty rate g (Iw) can be calculated as: g=|1/Up−UB−RL*Iw|/(|1/(UP−UB−RL*Iw)|+|−1/(UB−RL*Iw)|)=(UB+RL*Iw)/UP(Equation 3). Under the assumption that Upis kept constant, UB(Iw)/Up=g−(RL*Iw)/Upis a measure for the velocity ω of the motor14. Using the chopper duty rate g for the velocity measurement has the benefit that no knowledge about the value of the inductor L is required.

An input104of the speed monitoring circuit18is connected to the output38of the Schmitt-Trigger40and senses the gate voltage Vgof the chopper switch30. The speed monitoring circuit18comprises a pulse detector106and a pulse counter112. The pulse counter112has an output113to convey a counting result to an input114of a comparator115. The comparator115is designed for comparing the counting result with a limit value and to derive from the comparison result an estimation of the current motor speed ω. The circuit18for speed monitoring of an electric motor14comprises: a circuit106for generating a time-frame signal, which indicates when a time-frame of predefined length changes from an inactive state to an active state, and which indicates when the time-frame changes back from the active state to the inactive state; a circuit106for receiving a first signal Vgfrom a chopper driver circuit16designed to drive the electric motor14; a circuit106for detecting chopper pulses103in the first signal Vg; a pulse counter112designed to count the detected chopper pulses103while the active state is indicated by the circuit106for generating the time-frame signal; and a circuit115for at least one of outputting and evaluating a state of the pulse counter112, after the inactive state of the time-frame has been indicated. The pulse counter112is designed to be reset, when the time-frame signal indicates a change to the active state of the time-frame. The pulse counter112is designed to stop a counting of the chopper pulses103, when the time-frame signal indicates a change into the inactive state. The circuit18for speed monitoring comprises a circuit112for determining a frequency of the chopper pulses103. Alternatively or in addition, the circuit18for speed monitoring comprises a circuit112for determining a length 1/f of a period of a chopper duty cycle102of the chopper pulses103. Alternatively or in addition, the circuit18for speed monitoring comprises a circuit112for determining a chopper duty rate g of the chopper pulses103.

According to a second aspect of the invention, a phase shift α is measured when the winding voltage UW(coil voltage) is generated by pulse-width modulation (PWM). The magnitude of the driving voltage UWis known at all times because the PWM duty cycle is software-controlled by the current controller53. The phase shift α can be determined by measuring a delay α between zero crossings of the winding voltage UWand the winding current Iwor by measuring a delay between a peak winding voltage Uwpeakand a peak winding current Iwpeak. The value of the peak voltage Uwpeakis notified by the second signal Vuand the value of the current Iwpeakflowing through the winding12is notified by Vi. The moving rotor of the electric machine14increases the phase shift a between the driving current Iwand driving voltage UWin micro-step operation. This additional shift a of a moving motor14is caused by the inertia and the slip of the rotor. Due to its inertia, the rotor is lagging behind the electromagnetic field in the windings L for a given velocity ω. The BEMF VBis induced by the moving rotor and is therefore also delayed compared to the driving voltage Uw. The delay adds an additional phase shift a when the rotor is moving. In case of a stalled motor14, there is no BEMF VBsignal and the phase shift α is significantly lower than with the rotating motor14. The circuit18for speed monitoring of an electric motor14comprises: a circuit120for receiving a second signal Vufrom a chopper driver circuit16for the electric motor14, wherein the second signal Vuincludes a value of a voltage UWapplied to a winding12of the electric motor14; a circuit122for receiving a value Viof an electric current Iwflowing through the winding12; a circuit124for determining a phase angle α between the voltage UWapplied to the winding12and the electric current Iwflowing through the winding12; and a circuit126for at least one of outputting and evaluating the phase angle α. An example embodiment of the circuit18has all features according to both of the first and second aspect.

Within the described embodiments at least one of a chopper frequency, a length 1/f of a chopper duty cycle102, a chopper duty rate g, and a phase shift a between the winding voltage UWand the winding current Iwis measured and analyzed, in order to gain information about the angular velocity ω of the motor14. With the embodiments the chopper frequency f respectively length 1/f of a chopper duty cycle, respectively a chopper duty rate g, respectively a phase shift a can be measured continuously. Thereby, velocity measurement of those electric motors14is performable, which are driven by a chopper control or a pulse-width modulated voltage UWacross the motor windings12. This applies in particular to stepper motors14, in particular DC stepper motors14. In particular not only a rise time74is measured when the current Iwis commutated. The embodiments can be used for micro-step operation. The speed monitoring circuit18, respectively method, can be implemented in a motor control unit (MCU), in an on-chip motor controller, or in a dedicated motor controller, in particular in an electronically-commutated motor for automotive and non-automotive applications, e.g. for stepper motors and BLDG motors (BLDG=brushless direct current). The circuit18is designed for a least one of detecting a stall state of the electric motor14and of determining a speed of the electric motor14.

According to a third aspect a method for speed monitoring comprises following steps: generating a time-frame signal, which indicates when a time-frame of predefined length changes from an inactive state to an active state, and which indicates when the time-frame changes back from the active state to the inactive state; receiving a first signal Vgfrom a chopper driver circuit16controlling the electric motor14; detecting chopper pulses103in the first signal Vg; counting the detected chopper pulses103while the active state is indicated; and at least one of outputting and evaluating a count value, after the inactive state of the time-frame has been indicated.

According to a fourth aspect of the invention a method for speed monitoring of an electric motor14comprises following steps: receiving a second signal Vufrom a chopper driver circuit16driving the electric motor14, wherein the second signal Vuincludes a value of a voltage UWapplied to a winding12of the electric motor14; determining a value Viof an electric current Iwflowing through the winding12; and determining a phase angle α between the voltage UWapplied to the winding12and the electric current Iwflowing through the winding12.