GB2300711A

GB2300711A - Rotation sensing in DC electric motors

Info

Publication number: GB2300711A
Application number: GB9508073A
Authority: GB
Inventors: David Andrew Smith
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1995-04-20
Filing date: 1995-04-20
Publication date: 1996-11-13
Also published as: DE19615163A1; GB9508073D0

Description

ROTATION SENSING IN DC ELECTRIC MOTORS is 2300711

Field of the Invention:

This invention relates generally to rotation sensing in DC electric motors and has particular, though not exclusive, application to the automobile industry where the monitoring and control of window lift motors is increasingly of interest to manufacturers, inter alia as a result of safety considerations, there having in recent times been a number of fatalities involving small children and motor-driven car windows. Window lift motors should desirably stop quickly on encountering an increased load such as when the hand or neck of a child is caught between the rising window and its frame, and more desirably should automatically reverse their direction of operation.

Backaround of the Invention:

A commonly adopted method of rotation sensing in DC electric motors is to mount one or more magnets on a rotating part of the motor and sense the magnet position by use of a coil or a Hall sensor for example; such an arrangement is shown in DE-C-4142181 and involves undesirable modification of the motor construction. Other methods that have been proposed take advantage of the switching spikes that occur in the commutation of the motor, but such methods are 2 countered by the measures commonly taken to ef f ect radio-frequency suppression of the motor, namely the addition of inductors and capacitors to reduce switching spikes in order to suppress electrical noise transmission back into the electrical wiring harness of the automobile, which means that the sensing system again has to be incorporated into the motor. Furthermore, a system based upon the analysis of switching spikes is inherently at risk from unclean commutation and commutator brush resonances.

The present invention proposes to sense the rotation of a DC electric motor by use of a magnetic search coil to detect the changing magnetic fields generated by the coils of the rotating armature. Systems which make use of magnetic search coils are known from US-A-4 138 642, EP-A-0 529 131 and EP-A-0 626 748 for example. In the arrangement proposed in US- A-4 138 642 sensing coils are proposed to be located where the spatial variations in the magnetic conductivity of the motor structure as the armature rotates make themselves felt most strongly, which obviously maximizes the output of the coil, but otherwise there is little discussion of the signal processing applied to the output developed by the sensing coil. The arrangement of EP-A-0 529 131 uses an inductor comprising a coil and associated core to

3 sense magnetic flux variations, and couples the inductor with a capacitor in a resonant circuit which is tuned to resonate at a frequency corresponding to the rotational frequency of the rotor of the motor, and the output of the resonant circuit is connected to an amplifier having reverse feedback designed to provide maximum amplification within a range embracing the speed range of the motor. The arrangement of EPA-0 626 748 also employs an LC resonant circuit tuned to the alternating magnetic circuit frequency.

The problem with the proposal of US-A-4 138 642 is that there is no teaching whatsoever of the signal conditioning that is necessary to make practical utility of the outputs of the sensing coils. In EP-A0 529 131 and EP-A-0 626 748 there is such teaching, but with the problem that the teaching is to utilize resonant circuits which assumes that the alternating magnetic circuit frequency is generally constant for any particular motor irrespective of its operating conditions and for all of the motors coming of f a particular production line. This, however, is not the case and as a practical matter frequency variations arise, both during operation and as a result of manufacturing variations and tolerances, which are not satisfactorily accommodated by use of resonant circuits.

is 4 Summary of the Invention:

According to the present invention, the motor rotation is sensed by means of a search coil which provides an output in response to magnetic field variations occurring during rotation, and signal conditioning of the signal produced by the search coil is effected by circuitry having both low-pass and high-pass characteristics. As will be described in the following, by appropriate selection -of the lowpass and high-pass characteristics, the variations which occur in practice in the frequency of the coil output can be satisfactorily accommodated.

In accordance with a preferred embodiment of the present invention, the low-pass and high-pass filtering is achieved by use of an operational amplifier having two feedback loops R1C1 and R2C2. Before application to the operational amplifier, the output of the sensing coil is high-pass filtered for suppression of low-frequency noise and, after processing by the operational amplifier, the signal is applied to a Schmitt trigger which transforms it into a pulse train suitable for application to a microprocessor programmed appropriately having regard to the intended application.

The above and further features of the present invention are set forth with particularity in the appended claims and, together with advantages thereof, will be well understood from consideration of the following description which is given with reference to the accompanying drawings.

Description of the Drawings:

Figure 1 is a circuit diagram showing an exemplary embodiment of a rotation sensor according to the present invention for sensing rotation in a DC electric motor; Figure 2 shows the response characteristics of filter circuits incorporated into the rotation sensor of Figure 1; Figure 3 shows a theoretical "ideal" response characteristic; Figure 4 shows a modified response characteristic obtained by changing component values in the filter circuits incorporated into the rotation sensor of Figure 1; and Figure 5 schematically illustrates how the search coil of the rotation sensor of Figure 1 might be associated with a DC electric motor.

Detailed Description of the Embodiment:

Referring to Figure 1, a search coil 1 is wound on a 35mm square former with 80 turns of 0.2mm diameter insulated copper wire (these details of the search coil construction being exemplary only) and may 6 be located outside of the casing of a DC electric motor in an advantageous position as regards convenience and signal strength as shown in Figure 5. The search coil 1 is AC coupled by capacitor C3 and provides an alternating signal to the mid-point of potential divider R3, R4 which biases the signal to a DC level. The capacitor C3 and resistors R3, R4 also act as a high-pass filter for removing low-frequency noise. From the mid-point of potential divider R3, R4 the signal accesses the non- inverting input of a first operational amplifier Al having its output connected in a feedback path constituted by parallel-connected capacitor Cl and resistor Rl to its inverting input and with its inverting input furthermore coupled to ground via series-connected resistor R2 and capacitor C2. The operational amplifier serves as an active filter and amplification stage, with Cl and Rl providing a low-pass type response to the feedback loop and C2 and R2 providing a high-pass type response. From the output of operational amplifier A1 the signal is fed to the inverting input of a second operational amplifier A2 configured as a Schmitt trigger with a feedback resistor R5 providing hysteresis in the switching points. The Schmitt trigger transforms its alternating input into a pulse train output suitable for inputting to a 7 microcontroller programmed to count incoming pulses and initiate predetermined programmed actions in response thereto.

More particularly, the search coil detects the alternating magnetic flux generated by current passing through the motor armature coils and produces an alternating voltage signal across the coil output according to Faradays law. The frequency of the signal depends on the speed of rotation of the motor and the number of separate coils on the armature. Each separate coil will contribute two additional periods to the search coil signal per revolution of the armature. If n is the number of independent coils wound on the armature and w is the rotational frequency of the motor, then the frequency of the search coil output is given by n=2nw. The parameters affecting the magnitude of the output signal can be derived from a consideration of Faradays law applied to the problem, namely:

V= -A dB dt:

where A is the area the coil occupies perpendicular to the generated flux lines and B is the flux density of the field generated from the current passing through the armature coils. If the search coil is assumed to

8 be a current carrying loop of N turns perpendicular to the generated flux lines then an approximation for the signal generated by the coil is given by:

- g offic x dI 2 dt where r represents the radius of the current carrying loop, A, is the permeability of free space and I is the alternating current generated in the search coil. This approximation does not take account of the fact that the sensor 1 in Figure 1 is a square search coil, however the general form of the equation will hold. The alternating current I generated in the search coil is directly related to the current in the armature coil. It can be assumed to be sinusoidal with a frequency n and of an amplitude much less than the motor current due to losses, so:

V=- [LONnzIoocosQ t: 3.

2 where 10 is proportional to the magnitude of the current in the motor armature. N and r can be optimised for signal amplitude and minimum cost. In general, however, provided the amplifier noise is low, any deficiency in signal amplitude can be compensated by additional amplifier gain.

1 9 It is also possible f rom equation 2 to identify the self inductance of the search coil:

L=201C1 2 so an estimate of L is given by L (4nxlO-7x8OxnxO.015)12=2.4gH.

In Figure 1, capacitor C3 and resistors R3 and R4 have a dual purpose; as mentioned hereinbefore, namely:

1.

C3 de-couples the search coil signal and R3 and R4 bias this signal at a d.c. level so that no signal is lost below the ground rail when the amplifier is supplied with a single rail supply; and 2. C3, R3 and R4 act as a high-pass f ilter thereby removing low frequency noise. The filter has -3dB point given by f=11(2zRC3) where R=R3R4/(R3+R4).

The search coil, C3, R3 and R4 do actually form an LCR resonant circuit, but this resonance is not used as part of the signal processing because matching of the resonance to the signal frequency would require careful tuning of the circuit. This could be achieved by precise tuning of the inductor via the use of ferrite core material and careful calculation of the number of turns on the former, but this is not considered to be practical as hereinbefore described. The resonance for the search coil circuit occurs at a frequency given by:

f= 1 - 5.

2nVILC which, for a coil inductance of 2.4MH and a capacitor C3 of 47nF, for example, produces:

1 21c47X10-9x2.4xl 0-6 f= - =473kHZ The resonance, furthermore, has a very low Q f actor given by:

f=-! 'L 6.

R C3 With R3=30K and R4=20K then Q=6x10-4. The high-pass function of C3, R3 and R4 gives a -3dB point of f=280Hz using these values. Typically a window lift motor with five or six independent coils on the armature gives a signal with a frequency n of 500 to 1000Hz in application, the speed varying with changing friction on the window glass during its travel.

Op-amp A1 serves as an active filter and amplification stage, wherein Cl, RI gives a low-pass 11 type response to the feedback loop and C2, R2 gives a high-pass type response to the feedback loop. The high and low frequency gains of the amplifier well out of the pass band are unity. The discrimination of unwanted noise from the required signal is thus best achieved with high gain from A1 and its associated feedback network in the pass band frequencies. The gain of A1 is given by:

G=1+ 7.1 X2 .... 7.

where -L =-.L +jw cl..... 8.

xl R7 and X2=R2+_ 1 _7 w C2 -....9.

The above would suggest that reducing the preamplified signal, so that additional gain could be introduced into the feedback loop of A1 without placing the amplifier into a possible saturation condition, would be advantageous in discriminating non amplifier noise. If A1 does clip the signal due to saturation then the signal to noise ratio of the output signal is seen to deteriorate due to additional 12 amplification of the noise without signal amplitude improvement. This, does however rely on low amplifier noise. Actually increasing the gain economically can also prove dif f icult as it is undesirable to seriously ef f ect the 3dB points of the f ilter roll of f 1 s. C2 tends to increase to compensate for a reduction in R2 and large capacitors usually mean higher prices. R1 can also become large possibly introducing bias current problems or EM pick-up.

Figure 2 shows a typical response curve for the complete filter circuit consisting of elements C3, R3, R4, C2, R2, Rl, Cl and Al for the values given in the figure. The values used to produce Figure 2 were used in a prototype system with a motor producing a signal with n over the range 500 to 1000Hz. Observation of equation 3 would suggest a more suitable "ideal."

response would be that of Figure 3. This response, if correctly set-up, will produce a constant magnitude output signal independent of frequency which is the ideal case for transforming into a pulse train with a Schmitt trigger. Equation 3, however, does not fully describe the actual behaviour of the signal in a practical application. The parameter I, does not remain constant as the amplitude of the current drawn by the armature windings is not constant. For low motor speeds brought about by frictional effects of 13 the window seals and upon start up of the motor, the current is higher than when the motor is running under low load. Under less load the motor also runs quicker. Essentially the motor behaves more as a constant voltage load than a constant current load.

The f ield generated by the armature is thus greater in magnitude for lower motor RPM than for higher motor RPM. This is in opposition to the increased search coil output signal amplitude brought -about by a greater rate of change of magnetic f ield. The overall result is that in all cases studied the search coil has given a larger signal when the motor is running under load at n=500Hz than when running relatively freely at n=1000Hz. This explains the chosen frequency response of Figure 2. The circuit depicted in Figure 1 has two high-pass stages C3, R3, R4 and R2, C2 and it is therefore possible to modify the slope of the response curve by moving the relative 3dB points of these filters to produce a more optimum response in order to remove unwanted amplitude variations in the signal. Figure 4 shows a modified response curve which was obtained by use of different component values and was more successful than the original curve. Figure 4 shows relatively more amplification at 1000Hz than at 500Hz compared with Figure 3.

14 Op-amp A2 is configured as a Schmitt trigger with feedback resistor R5 providing hysteresis in the switching points. The Schmitt trigger gives a pulse train output with 2n pulses per revolution of the motor. For noisy signals with constant signal amplitude the Schmitt trigger can prove very effective at discriminating the signal from the noise. This obviously relies on the amplitude of the noise being less than the signal amplitude. This is-achieved by setting the hysteresis just less than the peak to peak amplitude of the signal. If the signal does vary in amplitude the hysteresis limits have to be set to detect the smallest peak to peak amplitude observed, and this is not an optimum condition for noise suppression at the higher signal amplitudes. The output of the Schmitt trigger, V,,,, is passed to a microcontroller which counts the number of pulses from the Schmitt trigger. The microcontroller count is advantageously edge triggered on the negative going edges of the pulse train for the micro under consideration. C4 slows down the switching edges and reduces false triggering of the microcontroller.

The sensor circuit as described in the foregoing has advantages over, for example, the proposal of EPA-0 626 748. The detector circuit described in EP-A-0 626 748 is made up of an inductor (with or without a f errite core) and a capacitor. These elements produce a resonance circuit which is said to be tuned to resonate close to the alternating magnetic signal frequency. Without the ferrite core a considerable number of turns would have to be placed on the inductor for the circuit to resonate at 1000Hz. The only resistance shown is the self resistance of the inductor, though this is not actually mentioned as a contributing factor. The self resistance is clearly quite low so the Q f actor will be large. This will make a very sharp resonance which will be poor at detecting signals over a wide frequency range such as 500-1000Hz. As a comparison, to produce a 3dB band similar to that of Figure 2, namely from 500Hz to 2500Hz with a centre frequency of about 1500Hz, then, using the equations AW - 1 W Q and T.1Cr 2 _ [ION? I .... 10.

.... 11.

gives Q=0.75 assuming a fairly large capacitor value of 1MF and a suitable inductance L=limH with a resistance of R=141n. This would be difficult to 16 achieve in practice without the addition of an external resistor. A ferrite core would probably also have to be used as from equation 10 a free air solenoid of 5mm diameter and length 20mm would require approximately 2100 turns to achieve an inductance of limH with a coil resistance of 4.5n. The number of turns could be reduced by including a ferrite core and typically this would reduce N by a factor of 50. it would also reduce the coil resistance similarly. N in this case would be more practical but an external resistor would still have to be used. in comparison, the circuit depicted in Figure 1 does not rely on resonance, having a resonant frequency of 500kHz and the low Q required. It also does not use a f errite core. The inductor of EP-A-0 626 748 is depicted as solenoidal whereas that in Figure 1 is a low profile coil of large area to increase flux linkage and hence signal magnitude. The coil approximates to a current carrying loop of N turns in the calculations above. The low profile of the coil allows it to be mounted on the motor body as shown in Figure 5. This positioning places the search coil co- linear with the motor winding being energised, so maximising the flux linkage between the two coils.

Having thus described the present invention by reference to an exemplary embodiment, it is to be well 17 appreciated that modifications and variations are possible without departure from the scope of the invention. one possible modification that could be made would be to remove C3, R3 and R4. The consequence of this however would be to reduce the signal magnitude by 50%. With present motors this should not be a problem. However, there is a 1800 phase change in the signal on reversing the direction of the motor. If the signal was noisy and the noise was confined to a particular region of the period, then in one direction the system would function correctly and incorrectly in the other. Noise tends to be introduced when the motor has a worn commutator, arcing occurring across the segments as the brushes move from one to the next. The removal of C3, R3 and R4 is thus not desirable.

is is

Claims

CLAIMS:

1. A rotation sensor for a DC electric motor comprising a search coil adapted to provide an output signal in response to magnetic field variations occurring in the motor environment during rotation of the same, and signal conditioning circuitry coupled to the search coil output and having low-pass and highpass characteristics selected to provide noise discrimination outside of a frequency range corresponding to operational variations in the frequency of the coil output signal.

2. A rotation sensor as claimed in claim 1 wherein the search coil is adapted for location outside of the motor casing.

3. A rotation sensor as claimed in claim 1 or 2 wherein the signal conditioning circuitry comprises an operational amplifier having associated components providing both low-pass and high-pass responses to the amplifier transfer characteristics.

4. A rotation sensor as claimed in claim 3 wherein the operational amplifier has associated therewith a first, parallel-connected resistor capacitor network in its feedback loop for providing a low-pass 19 response, and a second, series-connected resistor capacitor network for providing a high-pass response.

5. A rotation sensor as claimed in claim 4 wherein the first resistor capacitor network is connected between the amplifier output and its feedback input, and the second resistor capacitor network is connected between the feedback input of the amplifier and signal ground.

6. A rotation sensor as claimed in claim 3 or 4 or 5 wherein the output of the operational amplifier is connected to the input of a pulse transformation circuit for transforming the amplifier output into a corresponding pulse train.

7. A rotation sensor as claimed in claim 6 wherein said pulse transformation circuit comprises a Schmitt trigger circuit.

8. A rotation sensor as claimed in claim 7 wherein said Schnitt trigger circuit has a feedback resistor providing switching point hysteresis.

9. A rotation sensor as claimed in any of the preceding claims wherein a decoupling capacitor is connected in the output of the search coil.

10. A rotation sensor as claimed in claim 9 wherein the decoupling capacitor f eeds the output signal of the search coil to a potential divider f or biassing the signal at a predetermined DC level.

11. A rotation sensor as claimed in claim 10 wherein the decoupling capacitor and the resistances of the potential divider are selected to act as a high-pass filter for low frequency noise discrimination.

is

12. A rotation sensor for a DC electric motor, said rotation sensor being substantially as herein described with reference to Figure 1 of the accompanying drawings.

13. A rotation sensor as claimed in any of the preceding claims wherein said low-pass and high-pass characteristics are selected so as together to provide preferential signal conditioning between about 500Hz and 1000Hz.

14. A rotation sensor as claimed in any of the preceding claims in combination with a microprocessor arranged to provide predetermined functional outputs 21 in response to inputs from the rotation sensor.

15. A DC electric motor having associated therewith a rotation sensor as claimed in any of claims 1 to 13, or a rotation sensor and microprocessor combination as claimed in claim 14.

16. A window-lift motor in or for an automobile, said motor comprising a DC electric motor having associated therewith a rotation sensor and microprocessor combination as claimed in claim 14, said microprocessor being arranged to control the operation of the motor.