ADAPTIVE FILTER FOR MOTOR SPEED MEASUREMENT SYSTEM

Abstract

A filter for motor speed measurement signals includes one or more resonators configured to filter signals having a frequency that is proportional by a predetermined factor to the frequency of the motor whose speed is measured.

Claims

1. A filter for motor speed measurement signals, the filter comprising: one or more resonators configured to filter signals having a frequency that is proportional by a predetermined factor to the frequency of the motor whose speed is measured.

2. The filter of claim 1, wherein the one or more resonators is/are configured to have a resonant frequency proportional to the frequency of the motor.

3. The filter of claim 1, wherein each resonator is configured as a closed loop system comprising two variable-gain integrators connected in anti-parallel.

4. The filter of claim 1, wherein one the one or more one resonators is configured to filter signals having a frequency equal to the frequency of the motor.

5. The filter of claim 4, wherein one the one or more one resonators is one resonator configured to filter signals having a frequency equal to two times the frequency of the motor.

6. A speed position measuring system comprising: a sensor arranged to determine the speed of rotation of a motor and to provide a speed measurement signal; and a filter as recited in claim 1, wherein the filter is arranged to receive the speed measurement signal and to filter ripple signals therefrom being signals having a frequency that is proportional by a predetermined factor to the frequency of the motor.

7. The speed position measuring system of claim 6, wherein the sensor comprises a resolver.

8. The speed position measuring system of claim 6, wherein the sensor comprises a Hall effect sensor.

9. The speed position measuring system of claim 6, further comprising a demodulator to demodulate the output of the sensor to provide the speed measurement signal.

10. A motor drive system comprising: motor drive circuitry to control a motor to rotate at a given speed; and a speed position measuring system that includes: a sensor arranged to determine the speed of rotation of a motor and to provide a speed measurement signal; and a filter as recited in claim 1, wherein the filter is arranged to receive the speed measurement signal and to filter ripple signals therefrom being signals having a frequency that is proportional by a predetermined factor to the frequency of the motor, wherein the filter is in a feedback loop between the sensor and the motor drive circuitry.

11. The system of claim 10, further comprising the motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a simple representation of a single notch filter configuration in accordance with an embodiment of this disclosure.

[0022] FIG. 2 is a simple representation of a double notch filter configuration in accordance with an embodiment of this disclosure.

[0023] FIG. 3 is a simple representation of the configuration of a resonator such as shown in FIG. 1 or 2.

[0024] FIG. 4 shows the poles and zeroes as well as the frequency response of a single-notch filter.

[0025] FIG. 5 shows a more detailed diagram of a single-notch filter.

DETAILED DESCRIPTION

[0026] FIG. 1 shows a filter configuration according to an embodiment of the present disclosure. The filter is configured to filter out ripple from a speed signal derived from a motor speed sensor (not shown) such as (as discussed above) a resolver or a Hall sensor. The measured speed signal will include ripple due to feedback distortions caused by the speed sensors and other analogue interface components.

[0027] The speed signal from the demodulation algorithm is usually provided as feedback to the speed control loop of the motor drive system (not shown).

[0028] The filter in the embodiment of FIG. 1, a so-called ‘single-notch’ filter includes a variable frequency resonator 1 provided on a feedback path. The resonator 1 is configured, based on the average measured motor speed, knowing how the ripple is related to that speed, to remove the ripple from the input signal applied to subtracter 2.

[0029] If two different ripple frequencies are to be removed, the filter can be designed as a double-notch filter as shown in FIG. 2 comprising two resonators (1A and 1B) and two adders/subtracters 2,3.

[0030] The resonator(s) has/have a structure as shown in FIG. 3. As shown, each resonator is a closed loop system comprising two variable-gain integrators 4,5 connected in anti-parallel.

[0031] As mentioned above, the ripple frequency is known to be proportional to the mechanical frequency of the motor by a factor dependent on the type of distortion as discussed above—for DC offsets, the ripple frequency is equal to the motor frequency; for other distortions it is double the motor frequency.

[0032] For a single notch filter as shown in FIG. 1, the gain of the resonator should be such that the resonant frequency of the resonator is the same as the motor mechanical frequency as that will be the frequency of the ripple to be removed from the speed signal (for the first type of distortion mentioned above).

[0033] The resonator 1 of FIG. 1 has a configuration as shown in FIG. 3. If both integrators 4,5 have the same gain G=ω.sub.M=2π.Math.f.sub.M (where f.sub.M is the motor mechanical frequency) then the transfer function of the resonator is as follows:

[00001]$H_{\mathrm{OSC}}\left(s\right)=\frac{\frac{{\omega }_M}{S}}{1+\frac{{\omega }_M}{S}.Math.\frac{{\omega }_M}{S}}=\frac{{\omega }_M.Math.S}{S^2+{\omega }_M^2}$

[0034] Thus, the resonant frequency is equal to the motor mechanical frequency, which covers the first type of signal distortion mentioned above. The closed loop filter transfer function is the following (where K.sub.3 is the filter gain in FIG. 1):

[00002]$H_{\mathrm{NJ}}\left(s\right)=\frac{1}{1+K_3.Math.\frac{{\omega }_M.Math.s}{s^2+{\omega }_M^2}}=\frac{s^2+{\omega }_M^2}{s^2+{\omega }_M^2+K_3.Math.{\omega }_M.Math.s}$

[0035] Gain K.sub.3 needs to be tuned such that the positions of the two filter poles are close to the positions of the filter zeroes in order to achieve very narrow frequency notches (see FIG. 4). The real components of the poles need to be small negative values. The imaginary components of the poles need to be in approximate alignment to the imaginary components of the zeroes (see again FIG. 4). The filter poles are:

[00003]$p_{1,2}=\frac{-K_3.Math.{\omega }_M\pm {\omega }_M.Math.\sqrt{K_3^2-4}}{2}$

[0036] The condition for the filter stability is K.sub.3<2. Very small K.sub.3 produces very good vertical alignment between poles and zeroes. However, this also reduces the stability margin of the filter by bringing the poles very close to the imaginary axis. Larger K.sub.3 improves the filter stability but also increases the width of the frequency notches.

[0037] A good compromise is produced by K.sub.3=0.25. The resulting poles are:

p.sub.1,2=−0.125.Math.ω.sub.M±0.99.Math.jω.sub.M

[0038] The amplitude of the speed oscillations caused by the Sin and Cos distortions is proportional to the motor speed. Therefore, almost no oscillations are present in the measured speed when the motor speed if very low. However the resonator inside the filter will always oscillate after any fast transient on the input signal. For instance, this can happen when the motor speed decreases rapidly from high speed to low speed.

[0039] The full filter configuration needs to include variable saturation limits for the two integrators to eliminate unwanted oscillations in the resonator. The integrator limits will be proportional to the absolute value of the motor speed as indicated in FIG. 5. The proportionality factors K.sub.1 and K.sub.2 are set based on the following considerations: [0040] Gain K.sub.1 needs to be set just above the amplitude of the oscillations in the input speed measurement in order to allow these oscillations to be removed by the negative feedback loop of the filter. [0041] The steady state value of the second integrator is “Input Speed×K.sub.3”.

[0042] Therefore gain K.sub.1 needs to be set slightly larger than K.sub.2 to provide margin for the oscillations that occur when speed ripple is being eliminated by the filter.

[0043] The recommended value is:

K.sub.1=1.251K.sub.2=0.3125

[0044] A second resonator can be added to the filter if two frequencies need to be removed from the input signal (see FIG. 2). The integrator gains of the second resonator are set to G=2.Math.ω.sub.M.

[0045] The filter of this disclosure provides improved motor speed measurement accuracy when the position sensor outputs are affected by sensor distortions. The filter is also immune to variations in analogue component parameters of the drive system.

[0046] The described embodiments are by way of example only. The scope of this disclosure is limited only by the claims.