1. Servo Motor Drive Velocity Tracking
Reducing tracking error

2. Framework PMSM vector control drive Velocity Feedback:
Encoder with finite pulses/rev (e.g )
Current feedback:
LEM sensor + onchip 12 bit ADC +/-15A full range
Voltage feedback:
DC Voltage sense + onchip 12 bit ADC 800 V full range

3. Framework… Application: Machine tool industry Objective:
Control motor velocity as precisely (from 0 to 100 % or more rated speed)
Under external disturbances

4. Challenges at low speed operation
Limit abs. Slow velocities (small value  large % err)
Limit Rel. faster velocities (small %  large err)
Rel. and Abs velocity tracking error
Abs err. required approx 10 rev/day

5. Limit factors ADC current resolution Velocity feedback resolution
Dead time
Inductance
Back emf distortion
Residual encoder offset

6. Limit factors. ADC Current feedback res  limits torque impulse res.
Kt = torque constant = Nm/A
Ts = sampling time = 1 ms
J = inertia = Kgm2
dI = ADC current resolution = 10 mA
dT = Kt. dI
Min abs velocity resolution = Ts. dT / J approx rev/day

7. Limit factors.. Mitigation :
Oversampling and averaging to reduce quantization noise
Disadvantage :
ADC sampling frequency and triggering limitations

8. Limit factors… Velocity feedback resolution
Fixed time sampling (# pulses in 1 sample period)
1 pulse/1ms = 6 rev per min
@ High velocities less troublesome
@ Low velocities major problem

9. Limit factors…. Mitigation : Fixed angular distance/measured time
Limited by processor timer resolution
@ High velocities more trouble some
@ Low velocity is OK
Disadvantage :
Non linear / non time invariant dynamics
(LTI analysis not valid)

10. Limit factors….. Deadtime 2 - 5% duty is wasted in deadtime
Leads to 6 step voltage to be electrical freq.
Non linear dynamics
Distorts 6X electrical frequency + harmonics
( open loop ripple torque)

11. Limit factors…… Inductance
Inductance varies with current due to saturation
Distorts 6X electrical frequency + harmonics
( open loop ripple torque)

12. Limit factors……. Back emf distortion
Back emf waveform is never ideal sine wave.
Some distortion always present
Distorts 6X electrical frequency + harmonics
( open loop ripple torque)

13. Limit factors…….. Residual encoder offset
Leads to direction dependent performance
The angular offset between encoder 0 and motor 0 position is
always an estimate. Some residual always remain

14. Limit factors……… Static friction
Non linear direction dependent component near zero velocity
Note: 3 – 4 – 5 – 6 – 7 are not easy to mitigate unless using repetitive control techniques

15. Other considerations:
Non linear dynamics with fixed distance variable time sampling
Dynamics change with velocity

16. Proposed method Kalman filter based data fusion
Motor Current and encoder data combined to estimate
instantaneous velocity
Estimate current sampling freq. approx (20KHz)
No gain changes

17. Proposed method. No change in sampling freq. based on velocity
(only pulses per sampling period measurement used)
Zero velocity control
Abs velocity resolution limited by current feedback
Relative resolution can be arbitrarily reduced (subject to
abs limit) trade off against dynamic performance
Other limit factors automatically taken care

18. Simulation results Typ. velocity ripple @ 300 rad/s
Abs err = rad/s = 57 rev/day
Rel. err. = 13ppm

20. Extrapolated estimated results
(simulation time is very long at slow speeds)
@ low rpm (6 rpm)
(abs error/ripple estimate based on current quantization)
= rad/s = 3 rev/day
Rel. err. = 100 ppm

21. Thank You