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# Filtering servo motor- Reducing velocity tracking error

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### Filtering servo motor- Reducing velocity tracking error

1. 1. Servo Motor Drive Velocity Tracking Reducing tracking error www.controltrix.comcopyright 2011 controltrix corp www. controltrix.com
2. 2. Framework PMSM vector control drive Velocity Feedback: • Encoder with finite pulses/rev (e.g. 10000) Current feedback: • LEM sensor + onchip 12 bit ADC +/-15A full range Voltage feedback: • DC Voltage sense + onchip 12 bit ADC 800 V full rangecopyright 2011 controltrix corp www. controltrix.com
3. 3. Framework… Application: • Machine tool industry Objective: • Control motor velocity as precisely (from 0 to 100 % or more rated speed) • Under external disturbancescopyright 2011 controltrix corp www. controltrix.com
4. 4. Challenges at low speed operation • Limit abs. err. @ Slow velocities (small value  large % err) • Limit Rel. err. @ faster velocities (small %  large err) • Rel. and Abs velocity tracking error • Abs err. required approx 10 rev/daycopyright 2011 controltrix corp www. controltrix.com
5. 5. Limit factors 1. ADC current resolution 2. Velocity feedback resolution 3. Dead time 4. Inductance 5. Back emf distortion 6. Residual encoder offsetcopyright 2011 controltrix corp www. controltrix.com
6. 6. Limit factors. ADC Current feedback res  limits torque impulse res. • Kt = torque constant = 0.75 Nm/A • Ts = sampling time = 1 ms • J = inertia = 0.00017 Kgm2 • I = ADC current resolution = 10 mA T = Kt. I Min abs velocity resolution = Ts. T / J approx 20 -50 rev/daycopyright 2011 controltrix corp www. controltrix.com
7. 7. Limit factors.. Mitigation : • Oversampling and averaging to reduce quantization noise Disadvantage : • ADC sampling frequency and triggering limitationscopyright 2011 controltrix corp www. controltrix.com
8. 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 problemcopyright 2011 controltrix corp www. controltrix.com
9. 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)copyright 2011 controltrix corp www. controltrix.com
10. 10. Limit factors….. Deadtime • 2 - 5% duty is wasted in deadtime • Leads to 6 step voltage to be applied @ electrical freq. • Non linear dynamics • Distorts dynamics @ 6X electrical frequency + harmonics ( open loop ripple torque)copyright 2011 controltrix corp www. controltrix.com
11. 11. Limit factors…… Inductance • Inductance varies with current due to saturation • Distorts dynamics @ 6X electrical frequency + harmonics ( open loop ripple torque)copyright 2011 controltrix corp www. controltrix.com
12. 12. Limit factors……. Back emf distortion • Back emf waveform is never ideal sine wave. • Some distortion always present • Distorts dynamics @ 6X electrical frequency + harmonics ( open loop ripple torque)copyright 2011 controltrix corp www. controltrix.com
13. 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 remaincopyright 2011 controltrix corp www. controltrix.com
14. 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 techniquescopyright 2011 controltrix corp www. controltrix.com
15. 15. Other considerations: • Non linear dynamics with fixed distance variable time sampling • Dynamics change with velocitycopyright 2011 controltrix corp www. controltrix.com
16. 16. Proposed method • Kalman filter based data fusion • Motor Current and encoder data combined to estimate instantaneous velocity • Estimate velocity @ current sampling freq. approx (20KHz) • No gain changescopyright 2011 controltrix corp www. controltrix.com
17. 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 carecopyright 2011 controltrix corp www. controltrix.com
18. 18. Simulation results • Typ. velocity ripple @ 300 rad/s • Abs err = 0.004 rad/s = 57 rev/day • Rel. err. = 13ppmcopyright 2011 controltrix corp www. controltrix.com
19. 19. copyright 2011 controltrix corp www. controltrix.com
20. 20. Extrapolated estimated results (simulation time is very long at slow speeds) • @ low rpm (6 rpm) (abs error/ripple estimate based on current quantization) = 0.0002 rad/s = 3 rev/day • Rel. err. = 100 ppmcopyright 2011 controltrix corp www. controltrix.com
21. 21. Thank You consulting@controltrix.comcopyright 2011 controltrix corp www. controltrix.com