1. Servo Motor Drive Velocity Tracking
Reducing tracking error
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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 range
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3. Framework…
Application:
• Machine tool industry
Objective:
• Control motor velocity as precisely (from 0 to 100 % or more
rated speed)
• Under external disturbances
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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/day
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5. Limit factors
1. ADC current resolution
2. Velocity feedback resolution
3. Dead time
4. Inductance
5. Back emf distortion
6. Residual encoder offset
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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/day
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7. Limit factors..
Mitigation :
• Oversampling and averaging to reduce quantization noise
Disadvantage :
• ADC sampling frequency and triggering limitations
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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
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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)
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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)
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11. Limit factors……
Inductance
• Inductance varies with current due to saturation
• Distorts dynamics @ 6X electrical frequency + harmonics
( open loop ripple torque)
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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)
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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
copyright 2011 controltrix corp www. controltrix.com

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
copyright 2011 controltrix corp www. controltrix.com

15. Other considerations:
• Non linear dynamics with fixed distance variable time sampling
• Dynamics change with velocity
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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 changes
copyright 2011 controltrix corp www. controltrix.com

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
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18. Simulation results
• Typ. velocity ripple @ 300 rad/s
• Abs err = 0.004 rad/s = 57 rev/day
• Rel. err. = 13ppm
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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 ppm
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21. Thank You
consulting@controltrix.com
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