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
copyright 2011 controltrix corp www. controltrix.com
3. Framework…
Application:
• Machine tool industry
Objective:
• Control motor velocity as precisely (from 0 to 100 % or more
rated speed)
• Under external disturbances
copyright 2011 controltrix corp www. controltrix.com
5. Limit factors
1. ADC current resolution
2. Velocity feedback resolution
3. Dead time
4. Inductance
5. Back emf distortion
6. Residual encoder offset
copyright 2011 controltrix corp www. controltrix.com
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
copyright 2011 controltrix corp www. controltrix.com
7. Limit factors..
Mitigation :
• Oversampling and averaging to reduce quantization noise
Disadvantage :
• ADC sampling frequency and triggering limitations
copyright 2011 controltrix corp www. controltrix.com
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
copyright 2011 controltrix corp www. controltrix.com
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. 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. 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. 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. 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
copyright 2011 controltrix corp www. controltrix.com
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
copyright 2011 controltrix corp www. controltrix.com
