www.controltrix.com Servo Motor Drive Velocity Tracking

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www.controltrix.com

Servo Motor Drive Velocity Tracking

Reducing tracking error

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

Framework…

Application:

Machine tool industry

Objective:

Control motor velocity as precisely (from 0 to 100 % or more rated speed)

Under external disturbances

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

Limit factors

ADC current resolutionVelocity feedback resolutionDead time

InductanceBack emf

distortion

Residual encoder offset

ADC Current

feedback res  limits torque impulse

res.

K

t

= torque constant = 0.75 Nm/A

T

s

= sampling time = 1 ms

J = inertia = 0.00017 Kgm

2

dI = ADC current resolution = 10 mAdT = Kt. dIMin abs velocity resolution = Ts. dT / J approx 20 -50 rev/day

Limit

factors.

Limit factors..

Mitigation :

Oversampling and averaging to reduce quantization noise

Disadvantage :

ADC sampling frequency and triggering limitations

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

Limit

factors…

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)

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)Limit factors…..

Inductance

Inductance varies with current due to saturation Distorts dynamics @ 6X electrical frequency + harmonics

( open

loop ripple torque)

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)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

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

Limit

factors………

Other considerations:

Non linear dynamics with fixed distance variable time sampling

Dynamics change with velocity

Kalman filter based data fusion

Motor Current and encoder

data

combined to estimate

instantaneous

velocity

Estimate

velocity @ current sampling

freq. approx (20KHz

) No gain changesProposed 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 careProposed method.

Simulation results

Typ. velocity ripple @ 300

rad/s

Abs err

= 0.004

rad

/s = 57 rev/day

Rel. err.

= 13ppm

@ low rpm (6 rpm) (abs error/ripple estimate based on current quantization)

= 0.0002 rad

/s = 3 rev/day

Rel. err.

= 100

ppm

Extrapolated estimated

results

(simulation time is very long at slow speeds)

Thank You

consulting@controltrix.com