average current PWM control stepper drivers

[Adam Seychell]

I am looking into stepper motor controller circuits and wondering why
no one makes a true bipolar current drivers. The drivers available use
peak current PWM control and sense current between the bridge and
ground, thus giving a unipolar current sensing.

What about the following bridge circuit;

+V --*---------*
| |
FET FET
| |
| |
*--MOTOR--*
| |
| |
FET FET
| |
*---- *--R3--*--R6----
| | | | |\ |
R1 | R2 |_|- \ |
| | | | \_|_
GND--*---------* -|+ /
| | | /
| |
-------R4--*--R5---GND


The op-amp will sense average current of the motor winding in both
positive and negative direction independent of the bridge state.
Average PWM current could be achieved by taking the current sensing
and current programmed signal into an error amplifier. The error
amplifier output is then compared to a triangle wave to produce PWM
control just as in switch mode power supplies. With bipolar current
control the motor would give better control during breaking periods.
Any thoughts ?

Adam

 

[Rich Grise]

Probably because for a stepper, they're not needed. You just
drive it full-on for however long a step needs to be. All the
acceleration and braking is done in software, by timing the
drive pulses. At least that's the way I understand it. (and
was a fundamental assumption in my stepper-driver design work.)

Cheers!
Rich

 

[John Larkin]

I am looking into stepper motor controller circuits and wondering why
no one makes a true bipolar current drivers. The drivers available use
peak current PWM control and sense current between the bridge and
ground, thus giving a unipolar current sensing.

What about the following bridge circuit;

+V --*---------*
| |
FET FET
| |
| |
*--MOTOR--*
| |
| |
FET FET
| |
*---- *--R3--*--R6----
| | | | |\ |
R1 | R2 |_|- \ |
| | | | \_|_
GND--*---------* -|+ /
| | | /
| |
-------R4--*--R5---GND


The op-amp will sense average current of the motor winding in both
positive and negative direction independent of the bridge state.
Average PWM current could be achieved by taking the current sensing
and current programmed signal into an error amplifier. The error
amplifier output is then compared to a triangle wave to produce PWM
control just as in switch mode power supplies. With bipolar current
control the motor would give better control during breaking periods.
Any thoughts ?

Adam

An interesting case is when there's motor current but both bottom (or
top) fets are on; this is a common situation. The opamp reports zero
current, and the wrong current in certain other duty-cycle situations.

I've done microstepping with a current sensor floating on the actual
nasty, noisy motor current leads, and it was ugly. But the stepping
was beautiful. Maybe one of the new cheap Hall-effect current sensors
would allow simple, true coil current sense.

John

 

[Genome]

I am looking into stepper motor controller circuits and wondering why
no one makes a true bipolar current drivers. The drivers available use
peak current PWM control and sense current between the bridge and
ground, thus giving a unipolar current sensing.

What about the following bridge circuit;

+V --*---------*
| |
FET FET
| |
| |
*--MOTOR--*
| |
| |
FET FET
| |
*---- *--R3--*--R6----
| | | | |\ |
R1 | R2 |_|- \ |
| | | | \_|_
GND--*---------* -|+ /
| | | /
| |
-------R4--*--R5---GND


The op-amp will sense average current of the motor winding in both
positive and negative direction independent of the bridge state.
Average PWM current could be achieved by taking the current sensing
and current programmed signal into an error amplifier. The error
amplifier output is then compared to a triangle wave to produce PWM
control just as in switch mode power supplies. With bipolar current
control the motor would give better control during breaking periods.
Any thoughts ?

Adam

Yes indeedy, that's the way it is done. Do a google for the UC3637 motor
control IC. I think the newer version is the UC3638. Have a hunt for the
relevent app notes on TI's website.

Also used for micro-stepping of stepper motors, there's an app note about
that somewhere too.

AND

 

[Russell Shaw]

Probably because for a stepper, they're not needed. You just
drive it full-on for however long a step needs to be. All the
acceleration and braking is done in software, by timing the
drive pulses. At least that's the way I understand it. (and
was a fundamental assumption in my stepper-driver design work.)

Full-on pulsing is only good for low performance, low rate
steppers where the leakage inductance can be ignored.