Subject: stepper motor phase current control problem
Date: Tue, 29 Oct 2002 15:21:26 +0000 (UTC)
Organization: BT Openworld
NNTP-Posting-Date: Tue, 29 Oct 2002 15:21:26 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.50.4807.1700
We are using the MAE type 17 stepper motor( 6 lead HY200-1713-040-B6
15.6R phase resistance) for development, previously we were using a unipolar
driver, namely a UCN5804B device from Allegro, but with the documented lack
of power efficiency, we decided to develop a bipolar drive. I used the
L297/L298 combination, as I felt this would give me a better grounding in
how the bipolar chopper functioned, layout requirements, power dissipation
that sort of thing.
The issue...currently when the motor is in a static position, a holding
applied, the motor is starting to overheat, hot to touch. However, when the
motor is moving
(typically at 600 steps/second) the motor behaves as it should and does not
exhibit any overheating. Allied to this, I am trying to reduce the current
at the static position to decrease the demand on the power supply, yet
provide enough holding torque,
this should also reduce the overheating!
If, in a bid to reduce the static position winding current and the demands
on the power supply, I attempt
to decrease the Vref (compared with sensed voltage to control chopping) to
near or lower than the sensed voltage
(sensed winding current), then the Vref and current sense inputs become
'noisy' and although the motor
maintains a holding torque, albeit with some vibration, the phases are now
being turned on and off.
How can I effectively reduce the static position winding current and thus
The datasheets talk about chopper stability and that if spikes on the
sensing inputs exceed the
Vref, then the comparator can be fooled into resetting the flip-flop; it
describes including an RC on the sense input to the comparator or to use the
inherent set dominance of the flip-flop to mask out the spike. I've tried
increasing the chopper
frequency and including an RC at the inputs to the L297 from the sense
any noticeable effect.
Further information...we are using a 15V power supply for the motor. The
controller from start is
always enabled, we're in half step mode using Enable Chopping (not phase
chopping). For the current sense, a pair of 0R5 resistors are employed with
a reference voltage for the comparator set at >0.2V (max phase current =
0.4A). The chopping frequency is about 20KHz.
I hope someone can help.