Re:GeckoDrive 201 and Bridgeport Stepper Motors

Ballendo,

Good afternoon! You have it very nearly correct. Here's how the
current control is done in the G201:

1) A 20kHz clock "sets" a set-reset latch causing current to increase
in the motor winding at a rate equal to V/L amps per second.

2) A comparator senses this current and compares it against the sine
reference value.

3) When the current sense matches the reference, the comparator
resets the latch, reversing the voltage on the winding and causing
current to decay at the V/L rate.

4) process repeats.

Because both windings are "set" simultanously by the clock, both are
phase-locked and do not generate audible sub-harmonics as free-
running chopper circuits do (constant off-time choppers).

There is no "time constant optimization" per se, Changing the power
supply voltage merely changes the rate of current change (V/L).


It is important to apply a frictional load to a full-step or half-
step driven motor; this first-order damping lessens the nasty low-
speed resonances these drives have. It is NOT necessary on a
microstep driven motor because there are no resonances to damp out.


There is modification necessary for double-stack 42 and larger
motors. This has to do with midband resonance suppresion. The G201
senses motor load (rate of change), ampilfies this signal and phase
modulates the internal step pulses to damp mid-band instability
(viscous, or second-order damping). Large motors give too big of a
load sense signal and saturate this amplifier. The amplifier gain has
to be lowered by changing the value of a gain-set resistor for those
motors.


Poor performance, motor singing etc. usually trace back to improper
current set (too low) or power supply problems (insufficient current
or too small main filter cap).


Size 42 motors come into their own at power supply votages higher
than 80VDC, typically 160 to 240VDC. The highest power output of a 7
Amp, 80VDC is about 1/4 horsepower (180 to 200W) at the motor shaft.
This is obtained from a triple-stack 34 motor. Size 42 motors have
much higher losses limiting them to 120 to 150 Watts max at these
supply levels (80VDC, 7A).


Mariss

> Rick,
>
> Mariss may correct me, but microstepping is a function of the

CURRENT

> RATIOS between adjacent coils. Higher voltage just PUSHES this
> current into the coils faster.
>
> I believe you may be dealing with an internal(to the drive) time
> constant which is set for typical 23/34 size steppers. This would
> have the effect of trying to move the rotor to a new postion BEFORE
> the current position is achieved. Net result would be loss of

torque

> and poor performance. ALSO, were the BM's hooked up (to the BP)

when

> testing? Steppers do better when they are driving something.
> Otherwise the spring-damped system that is a stepmotor is out of
> balance.
>
> The 56 volts used in the bridgeport surely HAS ALREADY taken into
> account the motor voltage rating. I don't believe your BP steppers
> are nominally rated at 56volts, and therefore need to be driven at
> 224 to 560 volts for good performance!
>
> Ballendo
>
> P.S. Tim mentioned gecko may have drives optimized for 42 size
> motors. This would probably correct the problem.