Re: 20 Amp 80 V $99 DC Servodrive

Ron,

Actually you would need 12,000 pulses. The encoder line count is
multiplied by 4 internally. A 1000 line encoder yeilds 4000 locations
per revolution.

The pulse multiplier is interesting. It puts out bursts of 10 pulses
from 0 to 10 full steps per second. The step motor "burps" on every
step pulse until the "burps" meld together into a smooth pulse train
at about 10 FPS. You are correct, it can't "know" what a subsequent
input period will be in order to parse it into 10 evenly spaced sub-
periods. What it employs is a PID compensation as part of its phase-
locked loop. It takes the derivative of the error amplifier to sense
any accel / decel of the step input pulse rate to "guess" or
anticipate what the next period will be based on the previous one.
The integrator term is sort of like an accountant and ensures the
total pulses sent exactly match 10 times the ones recieved. It is
kind of fun to have two pulse counters attached to the input/output
and see a perfect 10:1 ratio out to 10 decades of display. Takes
about a day though at 100kHz...

The servodrive is much more tolerant of flakey accel/decel ramps and
ragged pulse trains than a step motor. A step motor has a "servo-
lock" range of +/- 1.8 deg versus about +/- 11 deg for a 1000 line
encoder. The step motor resonates during accel/decel which subtracts
from the already meager 1.8 deg margin, the servo does not, so its
margin is preserved. Also the PID algorithm provides low-pass
filtering or averaging of the pulse train so a ragged step input
looks awfully smooth on the encoder, which of course shows what the
motor is doing.

You still need to accel/decel or you will over-shoot the target
position. It will settle into position, but that does no good if you
are milling something, since the damage would already be done. If
your ramp rates are reasonable (still much steeper than for a
stepper), the servo will follow within 1 count. If you have a 'scope
you can look at the position error TP and see exactly where the motor
is relative to the moving command position. Every 40 mV it is off the
5 volt reference is a 1 count error.

Right now we think it will stay around $99

Mariss

> OK, I think I see the problem.
> If I had a 1000 line encoder on the motor, and a 3:1 pulley ratio

to the

> screw, I would need 3000 steps per rotation, or 30,000 steps per

inch

> given 10tpi screw. So my software would have to generate 30,000
> steps/in, vs the 2,000 steps per rev for a 200 step stepper motor

direct

> coupled to the shaft. This might be byond the speed a PC running

windows

> could do. Your pulse multiplier of X10 would take each incoming

pulse

> and generate 10 pulses, so the software would only have to do 3,000
> steps per inch.
>
> Just curious, how do you do X10 scaling and keep the space between
> pulses uniform?
>
> In my stepper stuff ramp up and down is very important, is that

equally

> true with a servo operated in step/dir mode? I assume your servo

module

> does not know anything about ramp up and down? Could I just throw

the

> right number of step pulses out, at my max desired rate, and

expect the

> motor to follow- assuming of course I have a motor large enough to

do

> the job. Wouldnt this tend to overshoot at the end?
>
> Your introductory offer of $99 sounds very good to me, but if I get
> hooked on these what will the produciton versions cost?
>
> Thanks again for all the answers.
> ron