Gecko Motors and Drives

[gerald_d]

Steve, you already have more torque than the guys without gearboxes. If you do want to go for different motors, have you considered the servo route instead of steppers?

(A search on rack + grease is interesting)

[stevem]

Gerald, I believe the steppers without gearboxes are double stack and have the same torque at the pinion.

As for servos, I’m waiting for the G-rex external pulse generator and Mach4 to become available. This setup should give the same performance as the Alpha drives and motors while using regular steppers with encoders.

[gerald_d]

Steve, the guys with Alpha drives seem to be getting higher speeds, but they don't seem to be getting significantly higher quality. ie, they seem to have similar degrees of chatter/scallops/whatever. I somehow think that we already have as good as it can get with steppers and that a quantum leap to servos is necessary. I gave up on chasing the Mach numbers after burning my fingers with a couple of Mach3 versions. (Just had a peep and saw that Mach3 has had about 6 version changes in the last 4 days!) I wouldn't hold any breath for Mach4 yet.

[Brady Watson]

Gerald,
FYI Mach4 is the 'industrial version' that is supposed to cost upwards of $600 (last I read)...and they are not really 'there' yet with the GRex, but they are getting there...

-Brady

[richards]

Brady,
The GRex junior (G101/G102) model that I'm testing is an excellent module. The MachIV prototype software does an amazingly good job of spinning the steppers - at high speeds. The pulse rate is much smoother than the pulse rate produced via Windows and Mach3 - which should be expected, since the G1xx system produces its own interrupts and doesn't time slice the Windows XP operating system. Of course, I will be very surprised if the GRex and MachIV are released before summer of 2006.

Gerald,
I must be missing something in my understanding of servos. If I understand correctly, the resolution of a servo is determined by the encoder (and the ability of the servo driver to monitor the encoder). If the resolution of the encoder is the same as the step resolution of a stepper motor, and, if the stepper motor is not driven so hard that it misses steps, it seems to me that the movement generated would be identical between the two types of motors. I know that the 'big iron' machines use servos, but I've always assumed that they've choosen that type of motor because of the mass that they're moving. In a low mass machine, like the Shopbot, where a lot of structural weight is not being moved around, do you think that a servo would reduce chatter/scallops?

[gerald_d]

Mike, I don't know the answers, and I am throwing up things for discussion only.....

I know that you think the "scatters" are not related to speed, but my linked pictures reminded me why I believe that speed is a factor. Stepper torque reduces drastically with increased speed, while I believe that servo torque stays constant? Steppers are inherently rough with obvious magnetic "detents", servos are inherently smooth? Need to scratch around in here a bit......Steppers vs Servos

[Brady Watson]

Mike,
Thanks for the clarification. Great to hear about the smoothness of the Mach4/G101 setup. I wanted to use these for a project, but figured that I would wait until others wasted their money figuring it all out 1st


Gerald,
My understanding is that steppers and servos, from a torque perspective are inversely proportional. This is why you would be hard-pressed to find a servo system that does not implement some sort of gear reduction. Servos typically need to run 50-80% (from what I understand) of their max RPM in order to be efficient. So...a servo system still needs to be designed to have a 'sweet spot' in order to get the type of performance we are looking for. The gear reduction goes a long way for low-speed performance and cogging issues.

Selecting the right encoder can be tricky because the higher the cpr, the lower the possible top speed (but higher the resolution) depending on how much bandwidth your computer can handle when throwing out a pulse stream to the servo drivers. You may not need .00004" 'step' resolution with a 2000 cpr encoder that gives you a top speed of 8IPS(just as example)...but you may want to run a 500cpr encoder that gives you .00016 'step res' and 32IPS max speed...if you follow me. Servo performance is in many cases dependent upon your processor's ability to throw out pulses...this is where the G101/102 comes in...with serious pulse speed/'bandwidth'.

-Brady

[gerald_d]

My crude theory is that a stepper "chokes" itself to run out of breath (torque) at high speed because of the increasing impedance at the higher frequency - in other words, the current can't reach the motor because it is too busy going backwards and forwards in the cables!