Sprint on DC motors

[Al_The_Man]

A recent release by Sprint Electric on the advantages of DC motors.
Although I take issue with a few statements which appear a little flawed IMO.
http://www.sprint-electric.com/media...1277927958.pdf
Al.

[Boltz]

Hey, Al, what's the takeaway for us router guys?

The way I'm reading this, it's saying that the constant torque and lower inertia of DC motors translates into faster reversals, which should mean faster rastering when these motors are mounted on a router.

Do I have this right?

-Jim Hart

[Al_The_Man]

That is one of the things I have an issue with, many BLDC & AC servo motors I have used have a lower inertia rotor than its DC counterpart?
BTW, on your other post on peak torque, I would only look at continuous torque which is what you are going to use, you should not be going in the peak torque range as this is usually the destruction point.
Al.

[Boltz]

Well, I guess these 2 threads are related, because I'm interested in getting the fastest rastering speeds, which involves a lot of reversals. Basically you need to come to a dead stop and reverse as fast as possible. There can be some big transients when reversing a heavy gantry. I'm thinking maybe peak torque may be a lot more relevant to routers than mills.

-Jim Hart

[Al_The_Man]

Your motor control should always be under the control of the servo amplifier, otherwise you have lost control.
Go the Galil site and check out the instruction videos covering tuning and this will explain the significance of motor control at the point the controller has no effect due to over driving the motor.
Al.

[Boltz]

Okay, Al, you kinda lost me there... Are you saying that if there's a 20 millisecond spike, during which time the current exceeds the programmed maximum, that during that transient the drive has "lost control" of the motor?

I'm coming at this from a background in audio amplifiers. To me, "peak torque" is the ability to handle transients, while "continuous torque" is a bit more like RMS watts.

It's just common sense that the motor and drive need to have enough "headroom" to handle the transient load of sudden reversals.

-Jim Hart

[Al_The_Man]

I imagine that one of the differences between servo and audio amplifiers is that servos are are controlled and tuned with a PID loop.
Audio is mainly open loop?
It may be a bit tedious and it applies mainly to Galil but some principles are the same for all servo systems, if you watch the 3 part videos it gives a bit more insight to PID loop tuning.
http://www.galilmc.com/learning/tuto...-systems-part1
I have used the WDSK program to tune my motors and the program exercises the motor to the maximum of their ability, including detecting drive saturation in order to come up with the correct PID.
I found if I tuned manually it was extremelly conservative as to what the program came up with.
Al.

[Boltz]

In audio you can tell what you need to know about what's happening at the speaker by studying the waveform at the amplifier end. The feedback is right there in the wave.

The most useful thing I've gained from audio is a fine appreciation for transients, both electical and mechanical. In electrical, mechanical, and electromechanical systems, you can have extremely large instantaneous loads which last for a few milliseconds or microseconds, and you need to build a lot of "headroom" into the system to properly handle them.

Relay contact "debouncing" is a great example of how messy the conditions in the real world can be.

-Jim Hart