Some success with a servo drive/tachometer mode

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I've never played with a tach. I'm assuming you still give the AMC drive a -10 to 10 volt signal for direction and speed and the control does the PID. I had read somewhere that a tach would help in super slow applications, like a wire edm where feeds are .0X ipm. Let me know if you see benefits.

I ordered one of the crimpers you suggest, got to be better than mine.

Karl

Yes, you give the drive a signal and the control minimizes the error between the speed as reported by tach, and speed as commanded by the signal.

The drive is, therefore, able to produce very precise speed.

I could never get it to run at super slow speed in voltage mode, but could easily do so in tach mode. It was kind of nice to see.

I was told (and it makes sense) that ability to set velocity precisely is useful for a lot of machining operations, smooth finish etc. You could have your cutter move along some curvy curve and follow it precisely and dynamically. The servo drives closes the velocity loop continuously. The PC control does not have to close the velocity loop by itself 5000 times a second based on encoder readings.

I would say, waste about 5-10 connectors to get a hang of it. The connector can go in only one way. The crimper has an internal step, kind of.

I still have no estop stuff done, I just pull out a fuse holder by hand if something goes wrong.

I try to get at least something done every day.

Bought a BIG BOX of DIN terminals:

i mean 1,000 times pre second

Disclaimer:My control uses a Galil dedicted real time controller so your results may differ.

Torque mode is the preferred mode for most all applications, certainly for what you're doing. Precise smooth control is not an issue at all with lathe and mill applications.

Karl

Karl, could you explain to me, why do you think so? Wouldn't the milling table move at very high rates of speed if it encountered no resistance, in torque mode?

Karl, that does not really answer my question (about possible high speed).

Axis speed is controlled by the CNC control giving the motion commands, not by the servo drive. The CNC control doesn't say "move to position X" and the drive goes full tilt to that position. The CNC control manages the movement to move at the specified feed or rapid rate, i.e. G01 X10 F25 moves from the current X position to X10 at a 25 inch per minute rate.

But how does the control tell the drive what speed to use, if the drive is in torque mode?

I do not quite understand this.

The control is smart, the drive is dumb. The control only tells the drive what to until the next refresh. The control looks at actual position vs. command position, acceleration, etc. and makes a decision for how much motion till the next refresh to solve the positioning problem. The dumb drive just gets a voltage and does what its told.

karl

I'm sure a EE could give a far more elegant description.

When the amp is set to velocity mode, the setting when you're using a tach, it acts as a voltage to voltage amplifier. In torque mode, which as Karl said is what the vast majority of motion control apps use, the amp's output *current* is proportional to the command voltage. If the controller sees, via the encoder feedback, that the motor is moving too slowly it increases the command signal, the amp increases current, motor torque goes up, and speed increases. Repeat until the controller is satisfied.

I've designed and built 15 or 20 different systems with perhaps 50 servo axes over the last 20 years, and I can think of only one instance that used tach feedback, and even that was synthesized from an encoder signal. And in that case there was no traditional controller.

Re your earlier remark that tach feedback decreases the load on the controller -- every modern digital motion controller I've worked with is constantly updating the servo loop unless you specifically tell it not to. The typical rate is in the range of 10 KHz.

OK. So, I think that I am making a mistake by using tachometer and instead, I should go to torque mode. It will actually be easier to use torque mode, anyway. If everyone does it, so will I. It is also safer and I can remove a whole bunch of cables from the control that I do not need. Seems to be a win-win all around.

Karl, you and Ned have convinced me.

I am actually very happy, because I would get rid of a lot of cables inside the control box, less complexity, less fear of tachs going bad on me and the drives going berserk, etc.

I will use torque mode.

One last question, if someone could enlighten me, whi torque mode and not "voltage mode"?

I am not trying to invent a better wheel with 4 minutes of experience, or think that I know better than everyone, I just want to understand.

Make sure the encoder wiring is good or your control has no way to know you have an axis that is running away. The only thing your control can do is put your system in e-stop.

I had a waterjet cell that profiled headlinders for explorers. The drives were DC and used a tach. I always kept in mind that one wire broken could cause a lot of grief and danger. Not a real problem to the operators but if I'm inside teaching a profile, I better be somewhere a run away can't get me.

As far as your tach, have you opened the motor and looked at the brushes? If the brushes are worn down, trouble is on the horizon. I've never had problems with tach brushes during the time period I had to deal with dc drives, usually the motor brushes that have a more demanding task fail. Oh, while you are in there, blow out the dust.

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

Looks nice.

Without cutting load, of course.

Usually, there is a zero trimpot on the servo amp (drive) which will first off eliminate drive with zero input signal voltage. When this is done, it might also eliminate the runaway problem with no tach feedback connected -- but I'm not sure of that.

I presume the black module near the bottom marked 'X' is the servo amp?

So -- this is a motivation to do it right. :-)

Indeed -- congratulations.

Especially with the new motivation. :-)

Enjoy, DoN.

The place where this is most likely to make a difference is cutting a line which almost but not quite parallels an axis of the machine. This means that one axis is moving at a speed which is determined by the appropriate feed rate for the tool/material combination, and the other axis is moving at a very slow creep. An example would be turning (or milling) a shallow taper. While turning, maybe Morse, even more so for the taper of a tapered arbor.

If the controller is monitoring the encoders frequently enough, and the encoders have sufficient resolution, you are fine anyway. However, with a coarser resolution encoder, the ability to command a precise slow speed means that you can trust it to maintain the very shallow angle between encoder pulses. The Galil, with a set of sufficient resolution encoders probably does not truly need the tach feedback -- but even it might find the job easier with that.

Enjoy, DoN.

In which case, it is possible for the computer acting as the control to lock up (BSOD for Windows) just after issuing a command for high torque, leaving the motor running quite rapidly until it hits a stop.

Less likely to happen with the linux backed EMC, but still possible if there is a hardware failure in the computer.

Enjoy, DoN.

This is where watchdog timers come into play. I presume the I/O boards Iggy is using have such a feature since the ones for Mach3 do. The watchdog timer monitors a signal from the PC that is supposed to cycle continuously as the program operates normally, if the signal doesn't cycle in a second or so, the watchdog timer times out and forces an E-stop.

I think that Jon's PPMC has some kind of a watchdog.