Bought more powerful servo amps

That does look nice. Among other things, it has both directional inhibits (limit switch operation), and a master inhibit (panic shutdown).

That makes sense -- and leave room to mount the third one when you find it. Yes, I agree that the Z axis is the most forgiving of underpower.

Enjoy, DoN.

Yes, that's right. Anyway, for panic, I would rather have some contactor that turns off power to the amps and to the VFD.

Yes, Z sometimes does need a lot of torque (like when drilling large holes) but never a lot of speed.

Actually -- you would get the servo to stop moving more quickly with power on the servo amp and inhibit activated. However, another way to get the same effect is to have the contactor drop a short across the motor winding while dropping input power from the servo amp. Since the motor has a permanent magnet field, it will act as a generator when driven by inertia, and the short will put a big load on the motor to stop it quickly.

Agreed in general. The place where speed could win you a bit in the Z axis is in peck drilling, where the drill is rapidly withdrawn to clear chips, then plunged back at speed until just before contact and then fed slowly from there. Many cycles of this for a deep hole.

Enjoy, DoN.

I want the estop to be implemented in a totally idiot proof manner. (idiot, being myself).

That is, I want estop to act so clearly simply, that there could be no way of me messing ANYTHING up. So, in my view, estop should kill power to servos and turn off everything moving, and also tell EMC that estop occurred.

True, but I can live with that. It will be fast at 75 volts, just now quite as fast as it could be.

It's also a good way to de-magnetize the the field magnets. A resistor sized to limit the current to the motor's peak rating will prevent this.

This reminds me of a pick and place machine I built for a safety obsessed customer. They insisted that if an operator opens a door:

1. The motion controller commands the servo to zero velocity
2. Holding brakes on the motors are set
3. The amps are disabled
4. Power is removed from the amps

When the door is closed the sequence is reversed.

Their caution was a result of a maintenance guy who managed to reprise the final scene of The Fly in an injection molder.

So, what happened?

In which case add the crowbars across the motor windings for rapid stop. Perhaps a SPDT set of relay contacts which disconnect one side of the motor from the servo amp and short it to the other side of the motor.

Just for a test, try rotating the motor shaft by hand with no servo amp connected (or at least not a powered one connected) and then put a clip lead across the motor winding and see how much harder it is to turn -- and that is just at the low speeds which you can manage by hand. :-)

Understood -- just keep looking for the third amp, and leave the space for it.

Enjoy, DoN.

Hmm ... I was wondering about this. Can it really generate enough current to do this?

So a resistor sized for the peak voltage and peak current would do it, since the motor should not be spinning faster than the max on the label.

Of course the V/RPM would be different on the drive winding than on the tach winding.

No pins set through holes in the gears? :-)

Hopefully, there should be time for the motor to fully slow down before the power is dropped from the amp. :-)

This sort of reminds me of a high voltage test cage (for checking the PRV of potted high voltage diode stacks).

To open the door, you take hold of a knob about the size of that on a 7A Variac (about 4" diameter) and start unscrewing. About 24 TPI, and about 3" of screw length.

Within the first few turns, a lever lowers a discharging resistor onto the high voltage point.

Fifteen minutes later, you have finished unscrewing the knob and the door can be opened to change the DUT to the next one.

The screw is a buttress thread, and the nut is a pair of spring-loaded wires, so when you are *closing* the door, all you have to do is push, and it quickly closes. :-)

I've not seen "The Fly" -- but I'm willing to bet that it was not pretty.

Thanks for the warning about demagnetizing the pole magnets.

Enjoy, DoN.

Absolutely, if there's enough mass rotating at a good clip.

That sounds right to me.

Yes, the sequence had to be just right. It took some fussing with the PLC and motion controller programs to get it to work quickly without causing a fault.

That sounds like a more measured response to the actual danger. The pick and place device was not especially fast, masssive, or powerful.

In the 50's version Vincent Price botches an experiment switches his head with a fly's. His wife puts him out of his misery in a hydraulic press. I don't recall it being especially graphic -- I've not seen the remake.

He defeated the interlocks while presumably trying to fix a problem with one of the mold cavities. The press cycled unexpectedly and crushed his upper body and head.

Scary stuff.

There are three separate interlocks on modern injection molding machines. One is electrical (a switch), one is hydraulic (a hydraulic switch) and another is mechanical (a beefy part such as a notched bar that physically blocks the movement of the platen. There is usually also protection that stops the platen if the moving half of the mold encounters any significant resistance before it is more than a mm or so closed (more for protecting the surfaces of the mold than humans, but it might prevent amputations or deaths- IIRC the force is typically adjusted to a few hundred pounds). You have to try REALLY hard to get hurt, even if some idiot has bypassed one of the interlocks.

That said, I recall a case on a really big machine where the victim entered the danger zone from _below_ the machine (without opening the sliding door with all the safeties) and managed to um.. lose his head.

Best regards, Spehro Pefhany

And I thought that a proper procedure involves an electrical lockout?

Having to deal with this issue on a daily basis, I'm waiting for other comments before commenting myself.

Wes

I would love to hear what you think.

Chuckle..same here.

Gunner

One could not be a successful Leftwinger without realizing that, in contrast to the popular conception supported by newspapers and mothers of Leftwingers, a goodly number of Leftwingers are not only narrow-minded and dull, but also just stupid. Gunner Asch

For repairs, yes. But many things that require access to the inside of the platens are not repairs. It would be impractical to do a lockout of the 480/600VAC every time they needed to fiddle with stuff during mold changes, setup or running.

I suppose you could write a procedure that requires the operator to lock out a microwave oven before opening the door, but it wouldn't be very practical. Instead they make the door switches and interlocks so reliable that virtually nobody gets injured accidentally.

Best regards, Spehro Pefhany

I am surprised that no one other than Gunner who is playing coy like me has piped up.

Wes