[LZ1TWB]

Hello all,

I started to wire my e-stop circuit which will handle my 4 axis setup and just wanted to share my ideas and see your opinion on whether this will work or not.

So I have a 24V relay that is energized through all the E-stop buttons, limit switches and the charge pump from the computer. When I have a normal condition the relay is switched ON. A NC contact on the relay is wired from the UHU E-STOP IN pin through a diode to ground. All 4 UHU's are wired in a loop, so one's OUT pin goes to other's IN pin. This halts all the boards when a single one errors.

Another pin from the relay is used to energize a magnetic contactor, which puts resistive load directly on the motor leads. When the contactor is switched ON the resistors are disconnected from the outputs.

Another contactor which switches the primary of the power supply is also wired to the E-stop relay. When it is OFF it cuts the primary and puts a load on the capacitor bank. So far so good.

My question is, I know that when the UHU chip is on red /the error led is ON/ the output pulses are cut, and the motor is let to spin free till it stops. Can I safely put this load resistors on the output without damaging the drive, and eventually stop the motor fast enough? My motors accept 100A peak current, so with a power supply of 65V and full speed if I put 2 Ohm resistors on the leads it gives something like 32.5A in the first moment and descending after that.

In my understanding, there should be no problem to do this but I wanted to be sure before I blow some FET's.

Sorry for the text explanation, I will try to draw a schematic of this later to be more specific.

Thanks, Todor

EDIT: I am using the OLD UHU if this matters.

[Al_The_Man]

A question I have is do the drives have a drive-disable input? generally this turns off the output FET's or IGBT's, as the case may be.
Al.

[LZ1TWB]

The error condition is triggered when a low state is applied to the E-STOP IN pin of the controller or when the maximum deviation from the position is exceeded. This switches off the output and the motor free-spins after that. No braking action is made as I recall.

Thanks, Todor

[H.O]

Todor,
If you can be SURE that the output stage disables BEFORE you connect the resistor across the output then it will probably work. Otherwise there's a risk of damaging the MOSFETs since the current limit circuit doesn't operate properly on a resistive load. (There's no inductance to limit the rise time of the current so the limit circuit don't have any time to react, if the MOSFETs can stand the 32.5A then I think it should be OK)

You say that you switch off the primary of the powersupply and connects a resisotr across it to blead it out. Why not make the resistance of this resistor low enough to break the motors? The body diodes of the MOSFETs will conduct current from the motor back to the resistor so it should work just as well. As long as you don't have too low resistance (that'll damage the diodes) it should work fine.

/Henrik.

[LZ1TWB]

Hi,

I am working with the IRFP150N, which has a body diode rating of 42A. Additional to that there are external BYV27/200's , which are some 6 times faster and handle the voltage spikes better than the internals when working.

So I guess I will try your approach first and if I am satisfied with the results will work with it. So with a capacitor bank of 4x6800mfd + 8x1000mfd on the boards = 35200mfd at 65V. 2 Ohms bleeder will almost fully discharge it within 0.25sec. As long as the online calculator I found is true. This does not count all 4 motors spinning at full speed during that time. So the time will be more.

The problem is that my rapids are 10m/sec, which gives 166mm/sec. So for a time of 0.5sec it will move 83mm, but my hard stop is some 25-30mm away from the limit switches. I guess I have to use smaller resistor.

Todor

EDIT: At that particular rapid speed I have to stop the motors for a maximum of 0.12sec as they don't hit the metal. 2 revolutions here gives me 20mm of travel.

[H.O]

Hi Todor,
You should have a look at the maximum discharge current of the capacitors as well, they may not like being discharged TOO aggressively. Also, remember that you would get 83mm travel in 0.5 seconds IF the voltage remained at 65V - which it doesn't.

You have a delicate problem there, breaking "gently" risks damaging the machine, breaking hard risks damaging the drives.... Perhaps you need to consider moving the limit switches "inwards" a bit to get a larger safety zone and/or add some shock absorbers at the definitive end.

/Henrik.