Allen Bradley Servo Brake

Hi all, I'm having trouble with an Ultra 3000 Servo that I have. Once I activate the E/stop this cuts all power off to the drive and I get a drop of about 20mm from the boom that this Servo is controlling. I thought about puting a delay relay in for the brake but this would contravine safety regulations surely? any other ideas? or maybe I am just blatantly missing something!

How would a delay make the drop less than 20mm ? am I not seeing something here !

Why does E-Stop kill all power to the drive? Killing all power to the drive allows the motor to coast until the brake mechanism actuates.

Presumably, it's an electrically released brake. Have E-Stop kill brake power and take the drive out of run.

We have something around here that exibits the same behavour explained above, but I can not think what it is. Under a normal stop, I beleive the drive sets in a DC braking action and the thing stops normally. Under an e-stop condition, power is dropped or otherwise inhibited and the thing free wheels for ever. (Must be the drive on one of our presses and the flywheel runs on much longer under e-stop VS normally hitting the STOP button. Not positive it's a press flywheel, but as the original poster posted, the situation sounds very familliar.)

I am being told by our 'control' department that 'upon E/stop the power must be removed completely from the drive'.

Is the brake being driven directly by the contact on the drive or is the contact on the drive powering a relay that activates the brake? Make sure there are no time wasters in there (mechanical relays, etc.).

10baseT said:

How would a delay make the drop less than 20mm ? am I not seeing something here !

Click to expand...

He's talking about an inverted time delay relay - you know, the kind that can tell what's going to happen before it happens. I have looked several times for such a device - but never found one...

I would also like to see a similar instruction in PLC software.

Realistically, I think he meant a time delay relay on the contactor for power to the drive.

If the brake is controlled by the drive and your facility requires that E-Stop kill all power to the drive, then take an additional contact from the E-Stop relay and have it kill brake power to actuate the brake. That way you will be simultaneously commanding both the drive and the brake to go to E-Stop condition.

Currently, the brake doesn't actuate until the drive reacts to loss of power. With the additional E-Stop contact controlling the brake, you'll be actuating the brake some number of milliseconds sooner.

We had a similar situation on a piece of equipment we built recently. A large servo motor is direct-driving a large indexing table (no brake on the motor). A number of large steel mandrels are located on the circumference of the table, which adds not only mass but a lot of inertia. Cutting power to the drive during an index allows the table to freewheel for some time.

Our engineering specs require all stored energy to be released during an E-stop, which includes inertia in the table. We ended up connecting the power to the drive through an off-delay time relay (safety rated of course). So when the E-stop is activated, the drive enable line drops immediately, causing the drive to decelerate to a stop, then the drive power is cut off two seconds later.

This has worked out pretty well but we had to make sure that our guards were positioned such that a person could not get inside the machine within two seconds of tripping the E-stop.
-John

Is it an SEW brake that is powered (possibly incorrectly in this case) off the motor supply ? I have had a not dissimilar situation where the motor autogenerated enough back emf for a short time to hold the brake off , easy enough to cure .

I'm still looking for one of those before delay timers , I have had plenty of success with my S5T#21D timer , which was unfortunately entered as a typo in a project , gave the operators 3 weeks off .

I have an application that uses 4 Ultra 3000s on a SERCOS loop. On one axis, a sudden stop could cause damage to the machinery. I solved it by installing a delay timer on the contactors. So in an E-stop condition, I send a signal to the PLC to stop all motion,and drops the enable bit, which takes 3/4 of a second. 1 second after E-stop, the contactor drops the drive power, and all output power. I still keep power to the PLC and the Drive Aux power. The delay timer is from STI or AB. Hope this helps.

As two others have mentioned, use an off-delay relay to drop power to the drive. If you command the servo to do a controlled stop on detection of the Estop condition, you can stop the motor faster than if you let it coast.

On the older Ultra 100/200 drives there's a delay time parameter for applying the brake after disable. You can enter a negative number in this parameter to apply the brake before the drive opens the servo loop. Since the 3000's are virtually identical drives, they probably have the same parameter available. I have used this in the past to combat gravity.

Gerry said:

On the older Ultra 100/200 drives there's a delay time parameter for applying the brake after disable. You can enter a negative number in this parameter to apply the brake before the drive opens the servo loop. Since the 3000's are virtually identical drives, they probably have the same parameter available.

Click to expand...

Here's a screenshot of the delay setup screen, assuming the drive is a SERCOS Ultra3000.

Thanks for all thinput guys. The way I am getting arounfd it now is with a delay safety relay, pretty much as KOLYUR mentioned in his post. I was interested to see if their was another solution to this problem (I'm not a big fan of delayed E/stops!) but it seems to be quite universal.

There is a brake active delay and brake inactive delay setting that can be configured in milliseconds in the Ultraware software for your servo. It is in the Digital Outputs configuration tab.