Mapping an analog output to a vitual axis?

[scottsutton]

So I have a control scenario that uses a 4-20 ma output to control a hydraulic rotary motor in a sheet measuring and cutting system. I am not certain about the best practice for controlling this with a Do-More PLC. The drive (hydraulic motor) must ramp up to speed and back down again as it approaches the target length for the cut. One way that I am looking into is using the virtual pulse output with encoder feedback and mapping the velocity parameter to a scaled analog register to drive the motor. As the velocity ramps up/down it will continuously copy the data to the analog register so the motor speed follows.

Is that a proper use of the virtual axis or is there a better way to achieve this goal?

Thanks in advance.

SS

[BobO]

Depending on how fast everything is happening and whether the PLC's scan latency is short enough, that could work fine. Mostly just depends on the motor and axis characteristics.

[scottsutton]

Thanks Bob,
I will be back onsite with the machine Thursday and give it a go with this control scheme. I will report back my results.

SS

[JasonO]

I've done this once with great success, just as you've described. I was controlling a servo proportional valve driving a hydraulic motor, with an encoder for position feedback. I was impressed with the performance, and the ease of doing this with the Do-More motion commands.

[scottsutton]

The project is complete and I can say that I had the same result as Jason, that control scenario worked out perfectly. The system is stable with smooth motion control and the cuts are accurate. Best of all, the customer is happy. They report that it operates better with the new controls than it ever ran on the old obsolete control system.

Question for the Do-More development team? Can this "Hack" be incorporated into a baked in instruction? If so, would it offer even better performance potential that would not be bound to the scan time?

Thanks for the great engineering behind the Do-More PLC.

SS

[BobO]

The project is complete and I can say that I had the same result as Jason, that control scenario worked out perfectly. The system is stable with smooth motion control and the cuts are accurate. Best of all, the customer is happy. They report that it operates better with the new controls than it ever ran on the old obsolete control system.

Question for the Do-More development team? Can this "Hack" be incorporated into a baked in instruction? If so, would it offer even better performance potential that would not be bound to the scan time?

Thanks for the great engineering behind the Do-More PLC.

SS

My only concern would be setting expectations that cannot be supported with the current hardware. It works great as long as the speed requirements aren't very high, but for something canned we'd prefer tighter control and high speed outputs.

To that point, we have long talked about a variant of the HSIO with analog outputs that would do all of this onboard at very high speed. From ADC's product management perspective, BRX is going to be changing focus a bit, placing a bit more emphasis on motion than previously. While we don't know the full extent of that, we do expect to review our motion with the new product manager and look for things we can do that will enhance our offering.

Motion on BRX has been a bit perplexing. We feel like we created a very capable product and yet motion is really underused compared to other features. That will be a major topic of discussion. Certainly any feedback our users can provide would be a welcome addition to that discussion.

[scottsutton]

We feel like we created a very capable product and yet motion is really underused compared to other features

Motion has been the number one Do-More feature used in my applications for years. I came from the stepper motor world controlling high volume photo processing equipment so when I made the jump to servo/industrial grade machines the Step/Direction format just worked for me. I do a lot of Yaskawa servo projects and my servo rep says that I am the only one using this method to control his servos.

SS

[BobO]

Motion has been the number one Do-More feature used in my applications for years. I came from the stepper motor world controlling high volume photo processing equipment so when I made the jump to servo/industrial grade machines the Step/Direction format just worked for me. I do a lot of Yaskawa servo projects and my servo rep says that I am the only one using this method to control his servos.

SS

And that may well be at the heart of it. Pulse motor control is pretty narrow these days. My guess is that most of the Yaskawa users are using EtherCAT.