Material movement suggestions

Hey guys,

I need to move a unit about 14ft, drop it off at the same place every time and return. the accuracy i would like might be a little less than a rotation. I still have a bit of mechanics to work out but...

I'm thinking to use a plc to control a drive via modbus with either encoder to HSC or limit switches. I'm not very up to speed on today's drives and thought there might be a smarter way to do this considering the simplicity of the operation. Any suggestions are appreciated.
steps outlined below.
thanks,
Martin

1. Actuate Pick
2. quick Ramp to Speed1
3. Slow to Speed2 approaching destination
4. Stop
5. Actuate Drop
6. Reverse quick Ramp to Speed 3
7. Slow to Home

10hp motor

By accuracy, "a little less than a rotation", you mean the encoder, right? How fast does said rotation happen? If it is anywhere near the amount of time it takes to encode/send/decode a modbus command then you are going to have problems.

Is there any way to do it with limit switches that go directly into the drive? This will give you the fastest, most deterministic response.

Thanks for the response. 300ms. limit switches to the drive could be an option.
If there was a way to program a drive to go to point A, wait, and return home. I'd be set.
have to use induction motor and not sure if i can figure out how to stop close enough every time. the unit weighs about 1000lbs
thanks again

Depending on the accuracy you might be better off with an absolute encoder. An absolute encoder gives you a "value" for each encoder position instead of pulses. This will eliminate the need for limit switches. With limit switches you will have drift. I would recommend a home limit switch at your "pick" position so you can start the system off from a know position. And an over-travel limit switch at the place position so you can cut power if you go to far.

mscott421,

you also need to include the weight of the lift.
you also have the load / unload shock to consider.
then there's load inertia, gravity, accel / decel.
i would also over rate the unit 30% for good measure.

regards,
james

Clay, will have safety limit switches thanks
James, the lift is external to the transit mechanism. thanks for the info

with a closed loop vector drive, will i be able to use the encoder data in the drive for positioning information?

by definition closed loop means you have some form of feedback for motor movement. What closes the loop is what you have to define. An encoder can be close the loop but instead of counting pulses to tell you were you are at it can just count pulses per a time interval and give you a rate of travel.

pick up
move to top
when switch is reached, drop (without any PLC) so timing etc, is not important, the repeatability will be very good.
when switch is reached change direction.
when lower switch is reached change and pick.
without any PLC only use a ON contactor and a direction contactor.

Most of the VFD's that you get now have the ability to write logic within the drive to do motion control without the need for a separate controller.

Incremental or absolute encoders don't have any inherent accuracy advantages. Incremental means you will have to periodically establish a reference, usually only if logic power is lost.

I agree with previous reply. Get a drive that can do positioning. Some that I know can position an induction motor: ABB ACSM1, Gefran XVy, Gefran ADV200, Control Techniques Unidrive.

Highland Controls said:

Incremental or absolute encoders don't have any inherent accuracy advantages. Incremental means you will have to periodically establish a reference, usually only if logic power is lost.

Click to expand...

That is the advantage of an absolute encoder. It always reports back a FIXED position. Incremental encoders will give a more accurate speed reference.

As you stated there are several drives out there that do positioning fairly easy. I prefer my PLC hands the drive the data so I can manipulate it from my PLC via a HMI. This way if I need to "move" a position it is not a painful exercise.