Simple motion control

Hello,

I have never done any motion control and the job I am working on is very simple. I will be monitoring the top and bottom position of a spray nozzle as it moves up and down inside large engine cylinders that vary in height from 12" to 60". The nozzle is moved up and down by a screw that will turn 24 RPM for correct nozzle speed (6" per min.) while spraying. The motor (through a gear box) turning the screw will be controlled by a VFD. At proper honing speed the VFD should be operating at approx. 35 Hz, so I should have good torque.

The plan is to run drive at 120 to 180 Hz to bring nozzle down to top of cylinder quickly and stop to allow for further user setup and then, when ready, operator can start the honing process. I plan to program a routine for each cylinder type/height (HMI for operator selection), use some sort of counter to know my position for reversing the nozzle. I can set up something so I can use a prox switch to count revolutions of the screw and use a standard counter in the PLC (Siemens s7200) which seems simple to me but should I use an encoder and a high speed counter in the PLC? Are there things I may not be aware of in motion control such as this that you can help with?

Additional things that you may need/want to know: Precision is not important, +/- 1/4 is fine. Also I plan on using the PLC to control the VFD.

Any setup and programming suggestions for this simple application will be appreciated and I hope the above gives you a picture of what I am trying to do.

Hello Newboy,



You seem to have a good idea of what you are doing and have a good plan, what type of drive are you thinking of using? If you plan on using an encoder off the gearbox/motor read up on them Peter has made some very good articles on them

http://www.hydraulicspneumatics.com/200/Issue/Article/False/21744/Issue

I will either use a Siemens MM440 or a Automation Direct DuraPulse drive. We have be using Siemens products but are thinking of moving over to AD as they may suit our simpler needs better.

I agree, you do indeed seem to have a good idea of what you are doing and have a good plan. I have done a similar application in the past and find that adding an overtravel limit switch [or prox] at each end of the stroke is essential.

There are two issues you need to consider.
1.Where am I when I start up the system and has it been mechanically moved since I shutdown.
2.What happens if the VFD fails to stop where/when I think it should.

Whether you use a prox switch [coarse reolution] or encoder [fine resolution] I would recommend a limit switch at each end to prevent overtravel and potential damage to the leadscrew if the VFD fails to stop when you want it to. Wire the switches normally closed in series with a contactor located between the ouput of the VFD and your motor so that when the switches are OFF the contactor is ON.

Each limits signal is also wired to an input on the PLC to provide a method of recovery if they are tripped as well as giving you a defined home positions to move to when the system first starts up.

Should the VFD fail to stop [or someone inadvertantly changes the decel. parameter to a slower rate than you planned for] the contactor will open and the motor will be disconnected from the drive and coast to a stop. Most VFD's these days can tolerate opening up the output to the motor with a contactor. For added safety wire a normally closed auxiliary contact [open when energized] off the contactor to the control input on the drive that turns off the drive output stage.

You may also want to consider adding a dynamic brake to the drive if you wish to stop with more precision.

Hope this helps - Good Luck

Also, make sure your motor can handle what you are trying to do. If you are using a normal 4-pole AC motor, you will be running the better part of 5400 RPM at 180 Hz. Many motors aren't balanced for that kind of speed. Also keep in mind that above base speed the motor will operate constant horsepower, not constant torque. And this is only up to about twice base speed. After that the horsepower starts to drop off too. So you won't have as much torque as you think above base speed. Last, but not least, you said the screw will turn about 24 RPM at 35 Hz. That will get you a screw speed of about 120 RPM at 180 Hz. With a 60" or longer screw you may need to consider screw whip.


Keith

kamenges said:

...And this is only up to about twice base speed. After that the horsepower starts to drop off too. So you won't have as much torque as you think above base speed....

Click to expand...

Hi Keith,
Is there a rule of thumb (so to speak).....Not to exceed?

I have some drives that I run at 90hz, is there are breaking point? Or is it drive and motor dependent

I have gone to 120hz and can tell you that I had serious problems. Change your gearing, bump your motor size and stick around 75 hz max in my opinion. If you were going 1:1 then a servo motor may be more feasible

Originally posted by geniusintraining:

Is there a rule of thumb (so to speak).....Not to exceed?

Click to expand...

It tends to be motor specific. The effects are also very application specific. I have run motors quire effectively upwards of 3 times base speed. However, you have to know what you need to achieve. I do quite a few center winders. As the roll diameter decreases and the spindle speed comes up the torque goes down. The thing to remember is that you will start to lose torque as you go above base speed.

The constant horsepower to twice base speed is a motor thing. Different motors will go to different speeds before they are no longer constant horsepower. You need to check the motor for this.

Keith

You can use an encoder from the motor to feed the PLC and then use counts to preset positions. You can use an input to Zero the count or input a specific count. Once you have extablished one or two points, the rest will be easier to program in.

I did a ball valve welder doing similar things. It ran slow while welding. When it was time to move past the opening, the drive runs at 120 hertz. Here it is a matter of the gear box not being able to take the rpm. There is a slow to 30 hertz to creep to the stop point. This is based on the "zero" set when the operator begins operation. It too is not exact as it is not necessary. When it finally stops, the counter is auto reset for the next weld cycle.

You might be able to use an HMI to select specific points or use a button input to select it. Then have a bump to position button.

On the motor, you must make sure the MOTOR can run at specific frequencies and what the duty cycles are. Gear ratios can be changed to get you better control. BTW, if you use a VECTOR control you can get excellent low speed torque control. The Marathon BLACK MAX motors are excellent for low speed high torque applications. Some will do 4500 rpm intermittant duty too. My company runs them at 120 hz all the time for rapid traverse and then real slow for feed rates.

Hello,

I want to thank all for their thoughts and suggestions as they have been quite educational for me. Plan on printing your comments out for future reference.

I have been using Siemens PLC's (S7-200's) and having some problems understanding high speed counters. I have no problem using the wizard to set one up but don't quite understand how to use it to count the pluses and then conrol the outputs for turning motors off and on. (Have encoder and plc set up on work bench for experimenting.)

I have been trying to stay away from using them because of this but would like to conquer them for the day that I will definitely have to use them. There is just some piece or pieces that I am not seeing and is quite frustrating. Any suggestions or examples would be very much appreciated.

NewBoy