dc motor speed control

[Steve101]

Trying to control speed of dc motor with encoder feed back. I'm using KB, SCR drive with SI-5 signal isolator that takes a 0-10v signal from V350&V430 plcs with analog expansion modules. Measuring speed with basic frequency measurement in hardware configuration converted to feet per minute.

So far I have fairly decent control using increment/decrement. Comparing set speed with actual speed and inc/dec output based on that. With some fine tuning this

can probably give me acceptable control though I would prefer more stable control. Can somebody give me a general idea on how to ramp the motor up/down to set 

speed before inc/dec takes over.  I have not found a reasonable way to do this yet in reasonable amount of time.

 

Also, is incrementing/decrementing reasonable way to do this? What would "standard practice" be for this type of application, perhaps PID or just PI control

or other?

 

Any feed back is appreciated.

[Ausman]

Hi Steve,

 

Re the hunting issue, if you are down to variations bigger than you like occurring, and you are only moving your outputs by single digits in that time, then you are a bit restricted in what you can achieve.  That said, a number of suggestions:

 

I don't use PID etc when large load variations can occur.. have found it good for roughly constant stuff...but hard with big variables.  So I do much like what it sounds like you do.  I have had luck in introducing small time delays into the output equations, be it small number of scans based or fixed interval.

 

Another thing to check is that the drive actually recognises minor control voltage variations.  I have seen many drives not react to tiny changes, only adjusting once things go past an internal reference to a previous input...essentially they have their own interference filter on their control inputs.  Often there is a setting change to get a reaction no matter how small.

 

Also worth a check with a meter during operation to make sure your control change is reaching the drive properly, as well, given the use of the signal isolator....might be doing the same thing with filtering.

 

From memory those drives also have inbuilt ramping control up and down (memory=age=hazy!!) so I'd make sure that isn't interfering somehow as well...you'll likely have to turn it off completely and do all the ramping via adjustable rate maths in the plc.  If the drive has inbuilt ramping and it is on, I can definitely see it giving you problems due to the lag involved.

 

Hope my little bit helps,

 

cheers,

 

Aus

[cantcliff]

PID works best when you have a process running you want to keep stable around a set point. 

 

To match your case:

 

The belt could have heavy items added to it that would slow it down, PID would adjust the output of the motor to keep the speed the same with the extra load, or would reduce it when the load was removed.

 

Really you just want a ramp rate before you start your cycle, I believe the PTO functions might help you, but I might be wrong as I've never messed with them yet.