PWM for DC motor

I am new to plc's and am looking at using input from an encoder to sync the speed of a DC motor (0-90vdc .5 amps) the motor already
has a variable speed controller but no way to control it except with a dial. I was goin to use PWM to control the speed of the motor but leave the current drive intact for any fine tuning ( I would take the output from the dc drive and pulse it to the motor using a plc)
Is this an exceptable way to do this ( any alternative is welcome)
and should I use a solid state relay or is there a better way. The company does not want to spend much money on this( real shocker I'm sure). Any and all advice is welcome thanks.

Originally posted by Paulmatrix:

The company does not want to spend much money on this( real shocker I'm sure).

Click to expand...

Can you put some type of range on 'much money'. We could be thinking different things by a couple of orders of magnatude.

I wouldn't do what you are proposing. I really doubt you will get the performance you want if you can keep it stable at all.
What plc are you using? Does it have encoder input capability? Does it have analog I/O?

My first thought would be to replace the current drive with one you can supply and analog input to. If that's not possible, if the drive has a pot for speed control you could probably break into that and send in your own isolated anaolg signal. But by the time you get the isolator and toss in your labor you could have a new drive.

Look to KB Electronics for reasonably priced DC drives. If you have a plc that can actually produce a high frequency PWM (3 kHz or greater carrier) then you may be able to use one of the drives from Advanced Motion Control.

Keith

The PLC is a keyence it does support encoder(high speed) input.
and a 20 KHz output. It does not support analog output. But
I think I may be able to get them to spring for an analog card
I like the idea of using the speed pot it is a KBelectronics drive
I dont know the model # off hand but did look into it and it didnt
have any I/O cap. in it. I am going to check out the speed pot though.
I dont know about youre place but mine thinks labor is free so wasting time on an obsolete app as long as there no out of pocket expense they think is a savings. Did I say that out loud?
Thanks Keith I really appreciate the insight.

If you have more time than money and you like to play, you can probably do this with the existing PWM output, a solid state relay (if it's fast enough), a couple of resistors and a capacitor. It is possible to filter the PWM signal and get a reasonably stable voltage out of it as long as you don't want it to change real fast. If you can change your PWM carrier frequency to something slower it will be easier to find solid state relays.

Be careful with the KB drives. Most of the base models don't isolate the +/-15VDC from the supply. So the whole control section, along with it's terminals, floats on the AC supply. I cooked some analog output modules several years ago before I realized that one. That's why a solid state relay between the plc output and the drive (if you filter the PWM) or an analog isolator between the plc and drive (if you use an analog output) is pretty much required.

Keith

Personally I would'nt use the PLC to do the pulsing. There are a huge variaty of low cost PWM drives that will accept an analog or digital input for speed control. The encoder will feed back to the PLC counter and the PLC will send a speed reference to the drive.

Vague answer to a vague question really.... What are the motor specs. How many speeds? Two or three.. or infinate? That makes a big difference on the drive type and reference type.

The specs were all stated I believe.

I am new to plc's and am looking at using input from an encoder to sync the speed of a DC motor (0-90vdc .5 amps) the motor already has a variable speed controller but no way to control it except with a dial.

Click to expand...

That tells me its a small fractional horsepower (1/25 or 1/30 HP?) possibly a gearmotor.
It is using a KB Electronics DC speed controller (model unknown)
The existing control is done manually using a potentiometer.
The plc to be used is a Keyence.

Have them spring for the analog output module and a signal conditioner/filter for the speed control supply voltage from the drive.

Lets us know what happens.

Here is a very crude scheme...

                     Current

               SW    Limiter

                /

PWM Output-> --o  o--/\/\/\/\----+-------------+------>

                     Resistor    |             | R

                    or 15K POT   |             / e

                                 |             \ s          Speed

                              ___|____         / i        Controller 

                             Capacitor         \ s

                             _________         / t

                                 |             \ o

                                 |             / r

                                 |             |

        COM   -------------------+-------------+------> COM

.
The idea is to charge the capacitor to a particular voltage.

Of course, the Loading Resistor (Voltage Dropper) and the Speed Controller are bleeding the charge almost as quickly as you build it.

The idea is to maintain a reasonably stable charge on the capacitor while the motor is running.

Having a reasonably stable charge on the capacitor depends upon your component values and the timing/duration of the PWM signal.

The PWM signal applies voltage as often, and for as long, as your PWM output is on.

The greater the number of pulses per second the greater the average voltage. The average voltage approaches applied voltage.

The applied voltage is determined by the current limiting resistor and the voltage dropping resistor. They represent a voltage-divider network. The capacitor will try to charge up to the voltage dropped across the voltage dropper resistor. However, bear in mind that the speed controller is ALSO in parallel with the voltage-dropper resistor. If you change the resistance of the speed controller then the parallel resistance changes and the voltage across the Voltage Dropper changes.

You'll have to play games with the values because I don't have the formulas in mind (maybe someone else has them at hand).

At the very least, place a Volt-Meter across the capacitor as you play games. Set the Speed Controller to Mid-Range. Set the Current Limiting POT to maximum resistance and then begin to increase the number of pulses.

If you reach the maximum number of pulses and the voltage across the capacitor is still too low, then begin to reduce the POT resistance.

BTW, the voltage of the capacitor should be greater than the voltage of the source.

Capacitor Farads... I don't know, start big (too much MGD at this point).

If your pulses are at maximum count, and/or duration, and the POT resistance is minimum then you need to DECREASE the capacitor farads.

In this case, the capacitor is like a HUGE accumulator that never reaches set point.

On the other hand, if the pulse count is low, and of short duration, and the POT is set to a relatively high resistance, then you need to INCREASE the farads.

There are better, more refined, ways to do this with simple components. I just can't go that way right now... damn... LOVE that MGD!