Speed Control of a BLDC Motor with a PLC

P

Thread Starter

PSJ

For our engg. project we are developing a pilotless dc vehicle - which will follow a magnetic tape on ground. vehicle will have 2 bldc motors + a magnetic tape reader + 2 swivel casters at back. vehicle will run on differential.

As a first step we need to control a bldc motor [which has a encoder also] with a plc command.

do we need a PID control for this or can we just use scaling for this control - plc will give control voltage of 0 to 5volt through analog card. and this will control the rpm of motor with a range from 0 to 2500rpm. the encoder meanwhile will give 18pulses/rev.

once we control each of these 2 bldc motors that we will hook up the magnetic tape reader with these 2 motors. to keep vehicle on track.

we are using a delta plc.
look forward to suggestions.

cheers !!!
juncera [at] gmail.com
 
Using PLC for control BLDC motors isn't good idea due to necessity to commutation of motor and use 3-phase power stage. Well-known compact digital motion controller with build-in servo amp from Elmo, AMC or Copley will be better solution. Macro program inside controller with I/O may keep your vehicle on track also.
 
our bldc [24volt dc] has a controller also and this includes a 0~5volt analogue speed control too - we plan to hook this to the plc.
- will it be workable ??
 
J

So it sounds like the motors are already commutated and powered by a drive, and you are not trying to PWM the motors or anything like that.

Since you have the analog 5VDC out available, I would probably keep things simple and just use it a as a speed reference signal for both motors, and use one digital output from the PLC for each of the enable signals to the motors. This will allow you to do things like slow down the motors for turns, accelerate on the straights, etc. But it will not allow you to turn the motors at different speeds.

Without knowing anything about what your sensor(s) is picking up from the magnetic tape I am going to assuming that you probably have a couple sensors wired to digital inputs that tell you if you are one one side of it or the other.

If that is the case you will not need a PID loop for something this simple. You just need to provide direct feedback to the motors to correct direction. This could be considered as only using the "P" in PID.

Jim Miller
Application Engineer
Quantum Devices Inc.
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Barneveld WI, 53507
Ph: 608.924.3000
Fax: 608.924.3007
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Just to further explain the sensor:

Voltage is about 5V when a sensor is in line with the center of guide tape. Voltage will increase from 5V to 10V when sensor goes in right side and 5 down to 0 volt when sensor goes on left side.

BLDC Controller takes an input of 0 to 5volt
& this varies its speed from 0 to 2500rpm.
Further BLDC controller gives 18pulse/rev.

When we run BLDC motors only - say at fixed speed
we are able to run the vehicle in a straight line.

The problem starts the moment we try to integrate tape - the vehicle starts going left and right on tape and this amplifies within few meters.

Since we need to take turns etc. the motors speed should be regularly changed as required.

Any suggestion how to integrate tape with motors.
 
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