Dillemma: DC Motor Control vs. Torque Sensing

Hello everybody,

I am fairly new to PLCs, although I have one year left in my three-year Industrial Automation course so I have had some exerience. I have also worked with microcontrollers.

I am creating a project that has a motor (DC brush) and a lead screw to move a device in and out along the lead screw. I am trying to decide on my control options when it comes to speed control of the DC motor and torque sensing of the motor.

The device being moved on the screw must stop and retract 0.5" when a certain torque has been achieved (device putting pressure on something).

My dilemma is that torque sensors are too expensive, so I was considering a simple current sensing circuit with a level detector, and through experimentation determine the correct level at which to retract the motor. The only problem with this is that I was planning on using an H-bridge PWM setup for controlling the motor speed and direction (in which case the current sensing resistor option would be useless).

I have since been looking for some kind of way to control the motor speed without using PWM (strictly voltage control) but the options seem fairly sparse. I've been trying to look into some type of digital voltage controller of some kind. I've also been thinking of using an AB Pico controller for this project. Does anybody have any suggestions for me?

DC Motor is 1/8HP 12 or 24v. Torque being sensed is not much (between 0 and 10 lb/in, or 0 and 1.2 Nm).

Thank you everybody

i know there are stepper motor control cards for some plcs. Im not familiar with the torque sensing ability though but its worth looking into.

We were faced with the same problem once and a friend recommended, making our own torque sensing device using an air cylinder filled with oil and attaching a pressure transducer and
reading the voltage to adjust the output to the motor.

You might want to buy the DC motor controller. There are several suppliers out there, one being Dart. Since current and torque are directly related to each other in permanent magnet DC motors, either the controller will give you an analog current output or you can do your own current sensing on the armature leads.

I'd do a fractional hp DC motor controller search to find other suppliers.

If you want a practical solution the I would go with DickDV's

I am sure that Dart controller is an inexpensive way to go. I don't know how well they can control torque though. It should be easy though.

If you 'simply' ( I wish there was a smiley for this ) want to control the motor using voltage then here is more info than you can use:
http://www.deltamotion.com/peter/Mathcad/Mathcad - DC Motor Velocity.pdf
This is more than what a PLC can do.

Where else on the internet can you see the formulas for PID gains expressed in terms of inertia, inductance, friction and resistance? OK, I know I am geeking out but it is yet another slow Friday evening.

We use voltage to current amplifiers ( transconductance ). It is easy to control or limit current ( torque ) doing it this way. We have plenty of experience at torque and force control.

Dart, KB and Curtis all have battery voltage speed controls which will work. None that I am aware of have any analog outputs for current/torque. But, as Dick pointed out, current and torque are directly proportional, so it's an easy sensing project.

You will need to sense the current the motor is drawing, with a transducer or a set-point relay, depending on whether you want a hardware trip or an adjustable trip in your plc.

Something like this:
http://www.davis.com/catalog/product_view.asp?sku=6846961
or
http://www.crmagnetics.com/products/CR-5395-P9.aspx

Peter Nachtwey said:

If you 'simply' ( I wish there was a smiley for this ) want to control the motor using voltage then here is more info than you can use:
http://www.deltamotion.com/peter/Mathcad/Mathcad - DC Motor Velocity.pdf

Click to expand...

That must have taken at least 20 minutes to work out

Norm, that is my point exactly. It will take at least 20 minutes to figure it out

So why bother attempting to roll your own?
Buy an off the shelf solution.
If no one else can do it then I/we can.

Well, I've been having trouble finding a current sensing transducer small enough. $160 for a transducer is simply too much money to spend for this project, there is a budget to be met.

As far as the motor control, I'm going to be building a simple motor controller with a Microchip PIC, for forward/reverse and different speed settings using pwm.

With cost as a major factor, and extremely tight space requirements, off the shelf solutions haven't excited me thus far. Another reason I don't want to use an actual rotational torque sensor is due to the cost.

You could use a shunt resistor. Put one in line with your motor lead and read the mV across the resistor. A calibrated one will give you a usable reading, and they are cheap

http://cgi.ebay.com/Shunt-Resistor-...tem&pt=LH_DefaultDomain_0&hash=item335c8f43d4

Brian

JonHopper said:

I'm going to be building a simple motor controller with a Microchip PIC, for forward/reverse and different speed settings using pwm.

Click to expand...

Have you considered using a FPGA instead of a PIC?
With a FPGA you'll have more options and more I/Os wich means you could both controll start stop and sense the current wich is linear with the torque.

ojz0r said:

Have you considered using a FPGA instead of a PIC?

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Don't be ridiculous. It is bad enough JonHopper thinks he can save money by making his own system from scratch starting with a micro controller. Now you want JonHopper to design the micro controller too.

Peter Nachtwey said:

Don't be ridiculous. It is bad enough JonHopper thinks he can save money by making his own system from scratch starting with a micro controller. Now you want JonHopper to design the micro controller too.

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Well he said he was on a budget hehe
But FPGA isnt that hard, there are lots of online guides with already programmed examples.

It sounds like this is a school or personal project. If that is the case, time is not really of the essence so taking the time to "roll your own" PWM DC motor control and current sensor should be a fun and very educational project.

If this is for real use in industry, then values like $160 for pre-made a current sensor are a pittance compared to the salary of a person taking the time to develop and build their own. In some companies minds, they don't care about this and consider in-house time "free" especially if there is nothing else for the person to do at the time and they are drawing a salary anyway. They are also not considering the person 2 years from now who will have to support the custom made device and will have no clue what is going on or how to troubleshoot the device when it goes haywire.

I would not consider an FPGA for a project like this but perhaps the Propeller chip from Parallax would be a good choice. There are many published "objects" for it and even a section of them on motor control:
http://obex.parallax.com/objects/category/7/

Peter Nachtwey said:

Don't be ridiculous. It is bad enough JonHopper thinks he can save money by making his own system from scratch starting with a micro controller. Now you want JonHopper to design the micro controller too.

Click to expand...

I'm not making the entire system, simply the pwm controller. I have built / programmed a few in the past, it's not that difficult using software interrupts. I will be using (most likely) a Pico PLC from AB for main system control.

I have still been looking for ready made options, however, I haven't found a motor controller that will fit my needs yet. The load that will be pushed on the linear bearing needs to be switched between reverse/forward and from slow/fast fairly quickly (forward slow, torque limit reached, fast reverse), most fractional DC motor controller boards have manual speed controls, not different, switchable speed settings like I am requiring (from what i've found so far).

Like I said, this is not an industrial application, I am prototyping a system that will eventually be produced and sold. I apologize if I sound like I don't know anything, this is my first 'real' job.