Project Question - Motor Control

macgyver007

Hey All

Not to long ago I was given a "What's a Microcontroller?" kit and I got about half way through it and had to start up classes.· I have a bit of spare time right now and I stumbled uppon this cool r/c toy.· I thought to myself, wow that's pretty sweet, but I don't have almost a 1000 dollars to spend on that, I wonder if I can build anything similar·myself.· So the other day I decided to start step by step.· I have alot of programming knowledge, c++, java, VHDL etc but I'm a little weak on the hardware side.· So I have hit my first wall and I'm hoping this is an easy enough question for you all.· What I'd like to do is program my homework board to say power on four small brushless DC motors, I can get these really cheap and powerful at hobby stores.· But there are several problems I can think of....

• If i remember from a while ago those types of brushless motors can take up to 12V so I don't know how to supply that much power to it using the microcontroller?
• The pins on the microcontroller only go "low" or "high" any ideas on how I make the output voltage variable?

Maybe I'm just a little stupid for trying this but I thought it would be a cool idea and a fun project.· Any suggestions or ideas, I would greatly appreciate them.

Thanks all!

Paul Baker

Ok this is a bit outside of my expertise, but your question has gone unanswered for a few days so I'll try to help. Do you have one of these brushless DC motors on hand (or can you take a close look at them in your local store)? Brushless motors typically mean that controlling them requires feedback signals (meaning your motor will have > 2 leads coming from it), however lately I have seen some advertised brushless DC motors which only have two leads on them (meaning there is no feedback control), Im not sure if those were actual brushless motors or some marketing gimmick. If the motors you want to use have more than two leads on them you will have to either create a controller or purchase one (you may find purchasing them easier than creating one). Since DC brushless motors are one of the most difficult motors to operate and you admit weakness in the hardware arena, you may want to consider starting off with a standard (brushed) DC motor. Then revisit the issue of brushless motors afterwards. If you are set on going the brushless route and the motors do require a feedback controller, I can help point you in the direction of controller modules and chips (Its not something you want to do by building up from scratch). If tho motor only has two leads you can control the speed and direction using an H-bridge, since speed control using an H-bridge requires pretty much the complete attention of a stamp (for a single channel) you likely want to go with a PWM controller board. But until I know for sure what kind of motors these are, I cannot offer further advice.

macgyver007

I do not have one of these brushless motors yet....I was looking at something like.....

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXGKC7&P=7

I do not really need a feedback line, I don't really see a need for one, so I can go brushed.

So what you're saying is that I would need to use something like this...

http://www.parallax.com/detail.asp?product_id=28020

to control for example these types of motors...

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXEBD8&P=0


Sorry for the disorganization, and thank you very much for your help.

Paul Baker

macgyver007 said...
I do not have one of these brushless motors yet....I was looking at something like.....

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXGKC7&P=7

Perhaps I missed it but I dont see where this motor is listed as a brushless motor.

macgyver007 said...
I do not really need a feedback line, I don't really see a need for one, so I can go brushed.

So what you're saying is that I would need to use something like this...

http://www.parallax.com/detail.asp?product_id=28020

to control for example these types of motors...

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXEBD8&P=0

The feedback lines for brushless motors are there because there no brushes (so the rotor winding's polarity must be switched by an external controller), but thats unimportant. Standard DC motors work just fine unless you looking to go really fast (with greater likelyhood of your model disintegrating into a million pieces upon impact with a wall). Take a look at this page·of Parallax's products, they are all motor contol modules and many combine the PWM and the H-bridge. PWM is used to control the speed of the motor, the H-bridge allows you to operate the motor in forward and reverse. So depending on your needs, you'll need one or both. If you need both, get a motor contoller board that does both for you. Id suggest going through the specs of each of the products and see what thier voltage and current handling is, and get a feel for what each is capable of doing, then look at DC motors for thier voltage, current, speed and torque·then try to pick which motor and controller best suit your needs.

macgyver007 said...

Sorry for the disorganization, and thank you very much for your help.

no problem, glad to be of help.

Post Edited (Paul Baker) : 6/21/2005 3:50:19 AM GMT

Paul Baker

Seems like I misspoke about the brushless DC motors, there are some which only have two leads, they require some external positioning system such as an encoder to generate the feedback nessesary to drive them (some systems use back-emf which doesn't require the external positioning system but the controller is even more complex). Those can be even more difficult because you need to find a positioning system which is mechanically compatible with the brushless motor. This info is irrelevant since you are going the brushed route, but I thought I would correct this mistake.

Guenther Daubach

There are also brushless DC motors available with integrated drivers and control logic. These type of motors usually require DC power (of course), two logic signal (0/5 Volts) to select the direction of rotation, and to start/stop/brake the motor, plus an analog signal (e.g. 0...5 Volts) to control the RPMs.

Using a BS or an SX controller, you can nicely control such motors. I don't think you need to reverse the direction of rotation, so you'd only need to control the RPMs. To generate an analog signal, just generate a (digital) PWM signal, and run it through a low-pass filter. Depending on the input impedance of the RPM control line, an OP-Amp might be necessary to buffer the signal - in this case, you can use the OP-amp as an active low-pass filter.

On the other hand, such brushless DC motors with integrated controllers are more expensive than brushed motors with the required electronics to drive them. IMO, you just need brushed DC motors driven by PWM signals generated by a BS or an SX. As you can't directly drive the motors from BS/SX outputs, you will need some power drivers. You should have a look at the various types of available IGBTs (Isolated Gate Bipolar Transistors). These devices nicely combine the low on-resistance of bipolar transistors (i.e. less power dissipation) with the high-impedance inputs of power FETs. This means that you can control such IGBTs directly with BS/SX outputs.

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Greetings from Germany,

G