Motor control chip options?
ElectroHack
Posts: 13
Hi,
I am a semi knowledgeable, and sort of a dreamer and builder.
I got this idea that I was going to take a Stamps in Class board and build a robot, a simple one, from ground up. For starters I was going to control it with a TV remote. I have plans to get some trackers and gear boxes for mobility.
My basic plan was to let the stamp handle things (like taking commands from me!) and let it communicate with other subsystems, such as motorcontrol, rather then doing the PWM itself.
Motor control is the topic of this question. I have put together 2 TLC555's in such a way that I do PWM with them, and I can control duty cycle thru a digitial Pot and a transistor, the pot communicating with the stamp, serially. I have also built a transistor bridge, which all works.
My question, does the TLC555 solution seem all to inelegant?·Is there a chip that provides a cleaner solution than this?
Thanks
Post Edited (ElectroHack) : 7/11/2008 12:49:10 AM GMT
I am a semi knowledgeable, and sort of a dreamer and builder.
I got this idea that I was going to take a Stamps in Class board and build a robot, a simple one, from ground up. For starters I was going to control it with a TV remote. I have plans to get some trackers and gear boxes for mobility.
My basic plan was to let the stamp handle things (like taking commands from me!) and let it communicate with other subsystems, such as motorcontrol, rather then doing the PWM itself.
Motor control is the topic of this question. I have put together 2 TLC555's in such a way that I do PWM with them, and I can control duty cycle thru a digitial Pot and a transistor, the pot communicating with the stamp, serially. I have also built a transistor bridge, which all works.
My question, does the TLC555 solution seem all to inelegant?·Is there a chip that provides a cleaner solution than this?
Thanks
Post Edited (ElectroHack) : 7/11/2008 12:49:10 AM GMT
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- Stephen
The question remains, do you want analog input do the job or digital. The motor controller final stage is primarily an H-bridge that decides direction by digital binary control. The PWM is often used to control the overall speed of the motor by its duty cycle.
So, I guess you should be considering H-bridges next. For big electric motors, these are better off build from individual components as the amount of heat involved is more easily dissipated. Big MOSfets are quite popular as they switch the power on and off. This generates less heat than traditional transistors that can be in a half on mode.
I guess you have the 555 PWM controled by the BasicStamp rather than directly transmiting PWM from the BasicStamp. This isn't bad as the BasicStamp may actually have more free time to do other things. PWM really requires a lot of the processing time in the BasicStamps as the device is running in fast loops. On the other hand, the digital potentiometer can be a set and forget device until you want to change it.
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PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······
Post Edited (Kramer) : 7/12/2008 4:05:53 PM GMT
Is there something wrong with that?
I had been away from this thread for a bit. Thanks all for the input.
My plan is as follows for building this robot
I plan on buying one of these tread kits from the Robot Superstore as a base. (Tamiya 70108)·For directional control, I'll also be buying their dual motor kit.
The electronics I have been working on to support this are as follows
1 TLC555 running as a free multivibrator
2 TLC555's triggered by the first TLC555. When the first 555 (pin 3) is low, these are triggered. So TLC#1 has a very high duty cycle (it's high with a very brief low pulse)
2 Digital Pots control the timing on the dual 555's (the are controlled by my Basic Stamp) So this controls motor speed.
The 555's feed into two optoisolators (1 per motor)
2 bridges (which I built with NPN/PNP power transistors actuated by 2 3906's)
Directional·rotation (for each motor) is handled·by a relay. (NO/NC) Input to the relay·is the PWM from the OptoIsolator.·The N/C side of the relay (each relay) goes to the forward side of each bridge, and the N/O side goes to the reversed side of the bridge.···
It seems a little "Rube Goldbergish" ; My concern is that all of this stuff is taking up quite a bit of breadboard space, and I'm not sure it's all going to fit cleanly on my platform, so I opened up this question to the forum.
I'll check out the·SN754410 motor driver and see what it can do for my projects.
Thanks Again
Patrick Casey
The TLC393 consists of dual independent micropower voltage comparators designed to
operate from a single supply.
http://focus.ti.com/lit/ds/symlink/tlc393-q1.pdf
The photo is on page 12
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··Thanks for any·
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Sam
Post Edited (sam_sam_sam) : 7/20/2008 3:14:05 AM GMT
Regardless of which way you want to go with the control circuitry, the final stage will be an H-bridge. In your situation, you need two - one for each motor. Tamiya is a good choice and Parallax actually uses their tracks on thier own bot.
Building a lot of outboard circuitry will give your more available processing time for other things on your Basic Stamp. If you used the usual PWM to servo motors, much of your processing time would be tied up in control of the motors as they expect to be constantly updated. Your approach doesn't require that as you can set the digital pots and leave them in their settings until a correction is required.
Another consideration with all the extra circuitry is how much power is it wasting. Batteries can only do so much and a robot can only manage a limited amount of weight. The BS2 uses very little power in standby mode while the BS2px uses quite a bit more, maybe 35 times as much.
The tlc393 appears to provide the H-bridge and all the control in one package, but does it lead you back to constant PWM updates? There are other H-bridge ICs that don't. And you can use 2 half H-bridges in a package to make one full H-bridge.
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Post Edited (Kramer) : 7/20/2008 5:33:32 PM GMT
My biggest thing, and sometimes it's a liability, is I like to figure out how things work, so I'll get some idea to do something and I take off to build it. Oftentimes, it's something that's already been built . . . . . but I have learned a lot doing this.
With this little project, I'll probably control the robot with a TV controller (I bought the Parallax Ocilloscope and manual). Then when I get bored with that, I'll make it RC. Then maybe I'll put a camera on it and do a documentary about turtles or something (grin)
As I read the documentation, it says that I can control a motor up to 9v. I plan on using the Tamiya double motor gearbox, (motors safe to 8.6V at .5 A) Technically, if I run 6Volts, I could get away with no bridge. The rating that they put on the gearbox say "safe up to 8.6v " . . . don't know if that per motor or total. My guess is per motor.
I'm on the fence as to whether I would still need a bridge. First thought is if I have two motors running at 8.6 V, I'll need a pretty beefy battery pack. I guess I'll have to play some more.
Thanx