Low-cost controller for 10 small DC servo motors with quadrature encoder

I can't give expert advice, but it sounds like you need 10 DC gearmotors with encoders, 10 motor drivers (the amplifier electronics), and a microprocessor to direct the motor drivers. The microprocessor can easily drive the motors with a PWM signal, although you'll have to search for one with 6 timers. The problem I see is when you say "low cost". You might be able to get the motors in a surplus shop, but with encoders they will still probably be $30 to $50 US each (if you're lucky). Motor drive amplifiers will probably have to be new, and they are $30 to $100 US, each. With structure and everything else, you're probably looking at $1000US. Does that meet your definition of "low cost"?

Also, encoders don't do velocity all that well, the microprocessor has to compute it and noisy values result, which may have to be averaged, introducing a time lag in the data. A tachometer gives better velocity sensing, but isn't that great for positioning.

You'll scale back the cost a bunch if you just use 4 wheels. Once it is working, you could add the other two wheels if your requirements and budget dictate so.

Here's a good place for micros and drive amplifiers:

There are also serial servo controllers that can issue PWM commands to many motor drivers, the processor commands it over a single RS-232 line.

I think the Mars Rover double-bogie design is available on the net, but I don't have any pointers.

Mike Ross

Sorry, no wheel help here. But, for CAD files, I suspect that if you contact the people at one of the links you provided , they may have some CAD Files.

Joe Dunfee

You might want to look at our IsoPod(TM) and ServoPod(TM) line.

With a single one of those boards, you can have up to 6 hardware quadrature inputs, and either 6 complementary PWM outputs, or 12 independent (single line) LAP PWM outputs. If your four steering motors were analog feedback, rather than quadrature, you'd have enough quadrature and PWM to drive the whole robot off of a single processor board, with PID and position/velocity control on all.

Randy M. Dumse

Dear Randy,

Thank you for the information. I have just checked your web site with for your IsoPod(TM) and ServoPod(TM) product line. They sound FANTASTIC!!!

However, they seem to have 2 quadrature encoder inputs rather than 6. Am I missing something here? Can you tell me how to connect 6 quadrature encoders?

Can IsoMax (Virtually Parallel Machine Architecture Programming Language for IsoPod(TM), ServoPod(TM)) handle 10 PID concurrently (6 with quadrature encoder feedback and 4 with analog feedback)?

Your NMIH-0030 H-Bridge product (10 of those) also seems to be suitable for my application. I assume that in addition to 10 PWM signals, IsoPod(TM) and ServoPod(TM) can also concurrently produce Dir (direction) signals. (Can you confirm this please?)

Is it also possible on to decode multiple channel RC pulse streams (see

Thank you

Albert Goodwill albertgoodwill at gmail com

In my opinion, nothing else comes close for motion control.

The IsoPod(TM), ServoPod(TM), have two built in hardware quadrature inputs that only do quadrature decoding. They share pins with some of the timers. There are 16 timers, 14 with pins. The 16-bit timers are programmable and can do the function of quadrature as well. They can be concatenated to give a full 32-bits of quadrature.

So you can do the 6 channels of quadrature in hardware by using the two built-ins and four timer channels. You still have a couple timers left over for other purposes. By hardware, I mean, once set up, you only have to read them when you want the answer. There is no software burden on the processor to have these channels coming in. I have some code examples for setting up 32-quadrature decodes. I use 6 channels input on the Panavision Back Pan Plus movie crane, so I know it works and works well. In fact, I even use the other timers to pull the velocity of the signal by doing pulse width timing on all channels.

I'm not absolutely certain IsoMax(TM) can handle that many channels of PID. It rather matters how often you want updates. Perhaps at 100 updates per second, I'd think there is a good chance of operating in purely high level.

Then NMIH-0030 is not my favorite H-bridge because the PWM upper limit is low, in the audio range. I much prefer the NMIH-0050, which is more robust, and we've tested at higher rates. Depends on your motors. Coreless are low inductance and need higher PWM frequencies or they will be operating in the resistive range, which is pure waste of power.

Have you looked into Locked Anti Phase (LAP)? With a single PWM line you can send both direction and power. At 50% the wheel is locked, at

But either LAP, or complementary, or PWM on one line or another at a time, you have to control the direction of the motor, and may have to reverse a line when crossing zero, if you use, say, a PWM line for power and a port line for direction. Only LAP and complementary PWM take care of themselves that way.

Yes, it is also possible to decode RC pulse streams. The timers are again wonderfully configureable, and can be used in a processor hands- off mode to measure the width of a pulse. Being able to reliably pull a string of pulse widths out though, may depend on getting back to the stream often enough, and that may depend on how loaded your processor is with the PID routines. Again I have some code examples of pulling the pulse width on a single servo channel, and we've got support in the language for doing that measurement.

Randy

It is pretty easy to decode the entire stream of RC pulses with the DSP56F807, but you need to open up the RC receiver to do it. I started with a pair of timers, but I think it is just using one of them now (it has been several years since I coded that). All of the servo signals are available on a single signal just before the decoder part of the receiver. The decoder is either a shift register or a Johnson counter depending on the receiver design. In either case, the composite signal is used as the clock to the counter or shift register.

BobH

Our FPGA cards with SoftDMC firmware can currently drive 8 servo motors/card at ~20 KHz PID update rate, 10 would be a minor hack. about 16 axis worth of control hardware/CPU will fit in a 400K FPGA

Peter Wallace Mesa Electronics