Driving servos from a PC

Why not pull some stepping motors out of junked inkjet printers (or dot matrix printers for that matter, though nowadays I find the former more likely to be found as junk), and use those? They would obviously be cheap and avaialble (you aren't likely to find RC servos lying in the garbage), and the driving is easy. Use a parallel port. Or, if this is from a dedicated computer, find an older one with an ISA bus, and put two parallel ports in there for more control lines. Some counter ICs would shift the needed control to hardware, and then it would only need a direction line and step line from the parallel port for each stepping motor. (They work by sending pulses to the windings of the motor, and each pulse makes the motor step one step; the direction is determined by which of the two windings you send the pulse to first.)

Michael

The message from Pete Verdon contains these words:

Hi Pete

I stand to be corrected but I don't know of any simple way that you can interface a servo to a sound card, let alone two.

servos would be good but might be a little over kill, why only two to each PC? Could you use one PC for upto eight?

if so some ideas to make your own servo controller at:

or buy

I have the original SSC version two at

Cannot for the life of me remember where I bought it in the UK, and have also built the first one, both are ideal for controlling multiple servos and very easy to programs for the serial ones. Cannot comment on the usb ones you may find.

I will try and remember where I bought the kit from, hope this helps

Pete

not entirely, but unless you need PC control it'd be easier to just use a 555 and a pot for each.

variable duty-cycle cicuit:

----+--- vcc | .----------------------|--------+ | | | | +--------+ | V | | | .--[POT]-->|--. | . . . .|. . . . | | | | . VCC(8) . | | | | . . | `---------| out | . 555 . +-------TH(6) DIS(7)---- | . . +-------TR(2) CV(5)-- C1 | . . ===== . GND(1) . | . . . .|. . . . | | +-------------+ | ---+-- gnd

A servo controller like below is your best bet.

Not a square wave. They need a pulse between 1 to 2mS long (depending on the position) repeated about 50 times a second.

I believe most people use a PIC micro to interface servos to a PC - possibly because a PC running Windows doesn't have the real time capability to do it? I'm not sure.

Hi, Pete. You already have the capability in your parallel printer port -- output logic level signals. You can also use the PC to supply

You didn't mention the other jobs you're having the computer do, or even what operating system you're using or what programming language. If you're using DOS and your PC isn't doing anything else, you can just set your printer port for SPP in the BIOS and then use QBASIC or anything else that provides direct control of the PC's ports to program a timing loop to output the pulses (which are supposed to be 5V logic level, positive-going pulses, 1-2ms. pulse width every 20ms). Voilla

-- done. Additional cost = zero.

If you've got other things to do with your program, or if you're using Windows, it gets a little stickier. That's because the servo turns off unless it gets at least one pulse every 30ms. or so. Using Windows, you'll need help with DLLs and drivers.

You might want to try going to the Jan Axelson/Lakeview Research website for more information. Chances are you'll find what you need there (and you might want to get a copy of the book "Parallel Port Complete", provided with a CD that has the support software for various languages).

Good luck Chris

It seems unlikely that I would find six of them. However, I can buy packs of servos remarkably cheaply from some outfit advertising on eBay.

Pete

I don't see that two is any harder than one, given that I have a stereo channel to play with. Exactly how hard *one* is, of course, is something I came here to find out :-)

The event is actually three identical events running at the same time, each with two dials. It's not inconceivable that I might drive all three pairs of dials from a single computer and have the machines running the other two events talk to it over a network, but I'd much prefer to have them independent. They may in fact be running in different rooms.

Thanks. I don't know why I didn't think of using the serial port before

- clearly much more sensible than the soundcard!

Pete

I do need PC control. These dials will be part of the output for a simulation program running on a PC.

Pete

That's effectively what I wrote, isn't it? I suppose it's not quite a square wave because it's cut off at zero rather than going below, but presumably that just calls for a suitable diode.

True; I hadn't considered the "real time" aspect of the problem, which would presumably frustrate any attempts to drive them directly from a serial port as well. But I don't think I have a) the knowledge and b) the budget to start programming embedded chips.

Pete

Yes, I'm not sure why I didn't immediately think of serial/parallel ports. I suppose the idea of frequencies and waves put the idea of the soundcard into my head.

The OS and language are flexible to some degree. The core of the system will almost certainly be in Java (the servos are just an output device) but we're prepared to call out to C for talking to hardware. I don't know whether we're looking at Linux or Windows as an OS; nothing we've yet decided on has mandated either.

Unfortunately we're not going to be at a low enough level that we have total control of the processor. The machine's display is also going to be used for graphical output, which effectively means Windows or X.

I'm a software engineer in my day job, and working with another on this project. I don't normally get all that close to the hardware, but it's not inconceivable that we might be able to knock something up.

Thanks.

Pete

if you can set baud rate divisor directly that'll give provide acurate control of pulse widths without being worried by multitasking operating systems.

attempting software PWM will be succeptable to losing multitasking, but may be accurate enough,

linux has usleep() and nanosleep() for making delays measured in microseconds or nanoseconds, accuracy is probably closer to +/- 10 microseconds most of the time (depecding on what else the computer is doing and how fast the processor is)

windows has Sleep(), which delays for a number of miliseconds, but there may be something more precise.

with a little hardware you could drive any number of 8-bit DACs from a parallel port (or a single DAC and a number of sample-and-hold circuits) this wouldn't have the tight timing requirements of the software PWM.

these analogue voltages could then be used to either generate a PWM signal (by comparing them to a triangle wave) or to directly drive a moving-coil meter.

as has been said, two output pins (and a few transistors) are suffucuent to control a stepper motor another option you should consider.

you can easily control five from a single printer port,

you'll be hit by multitasking in that case, this could cause late, or stretched pulses on with software PWM - something you probably don't want.

the sound card is still looking good, if you can load your PWM waveform into the wavetable and play it continuously it'll only need a little filtering to undo the 10Hz high-pass typical of most soundcard outputs and recover the DC component of the pwm