Servo Cable Length

Claude

Greetings-

Just got as PSC for my BS2. The servos that came with the kit have a ~foot cable to connect them to the PSC. Can I extend the cable length to ~12 feet without damaging the PSC or servo? If that’s not possible (and I suspect that is the case), could you suggest any driver circuits?

BTW, great forum. Very insightful dialog.

TIA,
Claude

Post Edited By Moderator (Chris Savage (Parallax)) : 6/29/2005 4:05:21 AM GMT

dandreae

You'll have to experiment with the length.· I don't know that the signal will make it that far?



Dave

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Dave Andreae

Tech Support
dandreae@parallax.com
Http://www.parallax.com

·

LoopyByteloose

Model airplane suppliers sell extension cords for servos to accommodate larger radio controlled airplanes.

But, I have never seen a 12 foot cord. The main problem is the pulse width modulated signal as you can move to larger wire for the power supply, but PWM requires you maintain some quality as well.
You could try a coaxial cable to keep the signal from becoming noisy and could add a transistor or an op amp (depending on how much gain you want) to boost its power. I suspect a 2n2222 would provide enough gain if properly included.

To review, you have main two issues.
1. providing a powerful enough signal
2. avoiding collecting noise from the length of the cable.

and one minor issue
3. the voltage drop in the small wire size on the power supply

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G. Herzog in Taiwan

Post Edited (Kramer) : 6/29/2005 3:54:07 PM GMT

allanlane5

You can extend the cable without "damaging" anything, I would expect. The problem you may run into is that the pulse you are sending to the servo will drop in voltage, and will 'smear' in rise and fall times. I don't know how much flexibility the comparators inside the servo have. If you do go 'too far', then the servo will either stop operating, or go all the way to one side and stay there. Shouldn't damage anything, though.

Now, a MAX232 driver on one side, and a MAX232 on the reciever side, should allow a nice sharp signal to the servo. In this case, you'd be using the drivers merely as a TTL to +- 10 volt converter, and take advantage of the larger signal to go longer distances with less distortion. That should be good to 150 feet or so.

Either way, experimentation will tell you what will work. I would use a shielded cable in either case, as you want to minimize noise on this line.

LoopyByteloose

Yeah, you're right.· There wouldn't be any damage as the problem is voltage drop.· Damage would only occure from too high a voltage.

Couldn't you just drive the signal through an IC driver chip rather than a Max232?· If would be a much cheaper chip.· Some invert the signal, but I think you could invert it twice to get the output to be the original.· I am thinking of a 74HC240.

Wouldn't twist pairs enhance the clarity?· You could use four wire phone line and double the ground if that doesn't drop the power.· Also, it would enable you to use the telephone plugs for modularity.

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G. Herzog in Taiwan

Post Edited (Kramer) : 6/30/2005 11:35:09 AM GMT

allanlane5

Twisted pairs would only help if you are using a differential signal -- like RS485. With 485, one line goes high while the other line goes low, then they switch. Since the reciever only pays attention to the difference between the two lines, "common mode noise" is eliminated. "Common mode" noise would be noise that affects both wires -- bumping both up a volt, then down a volt.

Twisted pairs in 485 helps keep the common mode noise common to both conductors.

Now, RS232 is a 'single-ended' signal -- one signal wire goes positive and negative, compared to a ground wire. This does NOT eliminate common mode noise, thus the helpfulness of a shield.

Don Buczynski

I've got about 10-11 feet of·wire between the BS2 pin and a·servo on my model railroad. I have not noticed any servo positioning errors or erratic operation. I am using gray "telephone" cabling (unshielded and not twisted pair).·I do notice servo jitter if a short circuit occurs due to a derailment. If your environment is electrically noise free, 12 feet of cable should work ok.·····


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Don Buczynski

http://www.buczynski.com

Post Edited (Don Buczynski) : 6/30/2005 11:40:18 PM GMT

allanlane5

Whoa. Don is 'Da Man' when it comes to using BS2's for model train control. If it worked for him, it's got to work for you. When you get a chance, check out his web-site. It never fails to inspire me.