position sensor for harsh environment.

Hi All; I'm looking at building a 4 wheel steering system for an offroad truck. The rack has ~ 9" travel and ~3.5 turns at the pinion. I was thinking about a pot for position sensor, BUT this truck current runs, on occassion with the rear axle 12" under water. I was also thinking LVDT, but that is expensive. What would you do?

BTW this is for off highway use, so the truck won't lose control and knock a gas tanker into a school buss convoy. Pat

Reply to
Pat Ford
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There isn't a column, a worm gear driven by an electric motor is the input. I'm going to see if I can find a rotary encoder that is water proof. I'm hoping to come up with a simple voltage output device, then I can do the control system as a simple analog servo.

Thanks John Pat

Reply to
Pat Ford

If you're trying to drive something through a gearbox your control system won't be simple --

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You'll use up less board space (and likely tear out less hair) if you use a PIC with PWM output.

If it were me I'd do one of the following:

  1. Grit my teeth and spring for an LVDT. When you buy a commercial LVDT for 0 you're spending about for the transformer, for the rugged, and for the advertising and to have it sitting on the shelf so's you can buy it and have it shipped today.

  1. Roll my own LVDT. This is not that bad if you don't need rugged, and you may be able to arrange for rugged with larger size and less accuracy for less money (assuming your time isn't worth squat).

  2. Use an optical encoder an John suggested, with a home switch and a PIC to make it all work. Of course, a nice rugged optical encoder and a home switch costs as much as a nice rugged LVDT.

  1. OTOH, coarse optical encoders can be whomped up fairly easily.

  2. If I can use a pair of gear-tooth sensors in quadrature, plus a home switch:
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I think that in any of the encoder cases I'd put the "home" switch so that it's closed on one side of travel and open on the other, with the switch happening at the center of travel. This isn't tradition but it means that you start out by driving to the center, and it also means that you have a redundant sensor should your encoder crap out.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

I read in sci.electronics.design that Pat Ford wrote (in ) about 'position sensor for harsh environment.', on Mon, 31 Jan 2005:

Put a rotary encoder on the steering column shaft, above the operational water level limit. If the rack jumps a tooth you have more problems than just a wrong sensor reading!

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
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Reply to
John Woodgate

the motor is water proof, it started life as a trolling motor ( for a fishing boat) Pat

Reply to
Pat Ford

I read in sci.electronics.design that Pat Ford wrote (in ) about 'position sensor for harsh environment.', on Mon, 31 Jan 2005:

Is the motor waterproof? Is the non-drive end above the water level?

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
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Reply to
John Woodgate

I like the idea of a quadrature detector using magnetic Hall-effect sensors. Dust, dirt and water generally don't affect magnetic fields (I presume the mud doesn't contain iron ore or filings), whereas optical sensors can be disabled by the dirty water or mud that is likely to get into such a device near a wheel. You can put each Hall sensor halfway into a long heat-shrink tube that folds back up so that both open ends are well above the water/mud line.

And even after doing the best job you can to make it reliable, I'd feel a little more comfortable if there were also a backup consisting of two center-off spring-loaded toggle switches as no-feedback (except leaning over and looking at the actual angle of the wheels, or just seeing which way the vehicle is going!) steering control, each one switching power directly to each steering motor.

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Reply to
Ben Bradley

Whatever encoder you use, don't use an incremental one! Otherwise you'll have to re- initialise the thing when you turn on. If you've got any rotational element that makes a single turn, a resolver would be a good choice- very rugged, absolute and not tremendously expensive compared to your life.

Talking of which, I wouldn't voluntarily hang mine on an electronic system anyway...

Paul Burke

Reply to
Paul Burke

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