position sensor for harsh environment.

[Pat Ford]

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

 

[John Woodgate]

I read in sci.electronics.design that Pat Ford <pat.ford@nrc.ca> wrote
(in <ctl9av$c96$1@nrc-news.nrc.ca&gt about 'position sensor for harsh
environment.', on Mon, 31 Jan 2005:

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?

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!
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Pat Ford]

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
newsxoP6dFI4i$BFwhF@jmwa.demon.co.uk...

I read in sci.electronics.design that Pat Ford <pat.ford@nrc.ca> wrote
(in <ctl9av$c96$1@nrc-news.nrc.ca&gt about 'position sensor for harsh
environment.', on Mon, 31 Jan 2005:

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?

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!
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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

 

[John Woodgate]

I read in sci.electronics.design that Pat Ford <pat.ford@nrc.ca> wrote
(in <ctlckk$eul$1@nrc-news.nrc.ca&gt about 'position sensor for harsh
environment.', on Mon, 31 Jan 2005:

There isn't a column, a worm gear driven by an electric motor is the
input.

Is the motor waterproof? Is the non-drive end above the water level?
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Tim Wescott]

Pat Ford wrote:

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
newsxoP6dFI4i$BFwhF@jmwa.demon.co.uk...

I read in sci.electronics.design that Pat Ford <pat.ford@nrc.ca> wrote
(in <ctl9av$c96$1@nrc-news.nrc.ca&gt about 'position sensor for harsh
environment.', on Mon, 31 Jan 2005:


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?

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!
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk


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


If you're trying to drive something through a gearbox your control

system won't be simple --
http://www.wescottdesign.com/articles/Friction/friction.html. 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 $150 you're spending about $20 for the transformer, $70 for the
rugged, and $60 for the advertising and to have it sitting on the shelf
so's you can buy it and have it shipped today.

2. 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).

3. 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.

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

5. If I can use a pair of gear-tooth sensors in quadrature, plus a home
switch: http://www.allegromicro.com/.

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Pat Ford]

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:3QLYC9FGuj$BFwxQ@jmwa.demon.co.uk...

I read in sci.electronics.design that Pat Ford <pat.ford@nrc.ca> wrote
(in <ctlckk$eul$1@nrc-news.nrc.ca&gt about 'position sensor for harsh
environment.', on Mon, 31 Jan 2005:

There isn't a column, a worm gear driven by an electric motor is the
input.

Is the motor waterproof? Is the non-drive end above the water level?
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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

 

[Paul Burke]

Pat Ford wrote:

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

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