My FB project, Question on MOSFETs

mx270a · 04-12-2008, 08:44 PM

Well guys, today was the maiden voyage for the project I've been working on - steering a tractor via GPS. I'm using a fusion brain to read steering angle position, and triggering two outputs that control relays that drive an electro-hydraulic valve that steers the tractor. I was letting the software drive the tractor, which stayed within a range of about one foot of the line.

Can I use a MOSFET instead of a relay? Would the FB outputs be alright with that? The clicky clicky of the relays was a bit louder than I prefer. I could move them somewhere else for testing, but just checking out other options? The solenoids I'm driving are 12vDC at ~3.5 amps.

-Lance

More info (added by greenman100)
Here's a new video:
http://www.youtube.com/watch?v=YFqfTlqF2NY

The machine is a Case IH 2588 combine (~300HP) that is harvesting soybeans at 4 MPH. I'm using the FB to read a steering angle position sensor, a yaw sensor, and a tilt sensor. That is added to a GPS position to drive the machine in a straight line, with each pass parallel and at a consistent distance from the last pass. The fusion brain outputs steering commands, which go to a couple relays, which send power to solenoids on a hydraulic valve that is connected in parallel to the steering wheel orbital.

The header (thing on the front) is 25 feet wide. As you can see in the video, the yaw sensor makes the vehicle control very smooth. It still likes to drift off the line, then slowly come back to it (it can get up to about 8" away from the line.) I'm still working on revising the yaw sensor calibration algorithm, and the method used to calculate what angle the steer axle should be at. Yaw sensors are a royal pain due to their "zero" position changing as they warm up. As slow as the machine is moving, with subtle steering inputs, getting an accurate and precise yaw reading is a challenge.

-Lance

2k1Toaster · 04-13-2008, 12:44 AM

The output uses the ULN2003A which outputs 400mA to 500mA

greenman100 · 04-13-2008, 01:27 AM

You could, of course.

I'm not sure exactly what your question is.

mx270a · 04-13-2008, 09:48 AM

My question was if I could use a FET in place of a relay. I'm relatively sure it will work, just thought I'd run it by you guys to confirm.

-Lance

greenman100 · 04-13-2008, 09:56 AM

yes, it shouldn't be a problem

greenman100 · 04-13-2008, 12:28 PM

Also: could you post a video/writeup on what you did? It sounds really cool.

mx270a · 05-06-2008, 12:51 AM

I'm working on putting together some details on what's going on here - will probably post it in the 'show off your project' section of the site. Still working on getting some video of the system in action.

Still haven't tried the mosfets. We found that it's actually preferable to use the relays while the system is in development, as it allows the operator to hear that the system is working, as well as how active it is. Here's a picture of what we're doing:

The computer is steering the tractor along a straight line, the operator turns around at the end, and the computer steers again. We end up with straight passes that are all equally spaced.

-Lance

HiJackZX1 · 05-06-2008, 06:39 AM

Wow this is cool..... pretty soon Im sure someone will come up with a solution to drive a car on the streets on its own via GPS.

h3rk · 05-07-2008, 06:07 AM

Quote: Originally Posted by HiJackZX1

Wow this is cool..... pretty soon Im sure someone will come up with a solution to drive a car on the streets on its own via GPS.

What a scary thought.. At least this is way out in the open.

So, did you just happen to have a guide-a-tractor-by-gps program sitting around, waiting to be taken to the next step? That's the part that intrigues me the most. It's pretty impressive, what you did right thar.

bes51659 · 05-07-2008, 07:33 AM

Fastest way to goal is to buy solid state relays. And maybe safest...

Quick search: http://www.ab.com/industrialcontrols...ate_relay.html

mx270a · 05-07-2008, 09:17 AM

H3rk, I built the software from the ground up.

Bes, I have been thinking about getting a couple of those to test. The load is a coil on a hydraulic valve, and it was creating a voltage drop/spike that the FB didn't like. Moved the FB to a different power source, and it has behaved normally ever since. I'm also seeing some odd behavior from my GPS antenna, which I suspect is also being caused from the voltage drops/spikes. I'm going to change where they are powered from to isolate that spike, which means a mosfet won't work for me any more. For right now, I like the sound of the relays to know what they are doing, but ultimately I need something more resilient, and a solid state relay looks to be a good option.

-Lance

HiJackZX1 · 05-07-2008, 11:24 AM

Quote: Originally Posted by bes51659

Fastest way to goal is to buy solid state relays. And maybe safest...

Quick search: http://www.ab.com/industrialcontrols...ate_relay.html

Solid state relays have issues, they can easily send signals back up the wrong way causing more headache of problems. Solid states are good for HDD, but when it comes to solid state switches, i would stay away. If you google the issues Solid State Relays have you would get a whole list of issues.

sandmanx5 · 05-07-2008, 11:43 AM

Quote: Originally Posted by bes51659

Fastest way to goal is to buy solid state relays. And maybe safest...

Quick search: http://www.ab.com/industrialcontrols...ate_relay.html

If you go with solid state - look for ones that are built using opto-isolators or just look at using an opto-isolator directly...

http://en.wikipedia.org/wiki/Opto-isolator
and
http://homepages.which.net/~paul.hil...rol/Optos.html

These are likely the solution I'll be using in my setup.. Absolutely no chance of power leaking across the gate as there is no physical connection for it to happen to.

Brilliant idea for running a tractor BTW.. I should show your idea to my aunt.. Maybe she'll let me pimp out her tractor too!

jimp · 05-07-2008, 11:43 PM

Quote: Originally Posted by h3rk

What a scary thought.. At least this is way out in the open.

"SCARY" you obviously haven't driven through Houston lately! I think I'd prefer to know a computer was in charge of things than some of the idiots around here!