Help with Wheel Speed Sensor design

mstarr21

I am working on a small project to create a dynamically adjustable wing for an automobile based on several data variables. The idea is to get the wing to reduce its angle of attack when the wing is not needed, and to dynamically adjust when it is needed. I've separated the project in to a couple phases, the first being a much simpler design which only looks at vehicle speed to actuate the wing.

I would like to derive the speed by reading in the period of rotation for one of the vehicle's wheels. I am using my old SumoBot BS2 board for this, as of now I think it is capable of handling the job, but I am not certain.

I've been researching how to do this and right now I am a little confused and I'd like to see if someone may be able to point me in the right direction.

Initially I came up with the idea to use a hall-effect sensor at the wheel to deliver a clock signal to a JK flip-flop. The toggling HIGH output duration from the flip-flop could be read in to the BS2 w/ PULSIN. From there I have a time for the period of wheel rotation and derive a speed. I'm having trouble with the following:

1. Finding a hall-effect sensor which simply sends pulses (maybe I need a coil instead of hall-effect?)
2. Figuring out if I need a low-pass filter or not.
3. PULSIN for the BS2 is only good for pulses < ~130ms, which may cause problems when vehicle at low speed.
4. Possible sampling rate problem.

What do you guys think?

Hal Albach

Mouser has an omni-polar Hall-Effect switch that is not latched and works with either magnetic pole. http://www.diodes.com/datasheets/AH180N.pdf. Single piece price is $0.54 + SH.

garyg

130 ms is a pretty long time when you think about it.
If I were building this, I would either try out Hal Albach's Hall-Effect switch idea using 2 or more magnets attached
either to the drive shaft, or one of the wheels.
A year or so, I built a tach to monitor the speed of several motors in a robot project I was doing.
I used a 555 timer circuit to make a good square wave consistent pulse that fed my BS2 microcontroller.
my sensor was a PhotoDiode and my reflector was white duct tape.
When using two duct tape reflectors, I was able to read a motor shaft speed of 4200RPM consistently.

Just some food for thought

Garyg

pmrobert

Depending on your potential budget a simple GPS input gives you speed that should be precise enough for what you're doing.

Multiple ways to skin that poor cat!

mstarr21

I'd be happy to go with a GPS module since it is a more elegant solution that could be used on any car. The PMB-688 on the Parallax page seems affordable. I'm looking for documentation on people using the BS2 with GPS and the pickings seem to be slim. Is there a better source for learning how to interact with the GPS module?

pmrobert

There's sample BS2p(?) code on the 688 download page that may be what you're looking for. It says it's for the 648 but I doubt they would have placed it for download if it didn't work - it looks like it expects very, very generic NMEA GPS communications at a quick glance. I would say that even if that code requires a little massaging it's still easier than the wheel speed sensor - default GPS is 4800 bps with all ASCII communication so is very amenable to use with just about anything. A 5 minute call to Parallax support would probably confirm device and code usability - they are awesome.