Let's Make Robots!

Question - Rev counter

For this project here http://letsmakerobots.com/node/22474, I am needing a simple sensor to indicate the number of revolutions a tsecond that an axle makes.  However this will be in a dirty/oily place (under isde of carriage bogie) so I am wondering on the best method of sensor to use.  I think I have narrowed it down to three possibilites -

1/ Optical slot interupter with seperate encoding wheel.  Probably easiest option but how prone to dirt and mis reading.  Could be hard to clean.

2/Magnetic sensor - hall effect switch.  Magnet on axle , most robust mechanically, may not be fast enough to record all pulses (especially with pickaxe?)

3/ Optical reflective sensor. Thinking here to put the alternate black/white segments on the rail wheel. Probably somewhere in the middle as paint will get dirty but easier to clean than (1)


The axle can probably rotate up to 30 times a second.


Thoughts please on options


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Consider that it may be easy enough to find a mechanical switch that is rated for wet or dirty conditions.

how fast will this be going, you could use a continuous rotation sinusoidal potentiometer with its output connected to a comparator to generate a square wave.

Why would I say to go with the magnet? Because that's what cars use now. Billions of cars have been built, billions of dollars have been spent and in the end, cars use a magnet on a part of the drive train to detect speed. I am not saying that private industry is always right or just because everyone does it, it is the best way but I will say that I have rarely if ever heard of a speed sensor on a car ever failing.

Magnets work, it is a sure thing, go with what has already been proven to work.

Word to ya motha'

Hi Guys,


 Thanks for the replies. It sucks not to have internet at home but just too expensive here in Australia compared to other countries.  Do I spend money on hardware or access to find out how it works?  Luckily I have time and internat availability here at work. 

 There is currently no actual bogie to mount it on - I am sorting this out before I build it all, as there will probaly need to be modifications made during the build to accomodate it.

However the axle and wheels will be smooth (to answer Gareth), but there will be a lot of vibration, which along with the need of the bogies to be able to rotate removes anything with fixed contact.  Looks like the magnets have it.  Next stage is to build my bogies and mock up the sensor / magnet housings.







I thought I knew all my UK and Australian car lingo but alas, it seems no. What is a bogie? It has to be a mount or bushing of some kind? Or a drive shaft?

Bogie is a term for chassis that carries wheels. The term is used for railway cars, and also for some tracked vehicles.


The Mars rovers use a modified version called a rocker bogie.


Hi Chris,  A bogie (other than something green from a nose) is a wheeled asembly which is located near the ends of a railway carriage. It is pivoted in the centre to allow the carriage to negotiate bends in the track. In this instance the wheels are roughly 3" diameter and 3/4" thick, with the sideframes in 10mm (3/8") steel plate. Two of these bogies on a 4 foot cariage will handle 3 adults weight.

My sensor will be attached to the sideframe and the magnet in a collar around the axle.