Right.... update time...

I spent a couple of hours today making up a bracket to hold the sensor.

I originally planned in making it from aluminium but as soon as I bent it, it snapped (twice!). even with heating it. pictures below tell the story!

Plan b was to use some steel strip bent to suit. I used 24x1mm strip.

This simply bolts to the existing holes in the motor.

I had access to a tool for bending strip metal, it has dies for various different angles and different radius curves.

I then measured the that the target sticks out fro the motor, added a bit on for the sensor and bent the strip steel into shape.

I then drilled a hole to cater for the sensor.

Stupidly, I forget to take a picture of it all on the motor!

I diddnt get chance to test what type it is. I'll have a look tomorrow.

The math behind this is straightforward: the **linear velocity** on the surface of the disc is pi*d then multiply by RPM, divide by 60 to get the surface speed in cm (or inches) per second. Divide the width of the sensing target in cm (or inches) by the surface speed and the distance component cancels out, leaving you with the pulse width in seconds. Multiply that number by 10^3 to convert to milliseconds, or by 10^6 to convert to microseconds. The resulting pulse width must be comfortably larger than 200us at the highest RPM to be read.

Maths time...

Thanks Jeffrey, I'm a bit rusty with my maths so bear with me...

When you say linear velocity do you men the diameter between the targets, multiplying by PI to get the circumference?

It seems as though you are calculating the linear velocity in the first few steps..

Linear velocity being the distance a single point of an object would move at a certain speed (rotations) over a set amount of time (a minute). I Always remeber this as thinking how far a wheel would have rolled if it travelled for 1 minute at a certain RPM...

Anyway...

- Distance between centre of targets is 58mm
- Targets themselves are 6mm bolts so 6mm
- Max speed is 7,200RPM

58mm * PI is 182.212mm

182.212mm * 7,200RPM = 1311926.4mm (~1.3KM travelled by the targets each minute)

1311926.4mm / 60 (seconds) is 21865.44mm (~21M travelled by the targets each second)

21865.44mm / 6mm (width of target) = 3644.24mm

I make that a 364 uS pulse width at 7,200 RPM, way above the 200

Does somebody mind checking the math? (its great fun... I promise!)

piotrsko,

That was the idea of keeping it simple, a bigger nut on the thread makes for a bigger target!

DIYguy,

It's not threaded, I used threadlocker rather than lockwashers, I dont see there being any real load on the bolts so it should be more than adequate.

Unfortunately the sensor is quite long so it wouldnt really help. the space at the end of the motor wont be used for anything anyway as a crossmember sits just in front.

I'll post pictures and results up when I get chance to test everything.

Thanks for all the help.

Cheers,

Mike