LM2907

[dave T]

Help
We manufacture small electric vehicles and have been asked if it would be possible to speed limit these to 4MPH. Unfortunately our vehilces are deigned to go at 12MPH. I thought of using a sensor mounted on the motor, maybe a magnetic pick up and run it through a LM2907 to generate a voltage of between 0-12 volts. This would then be used to vary to input into our speed controllers. We would need an output of approx 500ma from this. anyone had any experience with doing this. If so any chance of a diagram.
many thanks

 

[Roff]

What is the rotational speed of your motor at 4MPH?

 

[dave T]

approximately 1000 RPM at 4 Mph

 

[Roff]

Could you use a frequency comparator and a bang-bang limiter? You can use the phase/frequency detector and the VCO in a CD4046 for this purpose. The output of the comparator would simply shut the motor off if the speed was above the preset limit, and allow power to be reapplied (with whatever proportional control you have) when speed fell below the limit. Do you think this would be too jerky? If you let the motor coast, as opposed to shorting it out, it might be OK.
I think this would be simpler than an LM2907. The LM2907 approach has lag due to the filtering required for the f-v conversion process. I have to confess that I haven't used the LM2907, but I do know that in order to have low ripple at low speeds (say 1 mph, ~ 4 Hz), the filter time constant is going to be large, probably greater than 1 second, which will, I think, also be the lag. This may be OK, but you know more about that than I do.

 

[ivancho]

how about a hall effect sensor and a magnet, what will give you pulses according to the speed.... once the top speed is reached then don't increase the PWM cycle. I suppose it uses some kind of PWM in the car right?

Ivancho

 

[Russlk]

I don't know why I can't add a reply to this topic with a schematic, but if you differentiate the motor speed signal (assuming that it is a square wave), rectify it and low pass filter, then feed that to a comparator, a frequency limit circuit will result.