Variable reluctance sensor analog signal splitting?

[popeye2003]

I'm going to put a electronically controlled engine and transmission in an older (hotrod) car. I want to take the vehicle speed sensor (VSS)->(variable reluctance sensor) and feed the analog sensor signal to the engine/ transmission computer as well maybe a CS1124 chip to PWM a DC motor to spin the original mechanical speedometer. Being a novice at electronics I don't know if I can just pigtail the sensor output (varies in frenquency and amplitude) to the computer and CS1124 or if I need to condition the signal somehow to make up for the splitting of the signal? Another possible option is to use a DRAC (digital ratio adapter controller) this is a OEM GM part used in engine/ transmissions of the era I intend to use. The DRAC puts out three 5 volt digital pulses which vary in frenquency only at a 50% duty cycle in responce to the VSS. The advantage here is the Drac can being reprogrammed for different wheel/ tire size and gear ratio but I don't know how to take the digital pulses that vary in frenquency and use that as a reference voltage to come out with a modulate pulse to control a DC motor. This should be a fairly easy task for someone with experience in the field of electronics but that's not me. Can some one please help me?

Thanks, Rod

 

[Ron07663]

You may be able to drive both devices from same sender. You will need to determine the loading of the signal and whether the inputs are single ended or differential into the computer when you design the speedometer circuit.
My main concern would be the accuracy of the DC motor drive, which I guess you are trying to simulate the old cable drive to the speedo. Cables typically turn 1000 rev per mile. I think it will be dificult to have an accurate speed and mileage odometer with this setup over the entire speed range, especially at low speeds.
I suggest it would be easier to go with a complete electronic speedo.


Ron

 

[Warpspeed]

Agree with Ron on the voltage/speed accuracy of a dc motor. If you want an original looking pointer type speedometer, how about adapting a tachometer movement and fitting a miles per hour scale to it ?

The odometer is a bit more tricky, but you could buy a small pulse counter that has the same appearance as a multi wheel odometer. These increment one digit every time you energise a solenoid.

Another way might be to use a small stepper motor to drive an original mechanical speedometer and odometer. Some fairly simple software would be able to convert any incoming pulse rate to any other outgoing pulse rate smoothly without jumps. It could be done by measuring the period between incoming pulses with very fine resolution, and generating output pulses at a multiple or sub multiple period, also with very fine resolution.

Really odd non integral ratio relationships with very high accuracy could be created that way.

 

[ChrisFBoyden]

Popeye2003

Check out National Semiconductors Ignition Interface Chips.
They sell a chip that will interface directly to a
VR coil. They handle to the extremely wide dynamic range 10mV to over 20V output of a VR sensor. From there you can get a conditioned signal that will be easy for other electronics to interface to. From there, you'll get a TTL level 5V 100us reference pulse for every pulse on the VR sensor. Then go to a Frequency to Voltage conversion. then go from voltage to pwm. you might be able to find a frequency to pwm conversion chip.

try the LM1815 for the VR interface.

Chris

 

[ChrisFBoyden]

Oh yea, from there you can split the VR signal through a common 74SN244 digital buffer if you need to drive a bunch of signals.

 

[Ron07663]

Check out this site. They are selling an adapter to convert the electronic speed signal to drive a stock mechanical speedometer.

http://www.atrol.com/cablex.htm

Ron