OEM Speed Sensors - Magnetoresistve

[JayDee]

Hi all,
Have you come across 'active wheel speed sensors'. They are becoming quite common on many road cars over in europe and it would be nice to be able to use these sensors, since fitment of alternative sensors in the hubs is problematic.

Sensor Overview
Many are based on a magneto-resistive element. (some are hall apparently) The trigger wheel is a disk of material with alternately magnetically polarised segments(N/S/N/S/N... ), this disk if often a rubber/plastic material containing magnetisable particles.

The sensors is a two wire sensor and is supplied with 5-15v and Sensor 0V. (note the lack of a separate signal wire) The sensor detects the pulsing fields of the trigger disk and outputs a signal voltage, the amplitude is independent of the wheel speed. The internal amplifer/conditioner circuit inside the sensor housing converts this signal to a current pulse with a varying frequency proportional to the wheel speed, this signal is then impressed onto the supply lines ( the sensor 0v side ??).

The receiving ECU/Logger must have to monitor these supply lines, detect the current pulse and convert the frequency into a speed signal...

We are seeing this type of sensor being mounted in many different wheel hubs, I have personally seen it in Mercedes and GM cars. I guess loads of people have seen these before..... is it somthing that will be useable by current/future motec products... ADLx

[Hugh]

Jay,

I have not seen these sensors, but am just about to wire up my car and have a problem with my front driveshafts.

I had to remove the ABS sensor rings, due to a hub clearance issue, so do not currently have a wheel RPM to input to the ADL.

Are there any flexabilities in the way the rings are mounted that would assist a one off application?

Thanks

Hugh

[JayDee]

Hi Hugh,
If im getting you correctly, you've removed the trigger disk from the drive shaft but now need a trigger wheel to work with the OEM ABS sensor? If you are lucky (luckier than me ) you will have conventional mag (inductive) two wire sensors. They output a simple AC waveforem, the both the wave's amplitude and frequency will vary with speed.
The down side ( in my experiance ) is that these sensors are very sensitive to gap changes and need a 'mechanically accurate and crisp' trigger wheel and also a solid sensor mounting to produce a reliable output. You might be able to move the whole assembly, trigger disk and sensor to an alternative position or attach an alterative custom machined trigger disk. The old trick of using bolt heads on the back of the brake disk as a trigger wheel can work but the results are variable with Mag type sensors. They like to see a crisp square edge and a bolt head does not always provide this, Cap head bolts are a classic pain!
Mounting wheel speed sensors to a custom application are very difficult to advise remotley, since many variables effect how good the result will be...John.

[Apex Speed Tech]

I'm pretty sure that these are the type of wheelspeed sensors found on Porsche 997's. They work brilliantly; in these applications the ABS computers we install have standard voltage square wave outputs so that's where we get our speed signal. to be able to use these sensors directly would be excellent.

[Hugh]

John,

Thanks for the response, I guess I was hoping for a magic solution to pop forward from your reply.

I will first have to sort out a trigger machnism and then get some way of mounting the sensors.

Given that the ADL will allow you to custom calibrate each sensor, things should be fairly easy once I have things done.

Do you have any photographs of the trigger discs you have seen?

Neel,

I agree it would be great if you could wire these sensors directly.

Thanks

Hugh

[Apex Speed Tech]

I agree it would be great if you could wire these sensors directly.

The Teves MKIV ABS was also used in a lot of E46 BMWs. The valving and control strategy is different that what we use in the race cars, but the ECUs are the same. You could get one and use it soley to translate these wheelspeed signals - hook up power, ground, the speed sensor inputs and the speed sensor outputs. You get can secondhand units for around $1000. It may be cheaper and easier than all the fabrication work to add other sensors. If you take the valve body off, its fairly compact. I can put together a kit with a flylead harness for anyone who's interested in trying this.

[Dragon]

I think it is possible to convert the signal from a magnetic speed sensor to Hall signal using the DMCB, Motec part number 53114.

The SDL can take signals from 2 magnetic wheel speed sensors directly.

[Apex Speed Tech]

I think it is possible to convert the signal from a magnetic speed sensor to Hall signal using the DMCB, Motec part number 53114.

Ah, but the sensors we're talking about are not MAG in the sense that the DMCB would work. If you scope them and look for a voltage trace, you see nothing, just flat lines. These are constant voltage variable current sensors.

[figgie]

Neel

do you have any information what the current is when the sensors "senses" v. when it is just sitting there?

I assume that there is minimal current but current none the less when there is no signal.

Now from a purely theoretical standpoint.

current should be able to be converted to voltage and vise versa as they are interchangeable with a bit of electronics. Now I am surprised the MoTeC reads voltage fine but not current? The equation should be interchangeagle

ie

Power = Amperage x Voltage ie 5watts = 1amp @ 5vdc or 2amp @ 2.5vdc or 2.5amps @ 2 vdc or 5 amps @ 1vdc.

[SprinterTRD]

The solution is simple,

From the original ECU work out what internal pullup resistor is used between the sensor's signal pin and 5V

Wire the sensor signal into the ADL2 speed input and the sensors zero volt pin to ADL2 0V

Wire the same value resistor you found in the first step between the ADL2 speed pin and 5V

Configure the ADL2 speed input as a mag sensor

Activate the sensor and measure the trigger levels between 0V and the sensor output.

Adjust the ADL2 Speed input trigger level to suit.