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Reading a vehicle's speed sensor? — Parallax Forums

Reading a vehicle's speed sensor?

eagletalontimeagletalontim Posts: 1,399
edited 2009-02-05 03:01 in General Discussion
I am working on a small project that needs to be able to read a vehicle's speed sensor. I am not sure how to go about wiring it up or what command to use to read it. From what I have read, the speed sensor is some kind of pulse generator that just has 2 wires coming from it and when a slotted disk passes over it, it sends some form of a signal. If anyone has any ideas on how to read it or how to wire it up, I would greatly appreciate the help!

Comments

  • BeanBean Posts: 8,129
    edited 2009-02-01 22:04
    Sounds like COUNT would be the way to go for that.

    Bean.

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    ·The next time you need a hero don't look up in the sky...Look in the mirror.


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  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-01 22:10
    Ok, that is what I figured. I have a meter that has a low tach setting and I hooked it up to the 2 wires that go to the pulse generator and it did move the needle based on how fast I was going. I tried checking for voltage or ohm changes while driving and there was no voltage or any change in resistance. Would I need to send voltage to the generator? If so, what would be the best way to hook it up to be able to read using the COUNT statement?
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-01 22:58
    I am guessing the sensor in the transmission is like a hall-effect sensor, but I have no idea how to wire it to the chip to get a steady reading. I hooked one end to ground and ran the count statement while driving and the numbers were all over the place [noparse]:([/noparse] If anyone has any quick ideas on how to get this to work please let me know. Daylight is running out and I work all next week [noparse]:([/noparse]
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-02 01:36
    ok, here is a schematic of the sensor hooked up to the vehicle. Could someone please help me figure out how to read this thing?
    pulse-generators.jpg
    133 x 431 - 13K
  • lincolnlincoln Posts: 23
    edited 2009-02-02 11:40
    It would be helpful to know what kind of car your working on. I know on 92 jeep wrangler it is a hall sensor that has 3 wires; +12v gnd and "tone". at the output of the transfer case there is a 3 segment tone ring so that hz =3 x rpm. depending on the transfer case it could also be a mechanical drive but same 3 wire connection. it just depends on the car.

    link
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-02 12:13
    It is a 93 eagle talon. The sensors only have 2 wires coming from them so I am not sure if it could be a hall sensor or not [noparse]:([/noparse]
  • lincolnlincoln Posts: 23
    edited 2009-02-02 18:15
    ok,
    I don't have a talon manuel but i will bet that is a reluctor type setup, basically it is an inductive pickup, sort of like a guitar. with reloctor type setups it is common to have a 40 tooth tone wheel, ie: hz/40=rpm. a good write up on the signal can be found here: http://www.megamanual.com/ms2/pickups.htm

    your going to need a manuel or an o-scope (or both) depending on how good you library is where you are they may have factory manuals although a chilton's many have the info. basically your looking for a wiring diagram.

    any help?
    link
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-03 01:04
    Well, I don't have an o-scope [noparse]:([/noparse] Can't afford one. Is there any way to read the frequency from the sensor using the SX? I have tried the COUNT statement, but the results are EXTREMELY jumpy so i have no idea how to actually read this sensor effectively. I tried putting 3 volts through the sensor and read the voltage on the other side, but the meter stayed steady at 3 volts from 0 to 60 MPH. Is there some kind of special circuitry that I would have to get a reading?
  • lincolnlincoln Posts: 23
    edited 2009-02-03 01:23
    what is the open circuit resistance of the sensor? should be no need to send volts, its passive. its also AC. check out the link! even thought the link is about distributors it is the same sensor setup. can you connect a meter in parallel with the sensor hood up to ecu/dash? also you will need to make some kind of isolation in the final circuit, think of trying to count how many times a second a relay/solenoid fires by measuring the back emf. depending on where you live you should be able to pick up a used oscope at a flee market or e-bone.

    Post Edited (lincoln) : 2/3/2009 1:33:00 AM GMT
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-03 01:26
    The resistance is a around 4.7K. It stays that way even while driving. I cannot connect the sensor in parallel with the computer because the computer was burnt out when I got the car so I built a new one to bypass it.
  • lincolnlincoln Posts: 23
    edited 2009-02-03 01:31
    any ac volts?
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-03 01:54
    not sure. my meter is for DC only. I was able to get PULSIN to get an accurate reading but for some reason, at 55 mph, it showed 40 and at 60 mph, it showed 35. Why would the numbers be going down while the speed is increasing?
  • lincolnlincoln Posts: 23
    edited 2009-02-03 03:22
    pulsin measures the width of the pulse, as the frequency increases the period decreases; f=1/p, careful hooking a mcu up to the sensor directly, it can generate 25vac or more. skull.gif

    Post Edited (lincoln) : 2/3/2009 3:27:58 AM GMT
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-03 03:24
    oh..ok. so now i just need some kind of math function that will convert the frequency to MPH.....this is going to suck tongue.gif
  • lincolnlincoln Posts: 23
    edited 2009-02-03 03:37
    1/pulsin =hz

    hz/(#teeth on tone wheel) = rpm(driveline)

    rpm(driveline) * diff ratio * circumference of tires / 12" /5280' * 60minuts= mph

    or have a pace car /gps and make table of raw pulsin data and you can figure a simplified formula

    Post Edited (lincoln) : 2/3/2009 3:44:22 AM GMT
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-05 01:23
    ok, i finally figure out how to convert the frequency to MPH accurately using a calculator I built online.... now. since there are sooooo many decimals, how would I be able to do the math functions in the SX since it cannot calculate actual decimals? I completely suck at HEX and have no idea how to use the scientific calculator on Windows.
  • BeanBean Posts: 8,129
    edited 2009-02-05 02:58
    If 55MPH gives a PULSIN value of 40, then try just taking 2200 and dividing by the PULSIN value and see how close that is.

    2200 / 40 = 55MPH
    2200 / 35 = 62MPH

    · tempW = 2200 / value ' tempW is a WORD var and value is a BYTE from PULSIN

    · If this works, then you can also use 22000 / value to get MPH * 10. This will give you one place past the decimal point.

    22000 / 40 = 550 ' really 55.0 MPH
    22000 / 35 = 628 ' really 62.8 MPH

    Bean.


    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    ·The next time you need a hero don't look up in the sky...Look in the mirror.




    Post Edited (Bean (Hitt Consulting)) : 2/5/2009 3:03:43 AM GMT
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-02-05 03:01
    wow!! that was simple tongue.gif Thank you sooooo much!
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