Extra revision on ABS TTEC 4815
Explain the difference between stability and steerability.
Stability is related to the rear wheels, their sideways traction and prevents the vehicle spinning out
Steerability is related to the sideways traction in the front wheels meaning you can control the path of the vehicle.
Graph showing tire traction slip angles, and how tire slip percentage relates to tire traction. Two lines are shown for dry pavement, both braking traction and sideways traction.The goal of ABS braking systems is to increase safety prevent accidents and save lives it does this by allowing the driver to apply full brakes in an emergency while still being able to steer around a obstacle.
A misconception of ABS brakes is that Locked-up wheels don’t stop a vehicle?
False: locked up wheels still have @80 % of stopping traction on dry pavement, as the Bosch graph shows. Locked up wheels are more efficient on snow or gravel for stopping power, but vehicle control is lost.
This gives an AC voltage and can be seen on a oscilloscope .
The different ways an ABS sensor can be damaged or its input to the ECU/ECM can be not what it should be: Improper installation, wear, metal filings, debris, or damage could affect it. A problem with the wheel speed sensor system could be the wires twisting/cable flexing at the ends with wheel movements causing bad connections and high resistance. Also the air gap(between sensor and rotor) can be to large due to a problem with the mounting this will result in smaller frequency, noise, low amplitude and hence low voltage output, the rotor could be bent from curbing, or missing teeth.
Digital hall effect, magneto-resistive
• Both Hall Effect and Magneto-resistive elements generate a voltage that varies by relation to the changing magnetic poles. The voltage is shaped by the internal electronics to become a square wave pattern.
• The frequency of the signal increases with wheel speed, but the voltage doesn’t increase like inductive sensors. So active sensors are more accurate and reliable at slow speeds. ABS operation can occur at slower speeds.
• The pattern of north – south polarities, allows them to even calculate when a vehicle is going backwards.
If you had ABS on your car, and you had an “on – off” switch for your ABS, you would like to turn the switch “off” for driving on gravel and snow because the wheels need to lock up to stop the vehicle quickly
The basic inputs needed for an ABS system are;
Wheel speed sensors, alternator charging input power the pump motor, brake light switch, + battery, - earth, negative, on/ off switch
The Hall Effect or Magneto-Resistive wheel speed sensors different from the inductive type sensor
The waveform you would see on a or magneto-resistive wheel speed sensor is a
The Hall Effect is active (has in input), and gives out a digital waveform (Square wave) and the voltage doesn’t increase like inductive sensors so active Hall Effect sensors are more accurate and reliable at slow speeds. This makes it possible for ABS operation to occur at slower speeds.
The pattern of north – south polarities allows them to even calculate when a vehicle is going backwards. This is better for rollback detection and electronic parking brake options).
The difference about the wiring to the Hall Effect or magneto-resistive wheel speed sensor has 3 wires (+,-, output) the inductive has only two wires as these are passive and require no input.
The Coil resistance of a wheel speed sensor is normally about 800 ohms to 1200 ohms, during the on car practical a reading of 1350 this was acceptable. If faulty OL or If there’s low Resistance = 0 ohms there’s a short to earth
Spin reluctor test: ACV is measured at the wheel sensor and the faster you spin the higher ACV.
But this could have noise so we use an oscilloscope this will show that the faster the rotor spins the higher amplitude and shorter intervals so frequency increases. At low speed a lower frequency is measured 50Hz and at higher rpm a frequency of 650Hz was measured.
References for this entire unit;
Automotive Mechanics, Ed May & Les Simpson, 8th edition
Driving tips and tricks:
Wiki http://en.wikipedia.org
Unitec Blackboard:
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