Making Sense of SensorsPosted 2/6/2004
By Rod Collard
Evolution of Speed Sensors
Since the early years of the automobile, a need to monitor vehicle speed has evolved. As vehicle speed increased and roads improved, the main objectives of a speedometer were to allow the driver to accurately view the vehicle speed, possibly to avoid a speeding ticket, and to be able to read the odometer to verify how many miles were on the vehicle. Most speedometers operated off the rear driveline but some, like certain 1924 Ford Model Ts, used a front wheel as its input. The speedometer was optional equipment in those days but became an industry standard (Figure 1).
This method was calculated by gear ratio, tire circumference and somewhat averaged how fast the vehicle was traveling. Later, when a vehicle had a different rear axle ratio installed or when different profile tires were used, the process of matching the plastic "speedo" gears was used to ensure accurate speed (Figure 2). This system of measuring speed did not have the capability of comparing individual wheel speed differences between two wheels like found between the inside and outside wheels during a turn. It was simply not necessary to know this information. This technology is still with us today, but modern vehicles mostly rely on electronic sensors to perform that job.
Variety of Speed Sensors
The operation of most speed sensors is similar and might fall into one of three categories: Variable Reluctance, Hall-effect and Magneto-Resistive.
As a result of the use of modern speed sensors, today's vehicles utilize this technology not only to monitor vehicle speed, but also to monitor component position or rate of speed change on virtually any moving part of the automobile (Figure 3). They can be mounted on the vehicle in a variety of locations to perform different tasks.
Automotive manufacturers check the speed of a variety of components to support different automotive systems. Some examples include cruise control, antilock brake systems, suspension stability systems, engine crank and cam sensors, traction control and even tire inflation monitor systems.
General Motors Corp. uses a system that will compare the speed of wheels on the same axle and determine if a tire has insufficient air pressure. This is not the same system used earlier to actually measure tire pressure using a monitor ring mounted in the tire. This system would operate by observing a faster wheel speed, which would indicate low tire pressure. This system would also look at other inputs, such as a steering wheel angle sensor, before determining a low tire pressure situation.
Speed sensors are also providing input to more systems on the vehicle. An example of this is the original rear wheel antilock (RWAL) brake system on GM pickup trucks. This speed sensor was mounted on the tailshaft of the transmission on two-wheel drive models. This 40-tooth tone wheel provided inputs for different systems including RWAL, torque converter clutch solenoid, cruise control module, and input for the speedometer. The two wires from the speed sensor are connected directly to the digital ratio adaptor mounted in the back of the instrument cluster, which converts the analog signal into usable digital signals. GM used this to accommodate different tire sizes and axle ratios.
On GM's RWAL systems, the vehicle speed sensor (VSS) input was used by several different systems on the vehicle. The signal to the electronic brake control module (EBCM) was converted to provide 128,000 pulses per mile and 4,000 pulses per mile to operate the cruise control.
You might remember the process to recalibrate the speedometer on these trucks. In short, it involved removing the dash cluster and installing a special clip, to program the system to match tire size. Later vehicles used a separate component called a digital ratio adaptor controller (DRAC), which was sometimes located behind the glove compartment. This box was specific for the tire size and axle ratio of the automobile. Now, the calibration of the speedometer can be changed with a scan tool without changing any hard parts (Figures 4 and 5).
Variable Reluctance Sensor Operation
The variable reluctance wheel speed sensor is basically a permanent magnet with wire wrapped around it. It is usually a simple circuit of only two wires where in most cases polarity is not important. The physics behind the operation include magnetic induction.
A toothed ring on the wheel passes by the speed sensor and disrupts this magnetic field. The disruption in the field causes the wheel speed sensor to produce a sinusoidal (AC) voltage signal. The frequency and amplitude of the AC voltage signal are proportional to the speed of the wheel. The amplitude of the wheel speed signal is also directly related to the distance between the wheel speed sensor coil and the toothed ring. The distance is referred to as the air gap. This gap is critical to ensure AC output at lower speeds.
It is important to remember that air gap can change on a vehicle with loose wheel bearings or worn parts. Improper air gap can cause what is called sensor dropout. This can occur when the sensor will not produce output at lower speeds. This situation was found on some vehicles with an electronic speedometer. The customer would complain that while slowing to a stop, the speedometer would "drop off" or return to zero, maybe at around 8 to 10 mph. Too large of an air gap was the problem.
The following are some examples of systems that use speed sensors as important components in their operation.
Antilock Brake System
When wheel slip is detected during a brake application, the ABS enters antilock mode. During antilock braking, hydraulic pressure in the individual wheel circuits is controlled to prevent any wheel from slipping.
During antilock braking, a series of rapid pulsations is felt in the brake pedal. These pulsations are caused by the rapid changes in position of the individual solenoid valves as the electric brake control module (EBCM) responds to wheel speed sensor inputs and attempts to prevent wheel slip. These pedal pulsations are present only during antilock braking and stop when normal braking is resumed or when the vehicle comes to a stop.
Many ABS systems have an initialization sequence or self-check function, which uses the speed sensor input. The EBCM performs one initialization test each ignition cycle. The initialization of the EBCM occurs when the following conditions are met:
The initialization sequence briefly cycles each solenoid and the pump motor to verify proper operation of the components. This is why on some vehicles, you might notice the pump motor noise after the engine starts and the vehicle has reached 4 or 5 mph.
Traction Control System (TCS)
When drive wheel slip is noted while the brake is not applied, the EBCM will enter traction control mode. This system uses the speed sensors to provide input to the computer during acceleration. Tire slip should be kept to a minimum to provide the best traction. Positive slip occurs while accelerating and negative slip occurs during braking.
There are many versions of traction control systems. These systems have various means of providing this control. As an example, some systems can individually cut out cylinders, retard ignition timing, pull back on the throttle, apply the brakes, or utilize any combination of these together. All of these variations continue to receive signals from speed sensors.
Automatic transmissions will sometimes have one or more speed sensors on them. Some will monitor both input (shaft) speed and output (shaft) speed with sensors. Sensors will be used to provide input for shift points and torque converter clutch lock-up. Scan tools can be used to view this type of information (Figure 6).
Variable effort steering (VES) systems will use an input from a speed sensor. This system will only provide maximum power steering pressure when needed. Higher speeds typically would not require as much pressure when steering at low speed, like when parallel parking. This system might use a sensor found in the steering column to inform the computer about position or angle of the steering wheel and how fast the driver is turning.
On some Cadillacs, the steering wheel position sensor produces two outputs, one analog and one digital. By interpreting the two inputs, the computer can also determine the direction of steering wheel rotation. These sensors might not always be called speed sensors.
The 2001 Toyota Avalon has a steering angle sensor that operates with its vehicle skid control (VSC), which must be calibrated after readjusting tie rods. With all the advances in the modern automobile, you can be sure that some form of these sensors are going to continue to provide inputs from a variety of sources, which will make future automobiles more comfortable and safer than ever before.
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