May 2009 Articles & Features
By Mike Dale | May 2009
There are many new transmission designs out there, and a broad range of sensors have been developed to help control them. Mike explains how some of the newest sensor types do their job.
When the topic of automotive sensors comes up, it most often focuses on those sensors needed for engine control. For sure there are antilock brake sensors, HVAC sensors and even bumper impact sensors, but what often gets left out of the conversation is that package of sensors used to control the automatic transmission.
Things are different for the transmission—down there, underneath the car, out of sight. The sensors used for position sensing, pressure sensing, temperature sensing and speed sensing have durability requirements that sometimes exceed those used for engine control. They may be located near a hot exhaust, in direct contact with hot automatic transmission fluid, inside the transmission or in the general path of road debris, water and salt.
The need for sensing in transmissions is a two-way street: The transmission needs information from the engine (throttle position, engine load) and from the antilock brake system. And it needs to send information to the cruise control and traction control systems.
For shift control, the transmission control unit/module (TCU/TCM) needs information regarding the transmission’s input and output speeds. Also needed is information about the turbine speed, the lockup torque converter and the position of the gear shift lever.
Improved shift feel is accomplished by the TCU software based on parameters such as ATF temperature, vehicle load and driving dynamics as reported by wheel speed sensors and traction control/stability control systems. Information from wheel speed sensors can be used to determine the slope the vehicle is traveling on. Because uphill and downhill shift quality depends partially on the inertia of the vehicle, the wheel speed sensors can actually give information that improves shift quality.
Before we get into how some of these sensors work, it’s worth mentioning something about what’s going on with sensor technology in general. The goal for some years now has been to transition to noncontact types of sensors throughout the vehicle. Old-technology sensors such as the neutral safety switch and the TPS had wiper arms or switch contacts inside that were prone to wear, contamination and unreliability. Almost all new sensors are required to be noncontact types. This allows both halves of the sensor to be fully encapsulated, thus immune to contamination.
There are at least three different methods for input and output shaft speed detection. The most common, and least expensive, is the variable reluctance (VR) type of sensor. This is much the same technology as that used to measure cam and crank rotation speeds. This sensor consists of a metallic core with a magnet at one end. Wrapped around the core are layers of single-strand magnet wire insulated with polyurethane. When there’s no movement of metal near the magnet that affects its magnetic field, there’s no voltage generated in the coil. However, when the tooth of a gear passes near the magnetic field, the shape of the field changes and causes movement in the magnetic flux. This moving magnetic field generates a voltage in the wire that rises and falls as the gear tooth passes.
VR sensors can be easily checked with an ohmmeter. Unless there’s a break in the magnet wire or the connections to it, the sensor is almost surely without fault since there are no moving parts or electronic components that can fail. VR sensors are self-powered and need no battery power.
The downside to VR sensors is that their output is directly related to the speed of the passing gear tooth. They’re not the zero-rpm sensors that would be useful for transmission applications. They’re also air-gap-sensitive in that to work correctly and to have a good output, the gear tooth must pass by closely enough to affect the magnetic field.
VR sensors also need waveform shaping for their output to be digitally readable. The normal output of a VR sensor is an analog signal, shaped much like a sine wave. This waveform needs to be squared up, and flattened off by an electronic chip called a comparator to be digitally readable.
A more accurate but more expensive sensor uses Hall effect technology. A Hall effect sensor requires battery power to operate. When a magnetic field passes through the silicon of the Hall cell, there’s a switching effect that forms the information passed on to the TCU. The output is normally a square wave, so no special wave shaping is needed, as with VR sensors. Hall effect sensors are true zero-rpm sensors and actively supply information even when there’s no transmission motion at all.
Hall effect sensors have gotten more complicated in recent years due to adaptive threshold sensing. This offers improved timing accuracy, optimized air gap and tilt. Some feature an open collector output with provision for either internal or external pull-up. Others have a high-state output that goes low when the field is detected. These options give greater flexibility to how the TCU is set up. Many current Hall effect designs have on-board protection against EMI, stray magnetic fields and system voltage transients.
Another type of speed sensor is based on the principle of Giant Magnetostriction, or GMR. Here, a special material actually changes shape or length in the presence of a magnetic field. When the field is conducted to the GMR element by the steel or iron of a gear tooth, the length changes. This is detected and the switching occurs.
A variation of GMR is MagnetoResistive (MR) technology. These devices are sensitive to the direction of a magnetic field. When a field is detected, the resistance across the MR device changes. The strength of the magnetic field is not so critical with this type of sensor. Magnets can age or be reduced in output as a function of temperature. This affects GMR sensors but not MR sensors. Another consequence is that MR sensors are also less air-gap-sensitive than GMR sensors. MR sensors are often used when the position information needed is an angular measurement.
Position sensing in a vehicle transmission is important in a number of applications, among the simplest of which is the kickdown switch. It tells the TCU that the position of the gas pedal or the electronic throttle control is requesting a downshift for added acceleration. Yes, technically, this switch is a sensor, albeit a very simple one.
A more complicated position sensor is the noncontact version of a transmission range selector switch. Delphi’s version of this uses Hall effect technology. It can convert rotary or linear motion into a ratiometric analog output signal or a pulse-width-modulated signal that indicates the various positions of the gearshift lever (PRNDL). This sensor does require power input leads (plus and minus) but has a single output line. Having only three leads in the connectors reduces the number of connections and the number of places where trouble could start.
The ZF Group in Germany has a new linear position sensor on the Mercedes-Benz E-Class that uses inductive sensor technology (see the illustration on page 16). This is a magnet-free sensor that gives information at any time with or without movement. Square patterns of wire are printed on a p-c board. As the wiper arm moves across, it changes the inductance of these coils, indicating the position of the range selector.
Temperature sensing in a transmission is important, as the flow rate and lubricity of the ATF depend on it. Delphi’s version is typical in that it’s thermistor-based (nominally 10K ohms at 77°F) and operates from a 5-volt power supply. These sensors don’t change real quickly due to their mass. It takes about 15 seconds for a fluid temperature change to register. This acts as a noise filter by keeping minor temperature blips out of the way, while still allowing the sensors to react to important changes.
So, just as there are many different transmission designs out there, so, too, are there a broad range of sensing technologies used to help control them. Being hidden out of sight down below doesn’t change the important work they do.