Table of Contents

An Advanced
Steering Wheel Sensor

A steering wheel sensor combining thick film conductive plastic and a
novel optoelectronics technology outputs both analog and digital data
for use by the General Motors StabiliTRAK system.

Asad M. Madni and Roger F. Wells,
BEI Technologies, Inc.

Passenger cars, light trucks, and sport utility vehicles produced for today's marketplace are far exceeding the original designs and concepts that prevailed throughout the first half of the 20th century. The movement away from basic transportation and toward safety and comfort
Figure 1. The front housing assembly of the steering wheel position sensor contains the LED, collimating lens, code disc, hub, and molded-in 6-pin Packard Electric connector that provides the electrical interface between the sensor and the user's controller. The code disc is insert molded into the hub. The front housing is sandwiched between the capturing spring, shaft locating ring, and the hub. The spring and ring provide a mechanical link between the steering shaft and the sensor.

The detector housing assembly contains the reverse polarity protection diode, wave forming capacitors, EMI filtering capacitors, the current limiting resistor for the LED, and the opto ASIC. The discrete components are reflow soldered. The ASIC is attached by using conventional die attach and wire bond followed by encapsulation of the exposed wires and chip. Two screws fasten the front and rear housings. Electrical connection is accomplished by soldering the five exposed terminals to the rear of the front housing's pins.

The slider assembly contains the wiper to the potentiometer. The alignment pin, inserted through the rear housing via a 0.047 in. hole stopping at the front housing, locks the slider and prevents the internal parts from rotating during sensor installation and alignment to the steering column. The pin is removed after alignment, freeing up the internal parts. No adjustments are necessary after the sensor is installed in the steering column.

The rear housing contains the potentiometer and a rubber grommet that seals the sensor against external debris. The potentiometer is attached to the housing by double-sided adhesive tape. The partially embedded circuitry connects the three terminal points from the potentiometer to the opening of a rectangular window where the output, power, and ground are resistance welded to the front housing assembly. A cover snaps into place to close the exposed terminals. A bead of epoxy is deposited onto the open trench around the perimeter of the sensor to connect and seal the two housings together.

The rotational movement is picked up by the hub, which then meshes with the larger side of the pinion gear. Further reduction is made between the smaller side of the pinion and the slider gear. The encoder thus senses a direct movement and the potentiometer senses a reduced movement of 5:1.

considerations began in the 1950s with the introduction of mechanical devices such as seat belts and collapsible steering wheels. Electronic components, introduced in the 1970s, were dedicated primarily to improving engine performance but led nevertheless to new sensor and microprocessor technologies. The first widely used safety feature based on these advances was adaptive braking systems, which allowed vehicles to accelerate more quickly and stop without skidding. To prevent the incautious or unskilled driver from losing control of the vehicle in steering maneuvers close to or beyond its limits of stability or adhesion to the road surface, active systems are being developed that sense speed, yaw rates, and other crucial parameters, and stabilize the vehicle by actuating brakes on individual wheels and dynamically adjusting the suspension system characteristics.

Vehicle Dynamic Control Systems

In response to an ever-increasing demand for more sophisticated vehicle systems, automotive manufacturers now require their component suppliers to shrink the sensors while maintaining and improving their functions. In addition, there is a strong trend toward combining multiple sensors into a single unit.

A road texture detection package, for example, consists of sensors at all four wheels that feed data to the onboard computer, which calculates the texture of the roadway in stages, from smooth to rough. If a rough or slick surface is detected, the computer regulates the performance of the anti-lock brake and traction control systems to respond precisely to each condition in only 0.006 s. The benefits are shorter stopping distances and prevention of skids or loss of directional control on curves, slippery surfaces (e.g., gravel, ice, or snow), or during severe steering maneuvers.

This article describes the design features and performance of a steering position sensor (see Figure 1) developed by Duncan Electronics. The steering sensor, together with a micromachined quartz yaw rate sensor developed by Systron Donner Inertial Division, is at the core of the General Motors StabiliTRAK, an integrated chassis control system (see Photo 1) offered on several models of Cadillac and on the Chevrolet Corvette.

These advanced sensor packages provide the StabiliTRAK with vital information that helps detect and correct deviations between the driver's intended course and the actual course of the vehicle. Such deviations are determined through the use of steering column sensors that measure the intended course, and yaw rate sensors that measure the actual course by detecting the degree of vehicle spin or rotation. The stability control computer receives the inputs of these and other sensors and depresses or releases the appropriate front brake to help keep the car on its desired course.

Steering Sensor Design
Photo 1. Steering and yaw sensors provide crucial information to the StabiliTRAK system.

Developing the steering sensor required combining both analog and digital technologies in a single self-contained thermoplastic module. Analog, or voltage-based, output of the steering wheel position is produced by a thick film, conductive plastic potentiometer and the gear train. Digital output of the steering wheel position is achieved by a single optical ASIC (opto ASIC) incremental encoder that measures steering angle in 1° increments.

The Potentiometer

The analog portion is a potentiometer made of a conductive plastic (CP) screened onto an epoxy and glass laminate. The wiper is connected to the input rotor through a 5:1 gear reduction. The nominal signal output is a linear function corresponding to 0.05 Vin at –225° to 0.95 Vin at +225° steering position.

The Encoder
Photo 2. The lead frame design and a monolithic ASIC create an "encoder on a chip" module.

The optical encoder (see sidebar) consists of an emitter and a detector. At the emitter, IR light is generated by a GaAlAs LED (see Photo 2) and collimated by an aspheric polycarbonate lens. At the detector is an opto ASIC that is die attached and wire bonded to a detector housing. The light beams, from the lens surface to the photodiodes on the ASIC, are interrupted by the 90-window pattern spaced in 4° increments on the code disc. The interruptions correspond to the shaft's rotations and are detected by the photodiodes, which produce A and B sine waves. The sine waves are fed first to amplifiers and then into onchip comparators. The resulting square waves have onchip resistors providing output signals with symmetry and quadrature. The encoder thus has two-channel quadrature outputs plus a third-channel index output. The index is five cycles wide with reference to the steering shaft mechanical position of 0° (see Figure 2).

Figure 2. The steering sensor provides an analog output for absolute position data as well as digital quadrature and index outputs for incremental position and direction of steer information.

Opto ASIC Technology. All of the encoder's electronics are incorporated into the opto ASIC, replacing discrete data detectors, index detectors, symmetry adjusting pots, feedback resistors, and other components (see Photo 3). The aspheric lens permits operation with a 0.020 in. air gap between the code disc and the ASIC. Configuring the phototransistors in an array eliminates the need for a reticle mask. There are additional advantages:

  • MTBF is >1 million hr, as opposed to the 60,000 hr offered by conventional designs. The reasons are shortened circuit paths, reduced number of solder functions, and fewer wire bonds.
  • GaAlAs LEDs draw 60 mA for the opto ASIC encoder, in contrast to the 120 mA draw of a conventional encoder. They are derated to a 30 mA current draw, which increases their reliability.
  • The fold-back current limit designed into the circuit prevents external shorts from damaging the encoder and provides short-circuit protection.
  • The ASICs are capable of self-diagnostics such as assembly or operational failures.
  • The reduced parts count and size translate into lower user cost, improved reliability, and significant space savings.
    Photo 3. A monolithic phased-array opto ASIC chip contains all the signal processing circuitry for the encoder.

All internal moving gears are constructed from PTFE lubricants embedded in a thermoplastic matrix for greater lubricity. The electrical traces are achieved by the insert molded brass leadframes substrate. Electrical interconnects are handled by three techniques: wire bonding, soldering, and resistance welding. Selective plating of the leadframes requires three process steps:

1. Wire-bonded areas are plated with 50 min. of gold over 150 min. of nickel.

2. Soldered areas are tin plated for increased solderability.

3. Resistance areas are left unplated.

Resistor Technology

For position sensing, the conductive plastic (CP) resistors are used as potential dividers, i.e., a DC voltage is applied across each end of the element and a ratiometric voltage measurement is obtained with a sliding contact that "taps" the voltage at any intermediate point along the resistor. Halfway along the resistor, the measured voltage will be 1/2 of the applied voltage; one-quarter of the distance from the zero volt terminal it will be 1/4, and so on. With this technique, the ohmic value of the resistor is not important. Provided that the resistor is linear from one end to the other, the output voltage is simply proportional to the ratio of the contact position from one end. Temperature coefficient effects on the resistive elements are inherently self-compensated because any increase or decrease caused by temperature change will affect the entire sensor, and the ratio of resistances on either side of the contact will remain the same.

Understanding Incremental Encoders

The two basic types of encoder are absolute and incremental. An absolute encoder provides a "whole word" output, with a unique code pattern representing each position. The code is derived
Figure 3. An 8-bit absolute encoder disc provides digital outputs.
from independent tracks on the encoder disc corresponding to individual photodetectors. The output from these detectors is HI or LO, depending on the code disc pattern for that particular position (see Figure 3). Absolute encoders are capable of using many thousands of different codes, but the most common are grey, natural binary, and binary coded decimal. Grey code is particularly suited to optical encoders because it is unambiguous, i.e., only one track changes at a time. This allows any indecision during edge transition to be limited to plus or minus one count (see Figure 4). Natural binary code is converted from the grey code through digital logic. A latch option is used to lock this code to prevent ambiguities should the output change during reading.
Figure 4. This is a typical example of the grey code used for optical encoders.

The incremental encoder, which was used in the steering sensor, creates a series of square waves whose number can be made to correspond to the mechanical increment required. To divide a shaft revolution into 1000 parts, for example, an encoder could be selected to supply 1000 square wave cycles per revolution. By using a counter to count those cycles the shaft rotation can be computed: 100 counts = 36°, 150 counts = 54°, etc. The number of cycles per revolution is limited by physical line spacing and quality of light transmission (see Figure 5). In general, incremental encoders provide more resolution at a lower cost than do absolute encoders. They also have fewer interface problems because they have fewer output lines. An incremental encoder typically has 4 lines: 2 quadrature (A and B) signals and 2 power lines. A 13-bit absolute encoder would require 13 output wires plus 2 power lines. Using complementary signals for noise immunity would require a 28 conductor cable. Cost and simplicity considerations were the primary influences in choosing an incremental over an absolute encoder for the steering sensor design.
Figure 5. An incremental encoder disc has equally spaced patterns.

There are two types of incremental encoder: the tachometer encoder and the quadrature encoder. A tachometer encoder is sometimes called a single-track incremental encoder because it has only one output and cannot detect direction. The output is usually a square wave (see Figure 6). Velocity information can be obtained by looking at the time interval between pulses or at the number of pulses within a time period. When using the interval between pulses, the encoder should provide good edge-to-edge accuracy. Any inaccuracy will cause the servo system to constantly correct "errors" caused by disc pattern irregularities.
Figure 6. A tachometer encoder's code track outputs a square wave.

Most incremental systems use two output channels in quadrature for position sensing (see Figure 7), an arrangement that allows counting the transitions and viewing the state of the opposite channel during these transitions. With this information it can be determined if A leads B and thus derive direction, an important consideration because of the vibration inherent in almost any system. An error in count will occur should an encoder using a single channel (tachometer type) stop on a transitional edge. As vibration forces the unit back and forth across this edge, the counter will up-count with each transition even though the system is virtually stopped.
Figure 7. This is a typical output waveform produced by the code track of a quadrature encoder.
Figure 8. Edge detection and electronic circuitry provide direction sensing (pulsed output).
Figure 9. The electronic circuitry provides multiplication of encoder output (interpolation).

Once the quadrature signal is decoded, pulses of fixed duration can be generated at selected edge transitions within a cycle. These pulses can be fed via clockwise and counterclockwise output lines to an up-down counter or programmable controller input port. Many counter and programmable controller manufacturers include a quadrature detection circuit as part of their electronics to allow the use of a two-channel quadrature input without further conditioning. Quadrature detection provides the ability to derive 1, 2, or 4 3 the basic code disc resolution. A 2500-cycle, two-channel encoder can generate 10,000 pulses/turn, or, in the case of the steering sensor, 360 pulses/turn can be generated from the original 90 cycles/turn disc. With a high-quality disc and properly phased encoder, this 4 3 signal will be accurate to better than 1/2 count (see Figure 8).

The output accuracy characteristics of interpolating encoders are somewhat different from standard incremental encoders. Electronic multiplication, or interpolation, provides a higher degree of angular resolution while trading off some pattern regularity. While this may not be suitable for velocity servos, it is ideal for position readouts and position sensors. The most important specifications for interpolating incremental encoders are transition accuracy and frequency response. In position readout situations the system usually does not know where it is within a particular count, so tight transition accuracy specs tend to be wasted. What is desired in these applications is the ability to subdivide a "count" into smaller divisions to improve system resolution. Interpolation provides this ability without any sacrifice in mechanical integrity or internal electrical signal strength, thereby retaining safety factors associated with lower resolution specifications (see Figure 9).

As a final note, while the encoder resolution of 90 pulses/cycle was adequate for the StabiliTRAK application, interpolation can be easily added externally or internally, or be designed into the opto ASIC.

At first it might appear that a continuous rubbing contact will cause the element to rapidly wear out, but with careful attention paid to lubrication, contact design, and correct composition and curing, resistive sensors will remain fully functional after more than 1 3 109 dither cycles and many miles of contact travel.

Sensor life depends on location and duty cycle. For example, a sensor mounted on an automotive engine will see dither, or small localized contact movement at frequencies in the 50–100 Hz range. For a 10-year automotive service life, 4000 hr is an average operation time used for life estimates. Therefore, for an engine with an average frequency of 75 Hz, dither life would need to be 1 3 109. Similarly, typical steering column frequencies are in the region of
5 Hz where the dither life requirement for 10 years of service is 75 3 106.

 TABLE 1
 Performance Specifications of the Steering Wheel Position Sensor
 ELECTRICAL SPECIFICATIONS
Supply voltage
Operating current
Overvoltage protection
Absolute position signal
Active electrical angle
Total resistance
Linearity
Output smoothness
 
 
5.0 ±0.25 VDC
40.0 mA (max.)
16.0 V max.
±72°
10.000 ±1800
±2.5%
±0.50% of Vin
 Absolute position range –720° to +720° mDEG REF from 0.0° center with ±1.0° accuracy
 
 PHA. PHB INDEX  
 Voh
 4.5 VDC (min.)
 Vol
 1.0 VDC (max.)
 Output sink current
 19 mA
 
PHA OR PHB
DATA
 UNITS
 Pulse/rev.
  90
 PPR
 Cycle length
 4.0
 mDegrees
 Pulse width
 2.0
 mDegrees
 Phase
 1.0
 mDegrees
 PHA-PHB quadr.
 1.0
 mDegrees
 State width
 1.0
 mDegrees
Mechanical specifications
xxRotating torque
xxSlip torque
xxInstallation force
xxxxxx
10.0 oz–in. (max.)
50.0 oz–in. (min.)
25.0 lb. (max.)
Environmental specifications
xxOperating temperature
 xxxxxxxxxxxxxxxxxxx
–40°C to 85°C

The second consideration for sensor durability is the total distance traveled by the sensor's contact over operational life. The contact travel limit for CP resistive sensors is on the order of 1000 miles. When considering the lifetime requirements of 75 3 106 dither cycles and 1 3 106 full travel cycles (30 miles) for the StabiliTRAK application, it is evident that the CP resistive sensor is the best choice.

CP resistors are made of a blend of carbon powder and various plastic modifiers. Standard resistive inks provide reasonable life and temperature ratings up to 100°C, but for higher performance sensor applications and operational temperatures up to 300°C, special inks of proprietary composition are blended. The ink is usually applied as a paste ~0.001 in. thick using silk-screening equipment and then temperature cured in ovens. Resistance values are obtained by first calculating the total surface area of the resistor and multiplying it by the ink resistivity factor known as "ohms per square." It is possible to blend inks in a range of 0.1 (omega) to 1 M(omega) per square, but the preferred practice is to keep within a 250–10,000 (omega) range.

Because these sensors operate as voltage dividers, current flow is generally very low, <20 mA. Fixed-value CP resistors at either end of the sensing element, plus a series contact resistor, should be used to limit the maximum current flow when the sliding contact is at or near the end of the element. High current flow will cause heat to be generated in the resistor, damaging the substrate and/or the element or exceeding the sensor's maximum allowable temperature rating. As a general rule, current flow and resistance values should be calculated to ensure that power dissipation in a resistive CP sensor does not exceed 3 W/in.2.
TABLE 2
Testing the Steering Wheel Position Sensor
TEST NAME
DESCRIPTION
Life cycle with environment 1 million mech. cycles with temperature
varying from –40°C to 85°C
Dust As per SAE J726 for 8 hr
Humidity 24 cycles (24 hr/cycle) from –40°C
to 38°C/95% RH to 85°C/10% RH
Salt fog As per ASTM B117-73 for 24 hr
Temperature cycle 666 cycles to (90 min./cycle) between –40°C
and 85°C
Resistance to contaminants Exposure to: engine oil, graphite oil, WD40,
vinyl plasticizer, ammonia base cleaner, alcohol
base cleaner
Temperature/altitude 8 hr of exposure to constant altitude of
14,859 ft with temperature cycle from –40°C
to 85°C
Ozone 8 hr of exposure to ozone concentration of
0.39 ppm/min.
Dither 75 million dither cycles at ±1°
displacement
Drop 6 drops from 1 m height onto a concrete
surface
Mechanical shock Nine 30.5 g, 14 ms shock pulses
Vibration 4 hr/axis from 10 Hz–55 Hz–10 Hz, at
1.0 mm p-p excursion
EMI As per GM9100P
ESD As per GM9119P

Contact Considerations. Contact design is necessarily a compromise between conflicting requirements. Each variable must be considered with respect both to interactions and performance requirements:

  • Contact material will affect cost, longevity, and current capacity. Stamped contacts should be multifingered for reliability and to ensure that maximum contact area is maintained on uneven surfaces. If one finger breaks off or is bent away from the resistor, the remaining fingers will still allow the sensor to perform. Noble metals such as gold or special alloys containing palladium are the most durable and exhibit the least contact resistance, but they are expensive. Base metal alloy contacts can provide a less expensive solution but their performance level is suitable only for sensors with low durability and life requirements. High-current applications can often be best served with brushes of bundled wires, which provide long life but at the expense of high output noise and poor positional accuracy.
  • Contact shape is also dictated by the application. A knuckle-shaped contact has a reasonably long element life but will tend to flatten out, increasing the contact area until it wears through the contact material. A properly made hoe- or rake-shaped contact will have fingers at 90° to the surface of the element. As the contact wears, the position and cross-sectional area of the contact will remain constant for its entire life. This feature is particularly important when positional repeatability is a necessary feature of the sensor.
  • Contact force must be accurately controlled and kept in the 3–5 gram range. If the force is too low, resistance will be high and output smoothness poor; too high, and both contact and resistive film will exhibit premature wear. The contact force is set by bending the flat spring portion of the contact to a specific deflection within the elastic limit of the contact material. Actual deflection is determined from the beam section and Young's modulus of elasticity for the contact material and is maintained by accurate tolerancing and construction of the sensor components.

For the StabiliTRAK system, the analog portion of the steering sensor uses a proprietary ink resistor, a rake-shaped noble metal contact, and a rotary resistive element with a nominal 10 kresistance.

The 5:1 ratio gear train was designed to eliminate the mechanical noise commonly generated by the parting lines on the gear tooth profile on plastic molded gear trains. Additionally, the shape and size of the sensor housing acts as a sound box that both generates and amplifies noise in a manner similar to the old steel-needle, mechanical phonograph pickups. A specialist molding company was therefore called on to produce gears with no discernable parting line, successfully reducing the mechanical gear noise to a very low and unobtrusive level.

Putting It All Together

The analog and digital portions function independently of each other, and are built and tested separately before being brought together for final assembly and test. This approach permits any of three sensor configurations to be selected, depending on the particular system requirements: combined digital and analog, digital only, or analog only. Table 1 lists the sensor's electrical specifications.

Sensor Testing and Compliance

The sensor was subjected to extreme testing under temperature, humidity, vibration, shock, salt spray, life cycling under temperature, and other harsh conditions (see Table 2). To date ~1 3 106 sensors have been shipped in compliance with QS-9000 certification, a form of ISO 9000 tailored specifically for the automotive industry.

Summary

A steering wheel sensor combining thick film conductive plastic and a novel optoelectronics technology outputs both analog and digital data for use by the General Motors StabiliTRAK system. The sensor provides incremental angular position change during steering wheel movement, quadrature outputs to determine the direction of steer, and an analog reading of the steering wheel position at any point over the "lock-to-lock" range.

For Further Reading

Grinter, M.K. Sept./Oct. 1995. "Opto ASIC:Encoder on a Chip," Motion, Vol. 11, No. 5:2-3.

Madni, A.M. et al. 9-12 Nov. 1997. "A Miniature Yaw Rate Sensor for Intelligent Chassis Control," No. 0002, Proc ITSC '97, IEEE Conference on Intelligent Transportation Systems, Boston, MA.

Wells, R.F. "Automotive Steering Sensors," SAE Paper 900493.

Acknowledgment

The authors wish to express their gratitude to Linet Aghassi for her help in preparing this article.

Dr. Asad M. Madni is President and CEO, BEI Sensors and Systems Co. (principal operating subsidiary of BEI Technologies, Inc.), 13100 Telfair Ave., Sylmar, CA 91342; 818-364-7215, fax 818-362-1836, bei1madni@ aol.com

Roger F. Wells is General Manager, Duncan Electronics (a div. of BEI Sensors and Systems Co.), 15771 Red Hill Ave., Tustin, CA 92780; 714-247-2531, fax 714-258-8120, roger .wells@beiduncan.com


Dr. Asad M. Madni is President and CEO, BEI Sensors and Systems Co. (principal operating subsidiary of BEI Technologies, Inc.), 13100 Telfair Ave., Sylmar, CA 91342; 818-364-7215, fax 818-362-1836, bei1madni@ aol.com

Roger F. Wells is General Manager, Duncan Electronics (a div. of BEI Sensors and Systems Co.), 15771 Red Hill Ave., Tustin, CA 92780; 714-247-2531, fax 714-258-8120,
roger .wells@beiduncan.com


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