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Automotive manufacturers have decided to gradually replace hydraulic power steering (HPS) systems with electrical powered steering systems, and marketing experts predict that by 2010, more than 50% of new vehicles worldwide will be so equipped. In Europe, this same percentage will likely be reached by 2006 or 2007. EPS systems are based on electrical power and provide assisted steering without the penalties of loading the engine or increasing fuel consumption. Furthermore, EPS enables truly dynamic steering that can be closely linked to the vehicle stability control system or electronic stability program to account for vehicle speed and driving conditions.
EPS SYSTEM OVERVIEW
The essential elements of an EPS system (Figure 1) consist of three main sensors, one motor and an electronic control unit (ECU). The main sensors are the steering torque sensor, the steering wheel position sensor and the motor position sensor. The ECU usually features two microcontrollers — one main and one “fail-safe” — in order to get a safe and reliable system. The ECU processes information from the main sensors and also has access to the vehicle speed and the engine speed via the ABS wheel speed and crankshaft sensors, respectively. The ECU also includes the motor driver. The motor is usually a brushless dc motor, but brushed motors are also used in EPS systems. The whole system is either placed on the column or directly on the steering rack.
STEERING TORQUE SENSOR
The torque sensor translates the torque applied to the steering wheel by the driver into an electrical signal processed by the ECU. The torque sensor output is safety-critical and it is often implemented in a full redundant architecture for the electronic circuitry in the form of a single module. The torque sensor usually relies on the torsion bar — or torsional spring — inserted along the steering shaft. The torsion bar decreases the rigidity of the steering system and is used by the torque sensor to measure a micro-displacement of ±3°. This small, relative angular displacement is measured with the following non-contacting sensing technologies: optical, Hall effect (linear sensor), magneto-resistive (MR) and inductive. Newer sensing technologies that do not directly rely on the torsion bar include surface acoustic wave (SAW) and magnetostrictive-based sensors. However, these technologies have not reached commercial maturity.
The key feature of the steering torque sensor is the stability of its offset (output signal when no torque is applied) over temperature and lifetime. To accommodate this offset, a dead zone is introduced in the control algorithm: in this zone, no torque is assumed and, therefore, no assist is provided. Given the impact of this dead zone on the overall performance, its width is a critical specification and it requires a high stability for the offset of the sensor. An excessive sensor offset is unacceptable because it would need to be permanently compensated by the driver.
STEERING WHEEL POSITION SENSOR
The steering wheel position sensor provides the absolute position of the steering wheel over ±3° steering wheel turns or ± 1080° with a resolution of 0.1° and an accuracy of ± 1°. A redundant architecture is also required here, and this function is performed by the same sensing technologies used for the torque sensor. In addition to the sensor IC, the steering wheel position sensor module includes a constellation of gears to translate the rotation of the column.
According to the true power on (TPO) requirement, the steering wheel position sensor is expected to provide the absolute position of the steering wheel immediately after power up without drawing any standby current when the engine is switched off. As illustrated in Figure 2, this is realized through the combination of two angular position sensors linked to two separate gears with different numbers of teeth. This configuration exploits the Nonius principle to achieve greater precision in the general manner of a Vernier Caliper. Optical solutions also use two separate sensors and coded targets.
The steering wheel position sensor is either integrated in the top column module or is used as a stand-alone sensor around or linked to the steering column. Regardless of the implementation, the output of the steering angle sensor is transmitted over the CAN bus and is used by multiple ECUs in addition to the ECU dedicated to the EPS system. The steering wheel's angular velocity is derived in the ECU from the steering wheel position sensor data and is used by the EPS control algorithm.
MOTOR POSITION SENSOR
For EPS implementations using three-phase BLDC motors, the motor position sensor controls the commutation of the three phases. A smooth commutation is needed to avoid torque ripples that can be felt by the driver. The motor position sensor is essentially a high-speed angular sensor that can operate above 400 rpms with better than 0.5° accuracy and better than 0.1° resolution. Optical encoders or inductive resolvers are suitable for this application. The less expensive magnetoresistive and Triaxis sensing technologies can also be used but generally meet the technical requirements with lower margin.
The practice of combining both steering wheel and motor position sensors into one position sensor located at the motor is becoming a popular trend. The only technical challenge is to realize the TPO feature, which requires zero standby current, while accommodating a gear ratio between the steering column and motor shaft of 20-to-1. Solutions with sleep modes are marginal. However, there are promising solutions based on low energy concepts such as combining a Wiegand magnetic wire sensor with non-volatile RAM, such as FRAM.
TRIAXIS HALL SOLUTIONS
IMC Hall technology consists of adding a passive amorphous high-permeability ferromagnetic layer on top of a conventional Hall sensor front-end equipped with planar Hall plates. This layer is called the integrated magneto-concentrator (IMC) as it concentrates the flux density (B) applied parallel to the IC surface. It actually converts this parallel flux density into an orthogonal component that can be measured by the planar Hall plates placed underneath the IMC (Figure 3).
Two orthogonal pairs of Hall plates are used to measure the flux components along the two axes parallel to the IC surface, BX and BY. The third component of the flux density orthogonal to the IC surface, BZ, can also be measured through the same Hall plate constellation. Given its sensitivity to the flux density applied parallel to the surface, the IMC Hall sensor is comparable to the Hall sensors based on vertical Hall plates.
The CMOS IC MLX90316 developed by Melexis is based on the previously described IMC Hall front-end. Figure 4 shows the block diagram for this device. Both raw Hall signals VX and VY are amplified through a multiplexed chopper amplifier prior to being digitized. A microcontroller-based digital signal-processing (DSP) core processes the digitized signals and computes the angular information. The angle is then provided to the output as either an analog signal using a digital-to-analog converter or digitally using pulse-width modulation or a serial data protocol.
When a diametrically magnetized magnet rotates above the IC, as shown in Figure 5, the flux density components BX and BY will describe two sine waves in quadrature, as shown in Figure 6, where BX is proportional to cos(α) and BY is proportional to sin(α).
The raw Hall signals VX and VY are proportional to BX and BY. The embedded DSP of the MLX90316 performs the following operation to get the angular information:
Thus, the MLX90316 is intrinsically a rotary position sensor IC as it directly provides the angular position of the magnet rotating above it.
TRIAXIS HALL SOLUTIONS FOR EPS APPLICATIONS
The MLX90316 is suitable for use as the steering wheel position sensor. Furthermore, the fully redundant construction, as shown in the micrograph in Figure 7, encapsulates two isolated dies within the same package. This can enable simple and compact steering angle sensor modules.
The MLX90316 response time is insufficient for motor position applications. However, the Triaxis front-end can be used in combination with a high-speed DSP to provide the required angular information for the commutation. This solution does not match the performance of inductive resolver solutions. However, improved noise immunity will improve performance of the Triaxis front end for this application.
While there is no specific magnetic design using a Triaxis Hall sensor to measure steering torque, the Triaxis Hall sensor can be readily adapted for a small stroke (± 10 mm) linear position sensor using a dedicated magnet arrangement. In this case, the linear position is obtained though the simple division of BX by BY and, consequently, it features the same benefits as the rotary position sensor design. This small stroke linear sensor can then form the basis of a torque sensor due to its sensitivity to small displacements.
EPS systems are booming in the automotive industry and are gradually replacing hydraulic power steering systems. EPS places no permanent load on the engine, improves fuel economy and design flexibility. It also enables adaptive steering features for various vehicle speeds. EPS relies on several sensors to determine steering component parameters. The MLX90316 is one solution for the steering wheel position sensor when configured as shown in Figure 8.
In today's EPS systems, there is still a mechanical linkage between the steering wheel and the steering rack. In case of a faulty system, a clutch mechanism disables the motor to permit manual steering. The next revolution in the steering world will be “steer-by-wire.” There, the mechanical linkage will be eliminated and this fail-safe strategy will need a serious revision. While the sensor technologies previously described will benefit this research, their continued development also suggests the EPS era will last for a long time.
ABOUT THE AUTHOR
Vincent Hiligsmann is with marketing & applications at Melexis. He can be reached at [email protected].