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Swapping hoses and pumps for electric motors and electronic control . . . Electric Power Steering By Julian Edgar T he conventional hydraulicallyassisted power steering used in most cars is soon to be replaced with electric power steering. Already, many manufacturers are using electronically controlled hydraulic systems, while some car manufacturers have recently introduced purely electric systems to their mass production vehicles. In addition to reducing parasitic loads, full electric power steering allows steering responsiveness to be automatically varied depending on speed, road conditions – and even the driver’s ability! Hydraulic Power-Assisted Steering Hydraulic Power Assisted Steering (HYPAS) has been used in automotive applications for about 50 years. The systems use an engine-driven hydraulic pump, a control valve, steering cylinder and connecting hydraulic hoses. The pump is usually of a vane design with an integrated internal bypass. It is sized so that, even at idle rpm, it delivers enough oil flow to provide a suitable degree of power assistance. The control valve uses a flexible torque-measuring device (such as a torsion bar, spiral spring or leaf spring) to convert the steering torque into a small control movement. This movement is transferred to a valve that regulates fluid flow to the power assistance mechanism. In rack and pinion steering, a double-ended hydraulic ram mounted parallel to the rack (within the rack as14  Silicon Chip sembly) is used, while recirculating ball systems incorporate the mechanism into the steering box. Fig. 1 shows an example of a traditional HYPAS recirculating ball steering system. Note that in this particular system, the fluid reservoir is incorporated into the pump. A major problem with simple HYPAS systems is that the assistance level is not reduced at high speeds, resulting in a lack of steering feel. American cars of the 1950s and 1960s were particularly noteworthy for their feather-light steering effort during parking, a trait which resulted in extreme vagueness at high speeds. To overcome this problem, most HYPAS systems of the last few decades have incorporated mechanisms that reduce steering assistance, either as engine speed increases or (less frequently) as road speed increases. The reason that engine speed was more commonly used as the control parameter to reduce steering effort is that such a system can remain purely hydraulic, whereas using road speed as the control variable requires the use of an electronic system. Electronically-Controlled HYPAS The introduction of electronic speedometers – and subsequently, full engine management – meant that an electronic road speed signal became available, allowing the widespread use of electronically-controlled HYPAS systems. These vary steering effort depending on road speed and also, in some cases, other parameters. A number of different hydraulic approaches to regulating steering assistance are used. These are: 1.Flow Control A solenoid valve is located on the discharge port of the hydraulic pump. Electronic control is used to control the solenoid valve opening, thus regulating the fluid flow. The flow is reduced at high road speeds, decreasing the degree of assistance provided. 2.Cylinder Bypass A solenoid valve and associated bypass line is located between the two Fig.1: traditional Hydraulic Power Assisted Steering (HYPAS) systems use an engine-driven hydraulic pump, fluid reservoir, connecting hoses and a hydraulic steering box or rack. [Nissan] www.siliconchip.com.au chambers of the hydraulic cylinder, allowing a reduction of the pressure differential. The solenoid valve opening is controlled electronically, its opening greater at high road speeds. This reduces the degree of assistance that is provided. 3.Hydraulic Reaction Force A hydraulic force is enabled that works against the power assistance. As speed increases, the reaction force is increased. Since fluid flow to the power cylinder is not affected, the steering response rate can remain high without reductions occurring in feel. In their electronically-controlled H Y PA S s y s t e m , Hyundai use an ECU equipped with an 8-bit microprocessor. Two major inputs – vehicle speed and steering angular velocity – are used. From these inputs the ECU determines the driving conditions and via a 3-dimensional look-up map, provides the appropriate current flow to a hydraulic solenoid valve. Three different driving conditions are recognised: Parking – maximum current is supplied to the solenoid valve, resulting in maximum steering assistance. High Speed – minimum current is supplied to the solenoid valve, resulting in minimum steering assistance. Evasive Steering – a large and sudden steering input causes the ECU to supply a current to the solenoid proportional to the angular veloc- · · · Fig.2: electronically-controlled HYPAS uses the inputs from both a road speed sensor and steering angle sensor. [Mazda] ity of the steering input. The control algorithm used in the system is as follows: IS = IV + IAW + IA + IT; where IS = Solenoid actuating current IV = Current according to vehicle  speed IAW = Current according to steering   angle velocity IA = Current according to steering  angle IT = Current according to time The purpose of IA is to prevent the driver from experiencing an excessive steering holding force on banked roads. This current is increased in proportion to steering input angle. IT provides additional assistance in situations where the vehicle enters a corner that follows a long straight driven at high speed. ECU ANGULAR VELOCITY SENSOR CALCULATION OF ANGULAR VELOCITY CPU VEHICLE SPEED SENSOR (FROM SPEEDOMETER) CALCULATION OF VEHICLE SPEED ENGINE SPEED CALCULATION OF ENGINE SPEED BASIC CONTROL MAP POWER CIRCUIT CONTROL VOLTAGE PUMP MOTOR GENERATION OF HYDRAULIC PRESSURE MONITORING OF MOTOR CURRENT Fig.3: a schematic diagram of a Honda hybrid HYPAS control system. The hydraulic pump speed is controlled on the basis of inputs from steering wheel movement and engine and road speed. [Automotive Electronics Handbook] www.siliconchip.com.au January 2002  15 Fig.4: this General Motors hybrid HYPAS system senses motor current to determine the actual steering loads and so the degree of assistance that needs to be provided. The 3-phase brushless DC motor (12) is supplied power by the Motor Power Circuit. The EHPS control provides a duty cycle control to the Motor Power Circuit in response to input signals from the motor angle sensor, motor current sensor and battery current sensor, as well as operating system voltage. The temperature of the hydraulic fluid is also measured. The scaler is used on the motor current input to allow high resolution at low values without requiring a more costly A/D converter. [General Motors Corporation] TF TEMP IMH SCALER IML EHPS CONTROL ANGLE DUTY MOTOR POWER CIRCUIT BATTERY 12 CURRENT CURRENT IB IM Fig.2 shows the layout of a Mazda HYPAS system where the degree of assistance is based on road speed and steering angle. Hybrid Hydraulic/Electric Power Steering Systems Hybrid HYPAS systems use an electric motor to drive the hydraulic pump, rather than having the pump driven directly by the engine. This approach allows the steering effort to be easily controlled by varying the pump speed. While the efficiency of such an approach is actually lower than a conventional belt-driven pump, because flow can be better matched to actual requirements, the overall parasitic power loss is reduced. Fuel economy savings of up to 0.2 litres/100 km are claimed to be possible by taking this approach. The control approach that is taken can be of three types: Driving Mode – where driving conditions (such as city, country, highway, etc) are automatically judged with appropriate levels of assistance then provided; Steering Wheel Input Mode – where · · Features Benefits Engine independence Reduced engine power drain Improved fuel economy and acceleration Instant-on power steering Assistance available even should the engine stall Elimination of pump, hoses, Simplified packaging fluid, drivebelt and pulley Environmental compatibility Reduced mass Modular design and integrated controller Reduced assembly time Design and packaging flexibility Multi vehicle use Design and packaging flexibility Software tuning Wide assistance range In-vehicle laptop PC tuning Tuning process reduced from months to hours Cost-effective advanced features Variable effort steering Assisted return to centre Steering damping capability Fig.5: some of the possible benefits of using Electric Power-Assisted Steering (EPAS) systems in place of traditional hydraulic power steering. [Delphi] 16  Silicon Chip the angular velocity of the steering wheel movement is used to determine the degree of assistance required. Steering Load Mode – where demand for power assistance is indicated by the counter-pressure of the hydraulic fluid, sensed through variations in the motor current load. Fig.3 shows the processes followed in one Steering Wheel Input Mode system to calculate the appropriate degree of assistance, while Fig.4 shows a schematic diagram of a control system that uses the Steering Load Mode. · Electric Power-Assisted Steering Electric Power-Assisted Steering (EPAS) completely replaces the hydraulic system that hitherto has always been associated with power steering. EPAS systems assist driver effort by the use of an electric motor which acts through a reversible gearbox and also, in some cases, an electromagnetic clutch. An electronic control unit determines the degree of assistance that is rendered. EPAS has some significant advantages over any form of HYPAS, both for the owner of the car and its manufacturer. The reduction in engine load of an EPAS system (it can be as low as 4W when the car is being driven in a straight line) means that the fuel economy of a car equipped with EPAS is very similar to that of a car with no form of power steering. Analyses provided by manufacturers of EPAS systems indicate potential www.siliconchip.com.au Fig.6: the electronic control system for a Honda EPAS system. [Automotive Electronics Handbook] fuel savings of 4-8 per cent over cars equipped with conventional HYPAS, with the lighter mass of an EPAS also having an impact. The independence of the system from engine operation also means that should the engine stall, steering assistance does not vary. (In a conventional HYPAS system, a stalled engine immediately reduces steering assistance to zero – a problem if this occurs part way around a tightening corner!) From a manufacturer’s perspective, it has cost benefits. Using EPAS reduces assembly line time, allows easy software tuning of the steering assistance characteristics to suit a variety of cars (eg, a sports car or a limousine) and has the potential to improve reliability – 53% of all power steering warranty claims are from pump and hose problems. Environmental gains are also possible from the decreased production and disposal of hydraulic fluid (world- wide, an estimated 40 million litres of power steering fluid was in use in 1995) and from the decreased requirement for the non-recyclable polymers used in hydraulic hoses. Fig.5 shows the range of benefits potentially realisable from EPAS. A number of EPAS systems are currently in production or in the final stages of prototyping. The LucasVarity system uses a brushless DC servo motor and gearbox to develop a torque that varies from GPS SYSTEM RAM SPEED SENSOR STEERING SENSOR BRAKE PEDAL SENSOR INPUT UNIT CPU OUTPUT UNIT SKILL RATING THROTTLE PEDAL SENSOR YAW RATE SENSOR RAM  ESTIMATING DEVICE Fig.7: a recently patented Honda EPAS system actively calculates the driver’s ability and provides steering feel and weight to match. Inputs to this system can include GPS navigation and yaw rate information, with the system comparing the actual path taken by the vehicle with its computed target trajectory. [Honda] www.siliconchip.com.au January 2002  17 Fig.8: calculation of the available road friction is carried out in the the Honda active EPAS system by spectrumanalysing the noise generated by the tyres on the road! [Honda] START SPEED INPUT transformer) techniques, with the twist of a torsion bar converted to a slider displacement. Other system inputs include vehicle speed and battery voltage. Fig.6 shows the schematic diagram of a Honda EPAS system. Driver Skill Estimation! SOUND PRESSURE INPUT FREQUENCY ANALYSIS EVALUATE ROAD CONDITION DRY, WET, SNOWY, POWDERY SNOWY, AND ICY One of the most interesting aspects of EPAS is the ability that the manufacturer has to ‘tune’ the system’s responsiveness. As indicated earlier, this allows the easy software matching of a single EPAS to applications as diverse as a two-seater sports car or luxury sedan but it also means that system responsiveness can be made to vary in different driving situations in the one car. When this approach is taken, the input by the driver of a certain amount of steering lock does not always result in the same degree of assistance – should the ECU determine that such a steering movement is not appropriate for the conditions that the vehicle is undergoing, the steering assistance may be reduced or the steering input even actively resisted! As an indication of the far-reaching implications of this, Honda has very recently developed an EPAS system that estimates the skill of the driver and provides steering assistance to match. In the Honda system, a ‘driver skill estimation device’ is used, as shown in Fig.7. This device has inputs from: a GPS system(!); a vehicle speed sensor; a steering sensor that provides information on steering angular speed, angular acceleration and torque input; a brake pedal sensor that detects braking stroke, speed and force; a throttle pedal sensor that detects accelerator stroke and speed; a yaw rate sensor; a road friction estimate input. The road friction estimate is deter- mined by yet another system, with the approach taken shown in Fig.8. Vehicle speed and a sound pressure signal are gained from appropriate sensors, with an audio frequency analysis of this data then undertaken to determine whether the road is dry, wet, snowy, powdery snow or icy. (Note that while the GPS and yaw rate inputs are included in the Honda patent of the system, Honda state that the system can still work effectively without them.) The ‘driver skill estimation device’ analyses the actual path taken by the vehicle and compares this with a computed target trajectory. Using this and data on the vehicle wheelbase, the distance that the front and rear wheels are from the vehicle centre of gravity and other factors, the system awards the driver an ability that varies on five levels from “very poor” to “very good”. A very good driver is rewarded with very little steering force resistance (the driver gets what he or she asks for), while a poor driver will encounter steering that actively does not allow major steering inputs to be made at high speed. According to Honda, this allows the skilled driver to “positively control the turning behaviour of the vehicle so as to briskly manoeuvre the vehicle. Conversely, if the vehicle operator is not skilled, the control system produces a reaction which prevents the vehicle operator from over-reacting to the vehicle response, and [so] stabilises the vehicle.” One wonders what happens when a ‘very poor’ driver suddenly needs to swerve around a child that runs out onto the road. . . That they are a poor driver becomes a self-fulfilling prophecy, perhaps? However, the Honda system does provide a very strong indication of the direction that EPAS systems can be expected to follow in the future. SC about 15Nm in a small car to 75Nm in a large sedan. Other manufacturers, such as TRW, use variable reluctance motor designs. The electric motor that is used requires low levels or ripple and “cogging”. LucasVarity achieve this by using a three-phase inverter to vary motor phase currents and so torque. Power mosfets are used to control the switching and pulse width modulation techniques are used. Depending on the location of the electric assist unit, drive can be transmitted to the steering mechanism by a number of means. These are shown in the table of Fig.9. In the LucasVarity system, a dual-channel optical device is used to sense steering input torque. Two optical discs are mounted 50mm apart at either end of a torsion bar, which is incorporated into the steering shaft. Torque applied to the steering wheel Method Electric Assist Unit Location Power Transmission causes a relative movement of the two Pinion assist Under the dashboard on Motor > worm gear > column discs, with the angular offset optically the steering column shaft > pinion shaft sensed. On the steering rack input pinion Motor > gear train > pinion shaft Comparison of the two output sigRack assist On the steering rack Motor > ball screw > rack shaft nals allows the calculation of steering On a second pinion on Motor > planetary geartrain > torque, steering wheel angular velocthe steering rack another shaft pinion > rack shaft ity, and steering angle. Anther torque sensor that can be used incorporates Fig.9: electric assist units can transmit drive in a variety of ways, depending on LVDT (linear variable differential their physical location in the vehicle. ·· · · · ·· 18  Silicon Chip www.siliconchip.com.au