Steering, torque sensors

One of the latest developments in the area of steering torque sensors is an optical concept created by Bishop Innovation with Bosch. As in conventional hydraulic steering systems, a torsion bar is a part of the steering column. A disc with a barcode on its surface is attached to each end of the torsion bar. A light source within the sensor module illuminates the surface of the coded discs, partially reflecting the light through a lens onto an optoelectronic ASIC. 

Fig. 7.12.7 Steering torque sensor using a magnetic encoder ring and an anisotropic magnetoresistive element...

The vision of braking and steering by wire will demand new, extremely reliable sensors. Even in early implementations of steer-by-wire systems, in which manual control can override any system failure, more than one sensor is normally used for the sake of redundancy. Many of the sensor principles required are already established in the market, including steering-angle sensors (e.g., for vehicle dynamics control) and pedal-position sensors. Mechanical action or feedback control, however, will drive the emergence of torque and force sensors. 

SAS will be needed in future applications such as EPS, AFS, and SbW. These systems need a torque sensor to measure the steering torque applied by the driver in order to control an assisting torque. Hence a combination of SAS and torque sensor seems to be obvious. Products combining angle and torque measurement have been proposed (e.g., an opto-electronic angle and torque sensor for integration in EPS and EHPS systems [29]). 

Force sensors are not used in general automotive applications. In agricultural machines a force sensor measures the load at the connection rod [1], Some force sensors are specially made for measuring forces during vehicle testing and validation. Often metal strain gauges are welded to the devices under test The data is sometimes needed for system development or to verify simulation results. The first and only mass production of torque sensors is in electrical steering systems [2]. 

A force sensor combines a structure that responds to force changes with a sensor that converts these changes into a signal. A torque sensor similarly consists of a torsion or tension element detector, but it also has to transmit the force. Consider the torque for a electric power steering system any failure would immediately mean loss of control of the vehicle, so this must be avoided under all circumstances. Steering column torque requires careful finite element calculation, durability and performance testing to failure, and validation. 

Electric power-assisted steering systems are more and more replacing hydraulic systems. To improve fuel economy, the power assistance is provided by an electric drive. Such a system only consumes energy when power is supplied, unlike hydraulic systems. A torque sensor in the steering column provides the input signal to the drive control unit. An example of an electric power steering system is shown in Fig. 7.12.4. 

A multipurpose torque sensor was developed by FAST Technology. The specifications of this sensor allows its use in electrically assisted steering systems. FAST Technology developed a method for permanently magnetizing ferromagnetic steel shafts. Fig. 7.12.11 shows the shape of the toroidal magnetic field. The magnetic field beside the shaft varies with the mechanical stress on the shaft. A sensor... 

The light intensity on the sensitive surface of the optical ASIC, an array of photodiodes, depends on the code of the discs. The intensity distribution over the photodiodes allows the optoelectronic ASIC to calculate the absolute angular position and the angular displacement of the steering shaft Fig. 7.12.6 shows the sensor concept. This concept allows measurement of the absolute angular position of the steering wheel and the torque applied to the steering system. 

The torque-vectoring system has the task of applying different torques to the rear axle to reduce the risk of under-steering and to increase agility while cornering. For this purpose, the technical system architecture is realized with the three main modules torque handling, torque position calculation and position control. In order to perform its functionality, the three modules together read several data sources, either sensor, e.g., the disk temperature, or external units, e.g., the nominal torque value, realize a set of computations and finally operate the actuators to distribute the calculated torque to both wheels of the rear axle. 

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