Torque sensors

Corvari, V. Pry, W.C. Seibert, W.L. Augsburger, L. Instrumentation of a high-shear mixer evaluation and comparison of a new capacitive sensor, a watt meter, and a strain-gage torque sensor for wet granulation. Pharm. Res. 1992, 9 (12), 1525-1533. 

Install preventive maintenance monitoring equipment (e.g., vibration monitors, torque sensors) to optimize maintenance schedules. 

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]. 

The distinction between a force sensor and a torque sensor is not clear, because in all cases a mechanical movement resulting from a force is converted into optical, electrical, or magnetic signals. This conversion defines the characteristics of the sensor. All sensors are made for limited ranges. 

While sensitivity defines the minimal response of a system, the operational range gives the spread between the minimal noise level and the harmful overload level. For a force or torque sensor these limits have to be defined for all circumstances in which the sensor will be used (Fig. 7.12.1). 

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. 

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...

Fig. 7.12.8 Concept of torque sensor using anisotropic magnetoresistive technology...

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... 

There are various ways to design a torque sensor. They all have their specific areas of application. Beside those basic needs there are other values or influences of significant importance. 

Speed limits the ability to transduce applied force or torque for all measurement methods. Apart from electrical limits of filtering and signal conversion effects like material internal friction, skin effects and non-homogeneous material behavior will define speed limits. In most cases the rotational speed defines the principle of signal transmission. This is one of the reasons why torque sensors are still not used with automatic transmission. The input torque is calculated by the engine management [5],... 

The worst case is damping above the operational frequency. This means that the sensing element response includes a delay. For force and torque sensors there is no standard equipment on the market, as for electrical circuits. For signal transmission only radio-frequency based systems can avoid mechanical influences (Fig. 7.12.12). 

Lubricants have a nonlinear viscosity coefficient against temperature. At very low or very high temperature this nonlinearity can create significant measurement failures. A careful analysis will show this effect within the transmission chain. Sticking is the worst case. It can happen that frozen water makes a short cut in torque sensors. In most cases damping factors are based on the electrical filter circuits (Fig. 7.12.13). 

All automotive torque sensors form part of a system that could potentially lead to dangerous situations for the driver. Using incorrect signals that do not reflect the real torque must therefore be avoided under all circumstances. Various monitoring features within the sensors or the controllers are implemented. The data transmission from the sensor to the controller must fulfill these safety requirements, such that erroneous data transmissions are detectable. Another important... 

In the case of a torque sensor for traction control, output torque can be calculated by the engine management system from engine data. But when sensor designs become simple, very robust, and highly accurate, measurement with sensors will have advantages over calculation. 

Generally the benefit from a force or torque sensor has to be significant in terms of system improvements in automotive applications. A replacement of hydraulic pressure sensors can be expected for the electromechanical brake system. 

K. Yoshida, Development of Custom ICfor EPS Torque Sensor, Koyo Engineering Journal English Edition E160, Nara, 2002. 

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