Displacement measurement on vehicle engines – Analog sensors for reliable measurements in harsh environments

Analog sensors have not become obsolete even in this day and age. There are many areas of application in which they are superior to their digital counterparts. Their ease of use and the seamless recording of all values within their measuring range are a major advantage. They can also be manufactured as very robust versions. Signal transmission is also very resistant to interference. They are therefore an ideal solution when reliable, particularly robust and durable sensors are required.
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The strictly symmetrical design of the coil halves ensures very good linearity, high reproducibility, freedom from hysteresis and temperature stability. Electronic temperature compensation and linearization are not required for these systems.

These sensors provide the user with very robust, easy-to-use, absolute measuring systems. The wiring of the sensors to be carried out by the user using the proven 3-wire technology is also simple and reliable. Each sensor is connected with a 3-wire, shielded cable. The low-impedance terminating resistor in the receiving electronics (PLC, IPC or display unit) ensures the interference-free transmission of the analog signals. With the wear-free measuring method, measurements of highly dynamic processes are possible without any problems.

One example of use is measurement technology on vehicle combustion engines, which measures the tension of the timing chain. The chain drive of combustion engines has the task of transmitting the rotary motion of the crankshaft to the camshafts. This ensures that the valves are opened and closed reliably and precisely at the right time. A chain, the so-called “timing chain”, is used as the transmission medium. Timing chains have been used in engine construction for over 100 years. They can transmit high forces and are therefore particularly popular for powerful engines. They also have a long service life and can bridge relatively large distances in relation to the distance covered by the chain during the numerous revolutions. If all the components involved work faultlessly, timing chains can last for many 100,000 kilometers.

The timing chain coordinates all processes. It connects the crankshaft, which controls the piston stroke, with the camshafts, which are responsible for opening and closing the valves. In this way, it ensures that they rotate in the correct ratio to each other and makes sure that they do not get in each other’s way during their work. If something goes wrong, this can result in serious and expensive engine damage. Because a combustion engine operates at high speeds, the chain must be guided and tensioned. So-called slide rails keep the chain on track. Tensioning rails exert pressure on the chain and ensure that it sits tightly on the sprockets. This linear movement of the rails – up to 20 mm – is measured with the inductive displacement transducers. The contact points are lubricated by the engine oil, so the sensor must be very well protected. The package of guide elements ensures that the timing chain moves in the intended areas and does not start to flutter. In the prevailing atmosphere, the sensors must function against pan and splash oil, different crankcase pressures and temperatures between + 20° C and + 135° C. Even long-term operation at 130° C has produced very good results. The voltage must be measured and checked continuously throughout engine operation. If the tension changes, i.e. the chain is too loose or too tight, the function of the motor will be adversely affected. If the values are outside the tolerance range, the engine may be damaged or may even fail completely.

Function of inductive displacement and angle transducers
The technology of these sensors is based on the differential choke principle. A NiFe core is moved axially within a coil body. The respective position of the core causes a corresponding inductance distribution in the two coil halves, which is converted by external or integrated electronics into a path-proportional, analog signal.

Simple and reliable
This simple method of absolute position detection enables a robust, reliable design of the sensor element. Installation in a stainless steel or NiFe housing with subsequent complete encapsulation results in an analog sensor that can be used in a temperature range between – 40 °C and + 125 °C, which easily achieves protection class IP68 (submersion up to 50 m) and is shock and vibration resistant up to 250 g. This wear-free measuring method can be used for distances of up to 360 mm.

The evaluation electronics
supplies the sensors with a high-constant AC voltage. The measurement signal is demodulated in the correct phase, amplified and output as a standardized output signal. 0(4) – 20 mA, 0 – 10 VDC or ± 10 VDC for further processing. Depending on the application, the electronics can be designed as a 1- or 2-channel version in various housings or integrated in the sensor. The AC voltage used to supply the inductive sensors with a frequency of 10 kHz enables highly dynamic measurement processes to be recorded. The standard cut-off frequency is 800 Hz and can be increased to 4 kHz on customer request.

Low-wear and maintenance-free
The low-wear and maintenance-free design of the sensors means reliable operation with a long service life. The required operating voltages are variable and can be adapted to the applications, e.g. 11 – 17 VDC for use in motor vehicles. The strictly symmetrical design of the coil halves ensures very good linearity, high reproducibility, freedom from hysteresis and temperature stability. Electronic temperature compensation and linearization are not required for these systems.

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