Issue 132 Jult 2001
Inductive Sensors

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ELECTRICAL INTERFACES

There are several interface modules available from CSEM; choose based on the type of signal you need. The functions and processing are simple for a microprocessor, however you also can use optimized processing modules. It’s a matter of economics. The cost of development may not be justifiable for small volume usage.

Figure 2 is a timing diagram of the output from the decoder shown in Figure 1. Two external capacitors set the low-pass filters’ critical frequency. Typically a 3-kHz bandwidth is used, but that can be extended up to 40 kHz. Frequency in this case is determined by the number of teeth the sensor will see per second and, obviously, is an engineering trade-off. The more teeth, the better resolution, but speed decreases. The sensor signals calculate the relative position by counting teeth, determine direction by the quadrature offset of the outputs, and measure velocity. The interpolation signal used in velocity measurement is usually four pulses between teeth, but up to 256 pulses are available.

Figure 2—The timing diagram of the circuit in Figure 1 provides digital indication of relative target position.

A more complicated interface will enable you to measure absolute position. Figure 3 shows the timing of the signals. In this mode, the sensor can be used in place of a variable differential transformer, digital resolver, or three-wire analog interface.

Figure 3—As the timing diagram of the linear position interface shows, an absolute position signal is available to replace variable differential transformer or resolver-type sensors.

In addition to its small size and light weight, the CSEM sensor isn’t a power hog, as some resolvers and ’VDTs are. Working off a standard 5-VDC, TTL-compatible supply, the CSEM sensor draws about 20-mA current from it. The output in the low state can sink up to 1 mA. In the high state it can source 200 µA while guaranteeing minimum 2.4-V output.