Position Measurement of an Absorber Clamp
Chun Hao Ng
University of South Australia/ School of Electrical and Information Engineering, Mawson Lakes, Australia
Abstract —This paper reports on the development of the Remote Sensing of an Absorber Clamp system. An overview of the implementation method and background reasoning is given on the developed system.
Index Terms— Rotary Encoders, Position Measurement, Electromagnetic Compatibility, Sensors, Transducer I.
Electromagnetic compatibility (EMC) has been around since the early 1950s, predominantly regarding the motorised noise conducted over the power lines to sensitive equipments.  With the advancement of digital circuits, the switching speeds of equipments have increased dramatically. Since electromagnetic interference is caused by the changes in current with respect to time, the increase of switching frequency will cause false logic switching and improper operation of devices.  Wireless technologies and congestions of communication bands have also contribute to the awareness towards electromagnetic compatibility. 
The system being developed on is a motorised absorber clamp which moves along a rail detecting the radiated electromagnetic interferences (EMI) radiated from the equipment under test (EUT) trough its connecting cables. Given that the whole testing is done in a screen room, it is somewhat tedious to manually pin-point the exact location of maximum disturbance.
The absorber clamp consist of 36 sets of halved ferrite ring cores, 34 sets which acts as a current transformer and 2 sets which acts as an EMI measuring receivers, as seen in figure 1.
Figure 1. MDS-21 Absorbing Clamp
To measure the disturbance power, the cable under test is inserted horizontally so that the absorbing clamp surrounding the cable can be easily slid along the cable. Since there is no matching between the disturbance source, the cable, and the absorber clamp in this setup, the clamp is slid along the cable to obtain maximum current. The current transformer faces the EUT. A simple representation of the system is illustrated as in figure 2. 
Figure 2. Test setup
It is identified that the testing is sensitive against any type of electromagnetic radiation hence the following constrains are specified. First of all, the position sensor employed must operate without any considerable amount of radiation. In addition to that, minimal amount of cables are allowed to be placed inside the screen room.  This is to prevent any conductive coupling propagating in the introduced cables. Besides that, the system developed is expected to provide a measurement of 6 meter with error.
The main constrain of the project have short listed the choices of position sensors that can be applied. The most viable and cost efficient sensor is decided to be a rotary incremental optical encoder. The encoder operates on the logic of continuous photo detection of a light source. As a result it is electromagnetic radiation free. The operation of the optical encoder is shown in figure 3. When the coded disk lines up so the light from the light source is focused on the detector, the detector which in this case is a phototransistor, will go into saturation and an electrical square wave pulse will be produced. 
Figure 3. Operation of the optical encoder 
In most cases, a Schmitt trigger is embedded into the encoder for accurate digital output and noise immunity. The incremental optical encoder will utilise two pairs of the light source and detector to produce two channel of pulse train which is out of phase to each other.  These signals are called quadrature signals due to the out of phase relationship.  The relative phase of the signals indicates the direction which the shaft of the encoder is turning. Figure 4 shows the...
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