| 摘要 |
A radiation-type thickness measuring device has a radiation source for irradiating the object whose thickness is to be measured and a radiation detector for detecting the radiation transmitted by the object. A radiation absorber sheet is introduced into the radiation path, this sheet being of the appropriate thickness, according to the thickness of the object to be measured, to standardize the amount of radiation reaching the detector. Sheets of various thicknesses are carried circumferentially by a disk and the disk must be positioned accurately so that the correct sheet lies precisely where it should in the radiation beam. A positioning device is provided for the disk, having a bridge circuit of which two branches are formed by a potentiometer whose sliding tap is connected to the disk so that its position is dependent on the real position of the disk. The other two branches of the bridge are formed by a resistance chain whose intermediate tap can be selected in accordance with the desired position of the disk. A threshold switch in the form of a Schmitt trigger is controlled by the diagonal voltage of the bridge and controls a relay. When the diagonal voltage exceeds the threshold of the Schmitt trigger the relay connects the drive motor of the disk to a relatively high voltage source to operate the motor at a relatively high speed in the direction which reduces this diagonal voltage. When the bridge diagonal voltage falls within the threshold, the relay disconnects the motor from the high voltage source. A fine switch then takes over. This fine switch connects the motor to a relatively low voltage for driving it at a low speed. When the disk reaches its exact desired position, a cam which is connected to the disk causes the fine switch to disconnect the motor from the low voltage source so as to stop the motor.
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