Instrumentation design and precision analysis of the external diameter measurement system based on CCD parallel light projection method

The principle of the double parallel light path projection method is to image the workpiece' right and left edges onto two CCDs respectively, through two parallel light paths. Then the workpiece's external diameter can be obtained by calculating the length of the photosensitive units on the shadow field of the two CCDs, plus to the distance between two parallel light paths. In this way, double light path measurement system enlarges the measurement range while ensuring the measurement accuracy. To obtain exactly the result, the video signal output by CCD should be first filtered and amplified, then binary-value processed and at last pulse filled and counted. In the mechanical design of the system, screw pair drive is applied to achieve the up-and-down movement. Its precision mainly lies on the Abbe Error caused by the offset between the centerline and the mobile line of the capacitive-gate ruler, the heeling error of the guide mechanism, and the error caused by the dilatometric change of parts resulted from the temperature change. Rotary mechanism is achieved by stepper motor and gear drive. Its precision is determined by the stepping angle error of the stepper motor, the gear transmission error, and the heeling error of the piston relative to the rotation axis. The software mainly includes serial communication interface program, elevator motor control program, rotating motor control program, data processing program and error modification program.