This measurement system is designed to measure the lengths of steel pipes as they pass along a roller conveyor. The pipes range in length from 6 meters to 15 meters. The length is measured by means of a wheel of circumference 600mm, which is fitted with a shaft encoder producing 600 pulses /rev and is brought into contact with the pipe pneumatically.
As the conveyor is only 18m long, a set of three 'laser beam' detectors are used to start & stop the counting of the pulses from the encoder. One detector triggers count start and the other two are used to determine the count stop; one is used for pipes of 6m length and the other for the longer pipes. The distances between the three detectors are stored within the measuring system and are used to add the appropriate offset to the counts from the measuring wheel. The measuring system also requires a reset signal from the production PLC to indicate that a pipe is entering the system.
The measuring system comprises a dedicated real-time control computer which is housed in a metal enclosure. On the front of which are various operator controls and an LCD screen, on which the pipe lengths are displayed. The lengths are also transmitted via a serial link to a large electronic display, and via a separate serial link to a Pipe Marking Machine. In addition pipe lengths can also be sent, together with date/time, to a small panel-mounted printer located in a control cabinet adjacent to the measuring system.
The system enables various configuration parameters to be entered using a simple menu structure. This data is stored in battery-backed memory and is not lost if the system is powered down.
The control computer used in the length measuring system is programmed in a high-level multi-tasking control language, which is an object-orientated real-time control language similar to PASCAL. The system was shipped out to Saudi Arabia for the customer to self install.
The accuracy was such that repeated measurements on the same pipe, which precessed axially by a small amount at each pass, showed a sinusoidal variation in length which was found to corresponding to a slight angle of the cut end of the pipe.