Variable End-of-Travel Sensor

In a computer controlled machine, software time delays are used to allow full travel of actuated mechanisms before continuing the sequence. When the end of travel of the actuated member is constant and the time for full travel is predictable, the software time delay is cost effective and throughput efficient. If the time of full actuation is variable the time delay must be long enough to allow for the worst case. This results in throughput inefficiency. This situation can be improved by using a switch or other similar sensor at stroke's end. This eliminates the need for time delays, but adds cost to the apparatus, and possibly reduces reliability. If the end of the travel is variable, however, a switch or position sensor is not viable and a time delay which is long enough to account for the fullest travel may be necessary. The only alternative to the time delay in variable end-of-travel situations is an economical, reliable motion sensor. This paper focuses on the development of an integrated, economical motion sensor for the parts centering arms in the placement head of the 4621A Omniplace Surface Mounted Component Assembly System. The sensor is termed integrated because of it's streamlined incorporation with structures already present in the head and is economical due to this integral nature. In addition to these attributes, it is expected to be reliable and performance enhancing. The variable end-of-travel sensor is based on a 10,000 turn solenoid with a cylindrical magnet which moves in the air gap. The EMF produced by this transducer is amplified and presented to an absolute value comparator circuit. The open collector output of the comparator circuit is compatible with a current sourcing optically isolated computer input. The computer program executes the next sequential event upon determining end-of-centering arm motion in either direction. Thus, whether centering a small part (with greater travel) or a large part (with smaller travel) the next event after centering occurs with no delay. The previous design employed a time delay which allowed for reliable centering of the smallest part. This paper details the transducer and signal conditioning circuit, the improvement to throughput efficiency, and the cost effective integration of these elements into the existing design.