1. Field of the Invention
The present invention relates generally to the field of reed switch testing and in particular to an improved method and apparatus for use in detecting non-parallel reed switch contacts by measuring the duration or pulse width of each contact bounce after the initial operation or closure of the reed switch contacts and comparing each open contact duration to a programmed reference number corresponding to an acceptable duration.
2. Description of the Prior Art
In the manufacturing of reed switch devices it is necessary to identify reed switches that have non-parallel contacts, generally called bad mating parts. These non-parallel contact switches usually have one reed member or blade twisted axially which produces a line contact instead of the desired surface contact between the reed members. This undesirable line contact gives a high but acceptable electrical contact resistance and yet may still cause an ultimate failure in the operation of the switch after operating in the field.
Various testing methods have been developed to detect these potential failure parts including the visual sorting one-by-one which of course includes the inherent disadvantage of the varying visual perception of the testing personnel. Electrical testing apparatus has been developed that applies a linearly changing current simultaneously to an operating coil and to its reed blades and then differentiating the voltage developed across the contacts to determine the operating characteristics of the switch such as those of the type described in U.S. Pat. No. 3,586,961, which issued to R. M. Rovnyak on June 22, 1971 and U.S. Pat. No. 3,822,398, which issued to R. M. Rovnyak on July 2, 1974. These techniques however do not make use of the contact bounce time to determine badly mated contacts.
Other relay testing circuits of the prior art measure various parameters such as the length of time that the contacts continue to chatter after initial closure, the number of bounce pulses present after closure, and the accumulative bounce time such as disclosed in U.S. Pat. No. 2,236,157, which issued to W. T. Rea on Mar. 25, 1941, U.S. Pat. No. 1,677,157, which issued to A. Weaver on July 17, 1928, U.S. Pat. No. 3,612,985, which issued to A. F. Rockett on Oct. 12, 1971, and U.S. Pat. No. 3,241,063, which issued to C. L. Beattie et al. on Mar. 15, 1966.
Other prior art circuits have been devised to measure the pulse duration of a contact bounce as compared to a predetermined level by the use of capacitor charge and discharges such as disclosed in U.S. Pat. No. 3,364,419, which issued to G. D. Anderson on Jan. 16, 1968 and in an article entitled "Relay Contact Bounce Measurements", by Richard Jiu in Electronics Magazine, August, 1955 issue. Such circuits however do not include digital techniques that allow for the programming of various reference time durations. Further, this type of RC time constant measurement is also less accurate than those obtained by digital techniques.