Trends in Instrumentation
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IN a paper entitled "Recent Develop1 ments in Vehicle Instrumentation" read at a meeting of the Institute of Road Transport Engineers in London yesterday, Mr. I. W. Thompson, of the Smiths Motor Accessory Division of S. Smith and Sons (England) Ltd., reviewed current thinking on the subject.
He said that the particular requirements of commercial vehicle instrumentation could be summarized as follows: (1) Robust design for exposed working conditions. (2) Extended service life without attention between major vehicle overhauls. (3) Emphasis on strictly functional aspects of design and presentation. (4) Adaptability of design to meet special variations.
It was also necessary to pay particular regard to dust and weatherproofing and in some cases to insulation from shock and vibration. To some extent the requirements of commercial vehicle instrumentation differed from those of the private car, but the relatively small order quantities called for by large sections cr,' the commercial vehicle industry made it important that some product rationali zation be achieved for economy's sake. The current choice so far as speedometers were concerned lay between a mag magnetic instrument, mechanically driven by flexible shafting and an electro mechanical unit operated electrically by a transmitter Mounted Cin or near the vehicle gearbox. The choice between the two lay fundamentally in access to a suit able point of drive in the vehicle transmission. Underflooror rear-engined
vehicles were often unsuitable for flexible drives 'because of excessive length and resultant difficulties, in addition to which there was a cost. penalty when drive lengths exceed 25 ft. Extensive standardization of electrO-mechanical speedometer drives operating at 800 revolutions per vehicle mile had been made in the British commercial vehicle industry. Power-unit tachometers were not common on British commercial vehicles but the demand for them showed signs of increasing, he said. In addition to instruments operating on similar principles -to speedometers, electronic tachometers had a potential market in the commercial vehicle industry. On spark-ignition engines these were arranged to be sensitive to the rate of impulse generated by the distributor contact breaker. For diesel engines it was necessary to provide a simple pulse generator which might employ an existing part such as a gear wheel as its rotating element. So far as other instruments were concerned there had been considerable interest in using electrical units in order to eliminate the installation complications of the tubing required for Bourdon-tube instruments recording pressure or coolant temperature. But it was difficult for electrical units to match the accuracy or, in some eases, the cost of mechanical units.