RPM-measuring - Stroboscope
When did we first need such a device? It was an experimental set-up using a an adjustable-speed drill which drove an open distributor type pump. One could recognize the regulation by the centrifugal weights, what
interested us was, at which RPM they opened up.
There were already this type of rev counters. We marked the segments of the drill-chuck with a thick, black felt-tipped pen and now hoped, that the electronic device would show us the calculable RPM.
Unfortunately, we made our calculations without taking the neon laboratory lighting into consideration, whose on-and-off switching rhythm influenced the device more. It took a while, until we found the cause and
replaced the lighting with conventional hand-held-lamps.
Rev counters are devices, with which one can determine the RPM and the speed of a shaft, an engine, or in fact, any machine. Hand-held rev counters are also used for the counting of objects.
At that time, we still depended on a certain amount of reflection from the drill-chuck, there, between the felt-tipped pen markings. Nowadays, so-called adhesive reflection strips are provided.
Of course, such devices for continuous supervision can also be built in. There are the most varied input sensors, mechanical, electronic or, e.g., with photo-cells. Instead of showing the RPM, they can of course, also
show the time taken for one revolution.
Counting, by the way, is one of the simplest duties of the digital electronics. Simple and reasonably priced components (ICs) are available, which only have to be accordingly controlled and evaluated.
Surprise is one of the first experiences one makes as far as integrated curcuits are concerned, how often a unique switching operation is actually carried out. This type of counter is so fast, that it also counts the contact
bounces.
Digital hand-held rev counters are particularly suited for the preventative maintenance and servicing of machines and facilities. The measuring of speed or RPM can take place with- or without contact. Instead of the
reflection strips, the rotating object can also be marked with a felt-tipped pen.
For the purpose of the exact targeting of the measuring point, a few models of the hand-held devices have a laser aiming dot. Some devices have a digital display with digital- and analog output. With a suitable
interface, one can transfer the data to a PC.
The terminal W on the generator of somewhat older Diesel engines serves as an example of available sensors. However, the signal must be converted. Indeed, external rev counters are more suitable for veteran cars.
Modern cars are stuffed full of inductive- and/or echo-sensors.
In engines with a spark ignition, the method of measuring rotating components using a stroboscope (see figure 2) has been around for a long time. Thus e.g., the Porsche company has always adjusted the ignition at
relatively high RPM. The advantage: The dynamic ignition adjustment was taken into consideration, which was not the case with the earlier, widespread method of using a testing lamp.
A stroboscope is nothing other than a fast light-source. With this an RPM-balancing is possible, and it can be used to balance the reference marks. Thus, one can determine, if e.g., the ingition adjustment takes place
inside of the prescribed range.
This is what it looks like in the practice, the lamp is held on an index mark e.g., on the rotating flywheel and on a point on it's casing, both points are then flashed at the same time. Should the marking begin to wander at
high revs, one can bring the two points together again by adjusting the device, the adjustment value can then be read off. 07/11
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