Benefits of an Optical Detector

This page illustrates the benefits of using an optical detector, rather than an acoustic sensor, to measure clock rates.

In my development work with MicroSet, I was particularly interested in improving the accuracy of existing clock timing devices. One weak link that seemed obvious was the acoustic sensor. Since I wanted a resolution of single microseconds, an acoustic tick, which exists in the realm of milliseconds, seemed like trouble. It is.

The following graph shows the electrical signal of a typical acoustic clock tick. You will notice that it isn't a simple, precise event. It's a sequence of many sound waves. Where does the tick PRECISELY begin? It's impossible to say.

The next graph shows the electrical signal from same clock monitored with the optical detector. Here it's obvious where the tick begins: it begins with a sharp vertical line at the instant that the leading edge of the pendulum breaks the optical beam.

You will not be looking at electrical signals from the detectors, however. You'll be looking at beat times. The difference in beat measurements between these two types of sensors is just as dramatic as the signals themselves. The graph below shows rate measurements taken with MicroSet and captured with the Windows Interface Software. The same clock was used. The first half of the graph was captured with the acoustic pickup. The second half was captured with the optical sensor.

The first half of the data is obviously much more erratic than the second half. These readings were captured with a short beat count to emphasize the differences.

It's not surprising that the difference between these sensors is so dramatic. It is, after all, the purpose of a pendulum to control the rate of a clock. The raw gearwork does not produce precise timekeeping. When you monitor a clock with an acoustic sensor you're monitoring the gearwork - the pallets on the escape wheel. When you monitor a clock with an optical sensor on the pendulum, you're measuring the clock at the point where accuracy is produced, in the pendulum, with mass that mechanically stabilizes the "wild" rate of the escapement.