e. The seven outputs from the Sweeney Model 1000 are: 300, 500, 600,
720, 800, 1000, and 1800 rpm.
To extend these ranges, the unit is supplied with
speed converters for increasing or decreasing the speed of any output shaft. The
output of any shaft by a factor of 4, 10, or 30.
Thus, a tachometer can be
calibrated at almost any speed within its range.
(4) The Optical Tachometer. The optical tachometer is a very exact method
of measuring rotary motion. With the optical tachometer the number of revolutions
in a given time are literally counted.
The optical tachometer has no loading
effect upon the shaft being measured.
However, the optical tachometer is not as
simple to use as the mechanical tachometer and required the use of an electronic
counter as a readout device.
a. The optical tachometer is shown in Figure 5 as it is set up for a
rotary measurement. To make a rotary measurement of this type, a disc with evenly
spaced sections of reflective material is attached to the output shaft of the
rotating device. From the construction of the disc, you can see that the beam of
light is broken periodically. The end result in the development of pulses of light
which contact the photocell and cause pulses of energy to be generated in the
The Optical Tachometer