Manual 82510
Magnetic Pickups and Proximity Switches
Woodward
12
Chapter 2.
Proximity Switches
Be prepared to make an emergency shutdown when starting the
engine, turbine, or other type of prime mover, to protect against
runaway or overspeed with possible personal injury, loss of life, or
property damage.
General Information
Large engines usually have a larger air gap between the speed sensor probe and the monitored gear
than smaller engines. This is necessary because of the relatively large runout of the monitored gear in a
large engine. The monitored gears on large engines usually also have a slower surface speed. Because
of either or both of these conditions, a magnetic pickup may not perform satisfactorily on a large engine.
A proximity switch (zero velocity pickup) will, however, operate well on large engines because of its
abilities to operate with a large air gap and at low surface speeds.
The output of these pickups depends solely on the position of the gear tooth (or similar ferrous
discontinuity) and not the velocity at which the gear tooth passes the sensing end of the pickup.
When wired as a “sinking” type proximity switch, the output is normally zero. When a gear tooth is within the
sensing range of the proximity switch, the output of the switch goes “high” (nearly equal to the supply
voltage). After the tooth has passed, the output switches back to zero volts until another tooth comes into
position. Typical wiring for a “sinking” type proximity switch is shown in Figure 2-2.
When wired as a “source” type proximity switch, the output is normally “high” (nearly equal to the supply
voltage). When a gear tooth is within the sensing range of this proximity switch, the switch output goes to
zero volts. After the tooth has passed, the output switches back to nearly the supply voltage until another
tooth comes into position. Typical wiring for a “source” type proximity switch is shown in Figure 2-3.
The on-off nature of the output of a proximity switch produces a square wave which is compatible with
nearly all Woodward speed sensors and will directly replace the signal from a magnetic pickup as a
speed sensor input.
Proximity Switch Features
As long as the air gap between the sensor end of the proximity switch and the gear being monitored does
not exceed the sensing range of a particular proximity switch, changing the air gap will not change the
output voltage. The duty cycle of the signal from the proximity switch varies with the air gap, but this does
not change the reference signals produced in the Woodward control.
For proper operation there cannot be more than one tooth in the sensing field of a proximity switch, as the
distance between the teeth must be greater than the size of the sensing field.
Released
