Rockwell Automation System Design for the Control of Electrical Noise - Manual (Page 81)

Publication GMC-RM001A-EN-P — July 2001
High Speed Registration Inputs 9-5
Proximity Switches
Proximity switches are especially vulnerable in the off state since the
signal line is disconnected at the switch, forming an efficient antenna.
Observe the following guidelines when using proximity switches:
• Insulate the mounting, if possible, and connect the body to the
cable shield.
• Arrange to be normally on (i.e. hole-operated instead of
target-operated).
• Register on the falling edge. With the line effectively disconnected
(off condition) stray capacitance causes the signal voltage to fall
slowly. Even low levels of noise may then cause false triggering of
inputs without hysteresis.
• When the proximity switch is supplied with unshielded cable,
keep the unshielded length to a minimum by joining to shielded
cable inside a shielded terminal box mounted close to the switch.
Bond the terminal box to the sensor body.
Signal Noise Filter Options Most registration inputs have a response time of 1 microsecond or
less. In practice, such speed is rarely required. A simple, low-pass
filter will slow the response time but will increase the noise immunity.
Observe the following guidelines for best results in all configurations.
• Keep the length of cable between filter and control to a minimum.
• Bond the filter common securely to the controller chassis.