244 Unit Four Control Systems and Maintenance
Port
Motion
External Sensor Robot Controller Circuitry
Interface
Limit switch
Digital input
port transistor
Resistor
+24V dc
Figure 10-3. This illustration shows a limit switch connected to a typical digital input port. When no
object is present, the limit switch is open. When an object, such as an assembly part, is present, the
limit switch closes and conducts 24V direct current to the transistor. The controller circuitry detects
this change in condition (presence of an object and a change of transistor state), and passes the
information to the robot’s computer program to determine the appropriate action.
Interface
Relay
+24V dc
Conveyor
belt motor
120V ac power
Collector
Emitter
Base
Control signals
from computer
program
Robot Output Port
Controller Circuitry
External
Equipment
Figure 10-4. When enough voltage is applied to digital output port transistor Q1, the lower end of the
control relay is grounded. Grounding the lower end of the control relay coil causes the relay contacts
to close. This supplies ac voltage to the conveyor belt motor.