Chapter 10 Interfacing and Vision Systems 243
to initiate some action, Figure 10-3. A common use of digital input ports is
with safety devices. Safety devices, such as light curtains, motion detectors,
and door switches on safety fences, may be connected to a digital input
port, which sends a signal to the controller to shut down the robot when a
person or object enters the work envelope. Many robots are equipped with
16 or more digital input ports.
Digital Output Ports
One way that a robot interacts with external equipment, such as a
conveyor belt drive motor, solenoid, or another robot, is through digital
output ports. Digital output ports permit digital signals to be sent from a
robot controller to equipment controls, Figure 10-4.
Whenever a control signal from the robot’s computer program is at a low
voltage level, the base (control element) of transistor Q1 does not conduct.
The output current flows from the emitter (current source) to the collector
(output element). When the base is at a low voltage level, the output current
is zero and the circuit is high resistance. The motor control relay (CR) is not
energized and the motor is OFF.
When the computer sends a high-level signal (usually 12V or 24V) to
the base control element of transistor Q1, the current flow from the emitter
to the collector is at maximum. The resistance between the emitter and
collector is now very low. In this condition, the control relay is energized
and the motor is turned ON. Robot controllers are equipped with several
digital output ports and a matching number of input ports.
Limit
switches
Figure 10-2. Limit switches communicate information through input ports. This limit switch is used to
limit a robot’s rotary motion. (Motoman)