Page 1148 - The Mechatronics Handbook

loop:
btfss PORTA,0 Check digital input bit 0 of Port A
goto turnoff and disable motor if not 1
btfss PORTA,1 Check digital input bit 1 of Port A
goto turnoff and disable motor if not 1

bsf PORTB,5 Enable motor by setting bit 5 of Port B
goto loop and check inputs again

turnoff:
bcf PORTB,5 Disable motor by clearing bit 5 of Port B
goto loop and check inputs again

FIGURE 43.6 Fragment of assembly code for Microchip PIC 16F84A microcontroller. This code fragment examines
two digital inputs (bits 0 and 1 of input Port A) and sets bit 5 of output Port B if both inputs are at a logic 1 level.
The output can be used to enable or disable a DC motor with appropriate interface circuitry.

while (1) {
if (PA0 && PA1) {// Check status of bits 0 and 1 in Port A
PB5 = 1; // Set bit 5 of Port B
} else {
PB5 = 0; // Clear bit 5 of Port B
}
}

FIGURE 43.7 Fragment of C code to effect the same functionalilty as the code in Fig. 43.6.

43.3 Programmable Logic Controllers

The modern programmable logic controller (PLC) is the successor of relay-based controls. The techno-
logical shift began in the 1960s, when the limitations of electromechanical relay-based controllers drove
General Motors to search for electronic alternatives. The answer was provided in 1970 by Modicon, who
provided a microprocessor-based control system. The programming language was modeled after relay
ladder logic diagrams to ease the transition of designers, builders, and maintainers to these new controllers.
Throughout the 1970s the technology was refined and proven, and since the early 1980s they have become
ubiquitous on the factory floor.
Most PLC components are in card form that can be interchanged quickly in the event of a failure. A
typical PLC application has about one hundred inputs and outputs, but the scale of the applications varies
widely. A small PLC costing $200 might have six inputs and four outputs. A large application might
involve multiple PLCs working together over an entire plant and collectively have tens of thousands of
inputs and outputs. In general, the aggregated cost of PLC hardware per input and output is approximately
$10–$50. This does not include the cost of sensors (typically $50–$100), actuators (typically $50–$200),
installation (typically $10–$100), design, or programming.
Manufacturing control systems always require logical control and sometimes continuous control.
Logical control involves the examination of binary inputs (on or off) from sensors and setting binary
outputs to drive actuators. A simple example is a photosensor that detects a box on a conveyor and
actuates an air cylinder to divert the box. Continuous control systems are used less frequently because
of their higher costs and increased complexity. A typical continuous controller might use an analog
output card ($1000) to output a voltage to a variable frequency motor driver ($1000) to control the
velocity of a conveyor.

©2002 CRC Press LLC