Page 126 - Mechanical Engineers Reference Book

Interfacing of computers to systems 319
P +
Load
SUPPlV

Input signal 7- Relay
from computer Darlington
Driver

Relay Load
supply
voltage
(+I2 V)
Figure 3.6 Power switching with a Darlington Driver

Supply voltage
-

Figure 3.7 Opto-isolator

3.4.2 Controller output interface hardware
Figure 3.8 Inverter integrated circuit
The digital interfaces discussed above are suitable for switch-
ing in power loads in an ON/OFF control system. For a digital
control algorithm based on a PID strategy, some means is to the computer port where it is then transmitted to a DAC.
required of discretely varying the output power supplied to the The DAC converts the binary input into a proportional output
controlled device (Figure 3.10). A number of different me- voltage which may then be suitably amplified to drive the
thods are used to supply variable power to the system and controlled device. The controlled device could, for example,
these include the following. be a d.c. motor whose speed is directly related to the supply
voltage. The interface is illustrated in Figure 3.11.
Two basic types of DAC are available: the adder converter
3.4.2.1 The digital-to-analogue converter (DAC)
and the ladder converter. The adder converter can be illus-
The required control effort value, U, is calculated in the trated as a simple example of Ohm’s law. A 4-bit adder type
program according to the control strategy employed. This DAC is shown in figure 3.12. The resistance value of the line
value is converted to an equivalent binary number and output resistors are halved for each consecutive increasing ‘bit’ and