IDENTIFICATION OF THE SYSTEM
The system was identified by carrying out a simple step response test. Figure 11.6 shows the hardware set-up for the step response test. The port B output of the microcontroller is connected to data inputs of the D/A converter, and the converter is controlled from pin RC0 of the microcontroller. The output of the D/A converter is connected to the LM675 power amplifier which drives the pump. The value of the step was chosen as 200, which corresponds to a D/A voltage of 5000 × 200/256 = 3.9 V. The height of the water inside the tank (output of the level
sensor) was recorded in real time using a DrDaq type data logger unit and the Picolog software. Both of these products are manufactured by PICO Technology. DrDaq is a small electronic card which is plugged into the parallel port of a PC. The card is equipped with sensors to measure physical quantities such as the intensity, sound level, voltage, humidity, and temperature. Picolog runs on a PC and can be used to record the measurements of the DrDaq card in real time. The software includes a graphical option which enables the measurements to be plotted. The microcontroller program to send a step signal to the D/A converter is shown in Figure 11.7. At the beginning of the program the input–output ports are configured and then a step signal (200) is sent to port B. The D/A converter is then enabled by clearing its WR input. After writing data to the D/A converter it is disabled so that its output does not accidentally change. The program then waits in an endless loop.
Figure 11.8 shows the step response of the system, which is the response of a typical first- order system. It will be seen that the response contains noise. Also, since the DrDaq data logger is 8-bit, its resolution is about 19.5 mV with a reference input of 5V, and this causes the step discontinuities shown in the response (the steps can be eliminated either by using a data logger with a higher resolution, or by amplifying the output of the level sensor). The figure clearly shows that in practice the response of a system is not always a perfect textbook signal.
A smooth curve is drawn through the response by taking the midpoints of the steps, as shown in Figure 11.9.