Analogue control signal

Fig. 3.7 shows a comparison between a capillary thermostat in the right part of the figure and the ever more frequently used NTC temperature sensor whose analogue electrical signal can easily be processed by an electronic control system. The NTC sensor type is increasingly used, particularly in modern European machines that always have their own heater element and sometimes also an additional hot water connection. 

Analogue data signals (4—20 mA) firom the instruments are sent to the PLC input module located in the MCP and to a digital display. These displays provide operators with the necessary data. All analogue signals for control such as pressure and flow for process water temperature control are obtained firom the PLC. Alarms are produced by data received by the PLC from the instruments. Alarms are only produced when the process the instrument is monitoring is active. Most alarms often have several seconds delay associated with them to ensure the process has stabilised before activating the alarm. 

Figure 14.10 Schematic diagram for the signal flow and process control in an A/D converter device. The approximation of an analogue input signal by the digital conversion waveform isshown in thebottom part...

The Demodulator module is a microcontroller with necessary hardware to read and control ADC of the Analogue-Digital Converter module, control Signal Conditioner AGC and handle the communication with the Supervisor module, as shown in Fig. 4. It also must have the processing capacity to execute, each acquisition cycle, the following tasks ... 

Another approach to the control of batching systems is to use a relatively simple scale indicator which does nothing but supply weight data to a controUer such as a PLC, which in turn controls the fiH and discharge system. The communications between the scale and controUer can be serial such as RS-232, or it can be an analogue signal such as 4-20 m A. While this system minimises operator interaction with the scale, it sacrifices speed and accuracy because of the relatively slow communications. 

Once the desired control action has been transformed to an analogue signal, it is transmitted to the final control element over the transmission lines. However, the final control element s actuator may require a different type of signal and thus another transducer may be necessary. Many control valve actuators utilise a pressure signal so a current-to-pressure (I/P) transducer is used to provide a pressure signal to the actuator. 

Systems that develop contractile forces are very intriguing as analogues of physiological muscles. The idea for gel muscles was based upon the work of Katchalsky and Kuhn. They have prepared polyelectrolyte films or fibers which become elongated or contracted in response to a change in pH of the surrounding solution, and have estimated the induced force and response time. The contraction of gel fibers is also achieved by electric fields. Use of electric fields has the merit that the signals are easily controlled. 

The basic results from the individual units are processed and then combined to form the final result which is produced on the report printer. Results that deviate from an expected value by more than a preset tolerance may be marked or commented on. Additional information, such as sample identification and origin, is also made available. To ensure complete control by the analyst, the basic raw results may also be recorded in analogue form. Sample identification is provided so that the data can be re-analysed. Fully automatic systems require careful monitoring of the supply of reagents and the disposal of waste chemicals. To achieve this, fluid levels are monitored, and if they are low, an alarm signal is issued to the operator. 

TTie detailed information provided by the measurement of mercury is vital to the economic operation of the plant, so it is necessary to hnk the measurements directly into the process control computer. This is achieved in a simple manner by adding a digital-to-analogue converter to the control computer. Once the analytical result is calculated for any run, the result can be transferred directly by the D/A converter to the process control computer in a 0—20 mA signal. 

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