Control Systems: Data Monitoring

The transducer is used to sense the current condition of the installation. Information from the transducers is processed by the monitoring system, which gives a signal to an actuator, for example. A transducer for measuring the pressure or temperature often consists of a sensor and a measurement converter. The sensor senses the quantity to be measured. The measurement converter converts the sensor's output signal to an appropriate electrical signal that can be processed by the control system.

A pressure sensing body, for example, a diaphragm, is used to measure pressure. The mechanical signal from the diaphragm is then converted to an electrical signal, 4–20 mA or 0–5 V. The conversion from a mechanical to an electrical signal can take place in different measurement systems. In a capacitive system, pressure is transferred to a diaphragm. The position of the measurement diaphragm is sensed by a capacitor plate and is converted in a measurement converter to a direct voltage or direct current that is proportional to the pressure. The resistive measurement system consists of a strain gauge connected in a bridge connection and attached to the diaphragm. When the diaphragm is exposed to pressure, a low voltage (mV) is received. This is then amplified to a suitable level. The piezo-electric system is based on specific crystals (e.g. quartz) that generate electric charges on their surfaces. The charges are proportional to the force (pressure) on the surface.

In various compressor installations, there may be a need to monitor and control compressor operations from a remote location. On smaller installations, it is fairly easy to connect an alarm, operating indicator etc. from the compressor. It is also usually possible to perform remote starting and stopping. On larger installations, where significant financial investment is at stake, central monitoring is often desirable. It should consist of equipment that provides a continuous overview of the system, and which also provides access to individual machines in order to control details such as the intercooler pressure, oil temperature etc. The monitoring system should also have a memory in order to produce a log of what has happened over the last 24 hours. The log is used to plot trend curves, which serve to easily identify values that tend to deviate from the default. The curves can form the basis for continuing operations or planning a system stop. The system frequently presents compressor installation status reports at different levels, from a total overview to detailed status for individual machines.