SCADA (Supervisory Control And Data Acquisition) is a software package that is positioned on top of a real-time control system to control a process that is external to the SCADA system. Telemetry is a technology which allows the remote measurement and also reporting of information of interest to the system designer. This word is derived from Greek where tele means remote, and metron means measure.
While SCADA can effectively measure and control data, Telemetry only helps in measurement and analysis of the data. Telemetry cannot help in controlling the application. But they both have found their effective usage.
In telemetry system, there are two main components, the airborne system and the ground station system. The data acquisition starts in the airborne system, when sensors measure the amount of a physical attribute and transform the measurement to an engineering unit value. Some sensors also produce a voltage directly (thermocouples for temperature or piezoelectric strain gages for acceleration), while others require excitation (resistive strain gages, potentiometers for rotation, etc.).
Sensors attached to signal conditioners provide power for the sensors to operate or modify signals for compatibility with the next stage of acquisition. But maintaining a separate path for each source is time consuming and costly. So a multiplexer is used. It serially measures each of the analog voltages and outputs a single stream of pulses, each with a voltage relative to the respective measured channel.
The Output Formatter serializes the composite parallel data stream from the multiplexer (Mux) to a binary string of pulses i.e.1’s and 0’s. Later this binary signal is transmitted through copper wire or a fibre optic cable. These days, PCM or pulse code modulation system is used with the data stream, so that the original data can be sent more accurately, with minimum loss of data.
In the ground system, the received data stream from the transmission line is amplified. Since this transmission path is filled with noise, the original signal may get distorted. So to get the original signal, a bit synchronizer is used which reconstructs the distorted signal to the original serial signal. Then, a demultiplexer (DeMux) recognizes the synchronization pattern and returns the serial digital stream to parallel data. The DeMux also separates the PCM stream into its original format and data.
SCADA monitor and control entire sites or complexes of systems over large areas (on the scale of kilometres or miles). Most controls are performed automatically by remote terminal units ("RTUs") or by programmable logic controllers ("PLCs"). For example, a PLC may control the flow of cooling water in an industrial process, but the SCADA system allows the operators to change the set points for the flow, and also enable alarm conditions, such as loss of flow and high temperature, that can be monitored on a display and recorded. A feedback control loop passes through the RTU or PLC, while the SCADA system monitors the overall performance of the loop. SCADA itself is a software package, and it needs an input/output with the system. So this has to be done externally.
In this system, the SCADA package is installed in a computer system (most of the time in a server or mainframe), and through this other machines or components are connected. There are also alarm systems, which give green signal if the system is working perfectly and red signal if there is be some problem.
All the data, during operation of the machine, is stored in an internal database of SCADA, through which later, the operator or the engineer can analyse the system. The SCADA system gathers information from the PLCs and other controllers via some form of network, combines and formats the information. SCADA is popular, for its compatibility and reliability.
Telemetry can be applied to Agriculture, Defence, Water management, Enemy intelligence, Resource distribution etc. For example, in agriculture, to get healthy crops and a good yield, the farmers need to make decisions about timely availability of weather and soil data. Wireless weather stations, that use telemetry, play a major role in this. These stations transmit to a base station the major parameters or data needed for decisions such as: air temperature, relative humidity, solar radiation, wind speed etc. By analysing this data, the farmers can make informed decisions about the crops and they can predict in advance about the yield also.
Using RFID (radio frequency identification) tags in retail business and operations is another example of the application of telemetry technology. Through these tags, the retailer can know the label of the inventory, their movement and if there is any need for restocking of the inventory.
SCADA can be used in small applications, like controlling the temperature of a room, to large applications, like controlling a nuclear power plant. It can analyze the data acquired from remote equipment throughout a system, and command corresponding adjustments across the system to maintain safe, efficient operation. In utility applications, these adjustments can redistribute power, or pipeline flow, to serve current demands, and to bypass equipment failures and safeguard equipment and operations.
For example, in many chemical refineries, there may be some parts of the plant which are highly toxic or dangerous. In such situations, the operators can control this using SCADA system, by connecting it into the system. After successful installation of the SCADA system, the operator can check about the operation, the flow of chemicals, the pressure, temperature, composition of the chemicals in a display monitor. He can even control all the necessary parameters from the remote place, automatically, without being physically present there.