4.3 Sensors and Transmitters - Sensors and Transmitters
Key Concepts
- Sensors
- Transmitters
- Signal Conditioning
- Transmission of Signals
Sensors
Sensors are devices that detect and respond to some type of input from the physical environment. The input can be light, heat, motion, moisture, pressure, or any number of other environmental phenomena. The output is generally an electrical signal that is proportional to the input.
Example: A thermocouple is a sensor that converts temperature changes into electrical voltage changes. In a manufacturing plant, thermocouples are used to monitor the temperature of machinery, ensuring it operates within safe limits.
Transmitters
Transmitters are devices that convert the raw sensor output into a standardized signal that can be easily transmitted and interpreted by control systems. These signals are typically in the form of 4-20 mA current loops or 0-10 V DC voltage signals.
Example: A pressure transmitter converts the pressure reading from a pressure sensor into a 4-20 mA signal. This signal is then transmitted to a control room, where it is interpreted by a PLC (Programmable Logic Controller) to monitor and control the pressure in a system.
Signal Conditioning
Signal conditioning involves processing the raw sensor output to make it suitable for transmission and interpretation. This can include amplification, filtering, linearization, and conversion to a standardized signal.
Example: In a flow measurement system, the raw output from a differential pressure sensor may be very small and noisy. Signal conditioning circuitry amplifies this signal, filters out noise, and linearizes it to produce a clean, proportional 4-20 mA signal.
Transmission of Signals
The transmission of signals refers to the process of sending the conditioned sensor output to a control system or display. This is typically done using electrical, pneumatic, or digital communication methods.
Example: In a chemical plant, a level transmitter sends a 4-20 mA signal over a shielded twisted pair cable to a control room. The signal is received by a PLC, which uses it to monitor the liquid level in a storage tank and adjust the pump speed accordingly.