Instrumentation and Control Basics

Introduction

Instrumentation is defined as the art and science of measurement and control of process variables within a production or manufacturing area.

Instrument is a device that measures or manipulates variables such as

1. Flow
2. Temperature
3. Level
4. Pressure

Instruments may be as simple as direct reading thermometers or may be complex multi-variable process analyzers. Instruments are often part of a control system in refineries, factories, and vehicles. The control of processes is one of the main branches of applied instrumentation. Instrumentation can also refer to handheld devices that measure some desired variable. Diverse handheld instrumentation is common in laboratories, but can be found in the household as well. For example, a smoke detector is a common instrument found in most of the offices and homes.

Things that we commonly measure are:

1. Temperature 
2. Pressure
3. Speed
4. Flow rate
5. Velocity and Acceleration
6. Stress and Strain
7. Level or Depth
8. Mass or Weight Density
9. Size or Volume Acidity/Alkalinity

Basic instrument system consists of three elements as shown in Figure 1:

1. Sensor or Input
2. Signal Processor
3. Receiver or Output

Figure 1

Control systems consist of a control loop and it have four elements as shown in Figure 2:

1. A measurement of the state or condition of a process
2. A controller calculating an action based on this measured value against a pre-set or desired value (set point)
3. An output signal resulting from the controller calculation which is used to manipulate the process action through some form of actuator
4. The process itself reacting to this signal, and changing its state or condition.

Figure 2

In industrial process control, the Process Variable (PV) is measured by an instrument in the field and acts as an input to an automatic controller which takes action based on the value of it. Alternatively, the PV can be an input to a data display so that the operator can use the reading to adjust the process through manual control and supervision.