What is Automation?
Automation is a system concept utilizing the instrumentation system to control a process or sequence of activities in an automatic manner or without human intervention.
The process control loop:
Control Loop – is an organized instrumentation system set up from the process variable sensor, transmitter, controller, then back to the process through a final controlling element. It is consider as the basic and the simplest form of an instrumentation control system.
Process Control Strategies:
1. Closed Loop or FeedBack Control – Type of control loop with feedback to the process. Correction is base on the difference between the actual process value and the desired process value.
2. Open Loop Control or FeedForward Control – No feedback. Correction is based on the difference between the actual disturbance and the desired disturbance value.
Elements of Control Loop:
1. Measuring or Sensing Element – sensor is responsible in directly measuring the actual process value (PV) for transmission to the control room.
Sensor characteristic: Several factors determine how well sensors respond to a variable, including response time, accuracy and precision.
Sensor response time: Sensor do not respond immediately, a period of time is required for sensors to respond to change. This is called the Respond Time.
Response time is dependent on several factors including the type of sensor, and the sensor proximity to the measured variable.
For the most part, the requirements of the process determine the degree of response that is needed.
Sensor accuracy and Precision:
Accuracy and precision are also characteristics of a sensor. These have different meanings in the the context of process control.
Accuracy – is used to denote how closely a sensor indicates or read the actual value of the controlled variable.
Precision – is used to designate how consistently a sensor, respond to the same input value. Also called repeatability.
Sensor output: sensor output (not standard) depends upon the sensor type, the variable being measured and the technology being used.
Common example:
TC – Voltage
RTD – Resistance
Displacer – Torque
Turbine – Pulse
Quartz – Frequency
In general, sensors need to be reliable. The reliability of the sensor is determined by three characteristic it is the response time, accuracy and precision.
2. Transmitting Element – (Transmitter)
It is responsible for transmitting the measured process value from the sensor to the controlling element to the control room for comparison with the desired value.
Forms of transmitter output signal:
1. Analog signal – pneumatic and electric signal
Pneumatic : 3 to 15 psi
Electrical : 4mA to 20mA or 1Vdc to 5Vdc
2. Digital signal – two discrete value “ON” and “OFF” or “1” and “0”
Transmitter output:
Transmitter should be adjusted during calibration so that the transmitter output will vary its full range in proportion to the full range of the input.
Transmitter Accuracy:
Transmitter needs to be both accurate and precise depending upon the application, However linearity is highly required.
Transmitting sensor output signal:
A sensor is a device used to measure physical parameters such as pressure, temperature, level, flow, quality, etc. In order for the information to be more useful in controlling a physical process, the information must be transmitted to another readout/ control devices which directly or indirectly manipulates process parameters.
Transmission could be in the form of Analog (electric or pneumatic) signal format or Digital signal format.
Transmission by analog pneumatic format is the oldest method of transmitting process value using 3 – 15 psi convention.
whereas: 3 psi equals to zero % and 15 psi is equal to 100%
Electrical signal transmission:
Transmission by electrical analog format could either be in the form of voltage or current signal. However, for some valid reasons, transmitting information by voltage is not as practical as transmitting it by using current.
whereas: for Voltage 1-5V and for Current 4-20mA
However, transmitting voltage over long distance correspondingly lower voltages at the receiving end due to wiring and interconnection resistances. ” voltage drop ”
Why not use voltage signal for transmission?
Transmitting sensor’s output as a voltage over long distances has several drawbacks. Unless very high input impedance devices being used, transmitting voltage over long distances correspondingly lower voltages at the receiving end due to wiring and interconnection resistances. However, High input impedance instruments can be sensitive to noise pick up since the lengthy signal carrying wires often run in close proximity to other electrically noisy system wiring. Shielded wires can be used to minimize noise pick up, but their high cost may be prohibitive when long distances involved.
Why current signal transmission?
The 4 to 20mA current loop is a common method of transmitting sensor output in many industrial process monitoring or control applications. Transmitting sensor output via current loop is particularly useful when the information has to be sent to a remote location over long distances (1,000 ft or more..)
The loop information is straightforward, a sensor output current of 4 mA normally representing the sensor’s zero level output and 20 mA for the full scale output. Then a receiver or controller at the remote end converts 4-20mA current back into 1 – 5Vdc, which in turn can be processed by a computer/ display or control module.
Wiring resistance:
Sending current over long distances produces voltage losses proportional to wiring’s length. However, these voltage losses also known as loop drop do not reduce 4-20mA current as long as the transmitter and loop supply can compensate for these drops. The magnitude of the current in the loop is not affected by voltage drops in the system wiring since all the current (i.e electrons) originating at the (-) terminal of the loop power supply has to return back to its positive (+) terminal.
Roldan B. Sebastian 