Automation in Environmental Engineering

dr inż. Patryk Wójtowicz

Signals

•  Signal is a variable that conveys the value of a parameter (information) between the elements of a system

Signal types by physical form

General classification of signals

Signal

Analogue

Discrete Pulse

Analogue signals

• Analogue signals are continuous in nature •  Analogue signals may vary over some finite

range and may have any value within that range (determined by a physical phenomenon)

• Most process signals are analogue

Discrete signals

• Discrete signals have two or more states. At any point in time, may only exist as one or other of those states

• Discrete signals are used to convey status information such as auto/manual, on/off, open/closed, running/stopped

Analogue vs digital

Pulse signals

•  Pulse signals consist of trains of pulses, each pulse being equivalent to a fleeting discrete signal.

•  Pulse signals are typically associated with rotary devices such as turbine meters or agitator shafts

•  A known number of electrical pulses are generated with each revolution. Counting of the pulses with respect to time yields an average shaft speed

Standard signal ranges

•  Signals have some standard ranges. • Compatibility of input and output signals

enables systems to be configured from off-the-shelf elements

barg – gauge pressure

Signal interpretation

•  Knowledge of the behaviour or calibration of the elements and of the nature of the signal is necessary to determine the value of the parameter being transmitted

• Determine temperature reading assuming linear calibration (see diagram below)

Signal interpretation

•  Analogue signals have a non-zero lower limit to their range to enable zero valued signals to be distinguished from faulty ones (4 – 20 mA signal while 0 mA = fault)

Signal processing - Linearity

•  Signals which are linear are easy to interpret

•  In practice non-linear input-output relationships prevail

•  A basic strategy of control system design is to eliminate nonlinearities

Linearization - example

Orifice (ISO 5167)

orifice flow coefficient

mass flow rate

General instrument or function symbols

General instrument or function symbols contd

Symbols according to DIN 30600 and ISO 14617

Symbols according to DIN 30600 and ISO 14617

Symbols according to DIN 30600 and ISO 14617