Elements of Controls - Sensors - Page 1 2 3 4 5 6 7 8 9 10

Sensors

 

 

The sensor or detecting element of a control system sends a signal to the comparison device.

This is a negative feedback signal that is compared with the Desired value or Set point.

The comparison device compares the feedback signal from the sensor with the set point setting and works out the difference between them – this is the error signal.

The error signal is then fed to the controller.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


The siting of sensors is critical to achieve good control.

In pipework or ductwork, sensors must be arranged so that the active part of the device is immersed fully in the fluid and that the position senses the average condition.

 

Sometimes averaging devices are used to give an average reading of measured variable e.g. an averaging stat measuring temperature.

 

In sensing space conditions, the sensing device must not be in the path of direct solar radiation or be located on a surface which would give a false reading such as a poorly insulated external wall.

 

 

Local effects from heat sources, radiators or office equipment for example will also give unsatisfactory results and in the case of temperature sensors give higher measurements than the true values.

 

Temperature Sensors

 

Thermal expansion of metal or gas and a change in electrical characteristics due to temperature variation are common methods of detection.

Bi-Metallic Strip

The figure below shows a simple bi-metallic type thermostat with closing point contacts.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Sealed Bellows

 

The sealed bellows type is filled with a gas, vapour or liquid, which responds to change in temperature by variation in volume and pressure causing expansion or contraction.

 

 

 

 

 

 

 

 

 

 


Bulb & Capillary Sensor

 

Bulb and capillary elements are used where temperatures are to be measured in ducts, pipes, tanks or similar locations remote from the controller.

 

The bulb is filled with liquid, gas or refrigerant depending on the temperature range required.  

Expansion of the fluid in the heated bulb exerts a pressure which is transmitted by the capillary to the diaphragm and there translated into movement.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Electrical / Electronic Sensors

 

It is common to measure temperature, pressure or flow and convert these parameters to electrical signals, especially in building energy management systems (BEMS).

This measurement and conversion to an electrical signal is the function of the sensor, transmitter and transducer.

The sensor responds to a change in a measured parameter, the transducer changes the sensor signal to an electrical signal (e.g. a pressure into a voltage) and the transmitter is the electronic circuitry to enable a suitable strength voltage proportional to the sensed parameter to be sent to a controller or BEMS outstation.

 

Electronic sensors have no moving parts.

The resistance bulb type, normally a coil of nickel, copper or platinum wire around a core, produces a variation in electrical resistance with change in temperature.

Thermistors, which are semi-conductor devices, also produce a change in resistance so that resistance decreases with increase in temperature.

 

Thermocouples have two dissimilar metal wires joined at one end.

The output is a voltage proportional to the temperature difference between the junction and the free ends.

 

Dead-band thermostats have been developed to reduce energy use.

The principle is that for comfort applications a variation in temperature of about 3oC to 4oC is acceptable for some locations.

This type has a wide dead-band through which a change in temperature produces no change in the output.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Humidity Sensors

 

Fabrics which change dimension with humidity variation, such as hair, nylon or wood, are still in use as measuring elements but are unreliable - hygroscopic plastic tape is now more common.

 

These media may be used to open or close contacts or to operate a potentiometer.

For electronic applications, use is made of a hygroscopic salt such as lithium chloride, which will provide a change in resistance depending upon the amount of moisture absorbed. Solid state sensors use polymer film elements to produce variations in resistance or capacitance.

Thermoset polymer-based capacitive RH sensors directly detect changes in “relative saturation” as a change in sensor capacitance. Capacitive RH sensors dominate both atmospheric and process measurements and are capable of operating accurately down to 0% RH. Thermoset polymer-based capacitive sensors allow higher operating temperatures and provide better resistivity against chemical liquids and vapours.

 

Pressure Sensors

 

Bellow, diaphragms and bourdon tubes are typical sensing devices used. Bellows and diaphragms acting against a spring are quite common devices. This equipment can be sensitive to small changes in pressure, typically 10 Pascals.

The pressure sensing motion may then be transmitted directly to an electric or pneumatic control device.

In electronic systems, the diaphragm or the sensing element is connected to a solid state device which when distorted changes resistivity. This is known as the piezo-electric effect.

 

Flow Sensors

 

In water systems the most common method used to detect fluid rate is to measure pressure difference across a restriction to flow, such as an orifice plate or a venturi.

The Venturi meter is a device for measuring discharge in a pipe.

It consists of a rapidly converging section which increases the velocity of flow and hence reduces the pressure.

 

It then returns to the original dimensions of the pipe by a gently diverging 'diffuser' section.

By measuring the pressure differences the discharge can be calculated.

This is a particularly accurate method of flow measurement as energy losses are very small.

The diagram below shows a venturi meter, practically the meter is incorporated into an orifice plate or regulating valve.

 


         

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Extensive use is made of calibrated valves or regulating valves especially during commissioning.

Various devices are available to sense air velocity in ductwork.

The Pitot - static tube can be used to measure total pressure and static pressure.

The velocity can be found from the following formula:

 

Total pressure       =       Static pressure  +  Velocity pressure

Velocity pressure (V.P.)  =       Total pressure  -   Static pressure

                             V.P.   =       0.5  x   ra  x  v2.

Where:             V.P.     =          Velocity pressure (N/m2)

ra         =          density of air (about 1.2 kg/m3)

                        v          =          velocity of air (m/s)

Therefore     v        =       (  V.P.  /   ( 0.5  x   ra )  )0.5

 

The diagram below shows a typical Pitot-static tube.

The tube facing the air stream is called the facing tube and measures the total head.

The static head is obtained from the small tappings into the annulus.

The head difference as measured in the manometer is therefore the Velocity head. To convert from head to pressure use the following formula:

Pressure = density of liquid in manometer x acceleration due to gravity (9.81m/s2)  x  head (meters).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


In large ducts an array of sensing devices is required to obtain an average air velocity.

 

 

Another less accurate method of determining air velocity is to use a hot wire anemometer.

This tends to be less accurate than using a Pitot-static tube and is better suited to air movement measurements inside rooms or outside.

 

Paddle blade switches are used in water circuits, normally as a safety feature.

A paddle blade flow switch could be used, for example, in a chilled water circuit connected to the evaporator of a refrigeration plant so that the plant would not start until the switch sensed that water flow was established.

This is to ensure that damage to equipment is avoided if water in the evaporator freezes during a no-flow condition.

 

Elements of Controls - Sensors - Page 1 2 3 4 5 6 7 8 9 10