FIELD DEVICES, SENSORS AND CONTROL DEVICES

This article is about the field devices, sensors and controlling devices related to a building management systems of a building.

TYPES OF FIELD DEVICES

The BMS shall be capable of system monitoring, alarming, point trending, point adjustments, and overrides. Field devices such as sensors, actuators valves play a vital role in the BMS to function above operations. Field devices can be divided in to four categories

· Digital input field devices

· Analog input field devices

· Digital output field devices

· Analog output field devices

DIGITAL INPUT DEVICES

Differential Pressure Switch

This is an example of Digital Input. This is usually installed across a fan or a filter. If the contact is closed the DDC can detect either the fan is running or the filter is clogged.

Differential Pressure Switch

Smoke Detector

Smoke Detector installed in the duct to allow the controller to stop the Air Handling Unit in case of Fire.

Contactor

Auxiliary contact from contactor is used to indicate if the contactor is energized or not.

ANALOG INPUT DEVICES

Temperature Sensors

· Temperature Sensors/ Setpoint Modules for Rooms

· Temperature Sensors for Ducts

· Immersion temperature Sensors for water Pipes

Temperature Sensors

Humidity Transmitters for Rooms and Ducts

Humidity Transmitters for Rooms and Ducts

Differential Pressure Transmitters for Clean Rooms

Differential Pressure Transmitters for Clean Rooms

DIGITAL OUTPUT DEVICES

Digital output is a relay output controlled by DDC

· To energize contactor in the motor control centre in order to start Fan or Pump

· To start a condensing unit when using Air Handling Units.

· To energize Heater Battery Stages via contactors

Digital output devices

ANALOG OUTPUT EXAMPLES

DDC produce a voltage signal ranged from 0 to 10 Volt. According to the value the controlled devices respond.

· To Control the Fan Speed via inverter (Speed Drive)

· To Modulate water Valve

· To Modulate Damper Motor and control Air Flow

Analog output

examples

DIRECT DIGITAL CONTROLS

DDC use electrical pulses to send signals and interface directly with microprocessors and microcomputers (PCs).

Advantages

• Extremely flexible because the control algorithms are implemented in software instead of hardware. Changes are made by keyboard entries, not by adding or modifying hardware elements.

• Very precise; recalibration is not necessary.

• No controller drift.

• Costs have dropped dramatically for DDC components in recent years.

• Analog sensors may still require periodic recalibration, but early reliability problems have been cured.

Disadvantages

• Not well understood by many maintenance people and facility managers.

• Different programming languages also a problem. BACNET should help this concern.

BACNET: Building Automation Central Control System Network