6Sensors in Buildings

Whatever the decision-making process is, it is necessary to know the information about the state of the system and about the parameters that influence it. For people, this information is provided by human senses. In technical practice, to obtain an information about any parameter, we use sensors or transducers, which are able to measure certain physical or technical quantities and convert them into signals, which can be transmitted over long distances and further processed in measurement or control systems. Control systems perform evaluation of the information and they are able to transmit other signals to actuators. Information can be carried over by mechanical parts, gas or liquid, but in most applications, electrical signal is used, and therefore we will refer to electrical signals in the following text.

A sensor is basically a measuring instrument that converts the measured quantity to binary or analog electrical signal, which can be evaluated.

  1. Light barrier. It is an opto-electronic device consisting of a light source and a photodiode. Light barrier evaluates whether there is any obstacle in the space between the source and the photodiode.
  2. Strain gauge scale. The sensor is based on a strain gauge, which is basically a very thin electrical resistance wire attached to a component that allows its deformation. After deformation, the electrical resistance is changed. This resistance change is related to the strain by the quantity known as gauge factor.

An important parameter of each sensor is its sensitivity and measuring range. It is crucial for sensors that their design should not affect the measured value of the respective physical quantity. Due to this condition, sensors are becoming smaller and their sensitivity higher. Technological progress allows production of sensors in a microscopic scale, such as microsensors using MEMS (Microelectromechanical Systems) technology. In most cases, microsensors provide significantly higher signal feedback and sensitivity compared to macroscopic approaches. Recently, NEMS (NanoElectroMechanical Systems) technology has been also introduced to the markets.

In buildings there are used sensors that provide input information to a logic circuit or state controller. The following sensors are the most common ones:

Different types of readers for access control (chip or smart card, fingerprint reader, biometric readers, etc.).

Light barriers that interrupt the operation of machines if any object or person enters the working area of the machine.

Electromagnetic contacts for doors and windows, which determine their status. They raise an alarm when unauthorized opening is detected or when the doors are not closed in the prescribed time range. The sensors form a part of electronic security systems.

Fire alarms and smoke detectors, which are a part of fire protection systems.

Electromagnetic scanners, which are a part of the protection against shoplifters or prevent armed people from entering into restricted areas.

Motion sensors, which are most frequently used for automatic lighting or for security systems.

In buildings there are also sensors used for measurement of physical quantities and transducing them into electric signals, which are brought to the inputs of control logic With respect to the measurement physical principle, the following types of sensors are most common:

Dependence of electrical resistance on temperature

Any material changes its electrical resistance depending on the temperature. This principle is used for temperature measurement in resistance thermometers, also called resistance temperature detectors (RTDs). Because the resistance thermometer is in principle a passive element, power supply is necessary to operate it.

Resistance thermometer is basically a wire or a layered resistor. Materials with favourable characteristics are used, such as nickel (Ni) or platinum (Pt). Labelling of the sensors on the market consists of chemical symbols and numbers, e.g. Pt100 – it means a platinum sensor, which has electric resistance of 100 Ω at 0° C.

The change of electrical resistance is directly proportional to temperature. For platinum, change of the temperature by 3° C corresponds to 1 Ω of resistance change. For nickel, the change is twice higher, and therefore 3° C of temperature change correspond to 2 Ω of resistance change. Platinum thermometers are used in the range between -220 °C and 900 °C; the range for nickel thermometers is smaller, from -60 °C to 180 °C.

Thermoelectric effect

Thermoelectric effect is a physical process that generates voltage as direct conversion of temperature differences in a junction of the two metals, and vice-versa. Connection of two different metals is called a thermocouple, and it is an active sensor (i.e. it generates electricity).

A thermocouple is constructed as a combination of two metals, either iron (Fe) & constantan (an alloy of copper and nickel in a ratio of typically 55% copper and 45% nickel) for temperatures from -220 °C to 750 °C, or platinum (Pt) & rhodium (Rh) for measuring temperatures from 0 °C to 1600 °C. Because of the large measurement range and ability to measure high temperatures, thermocouples are widely used in combustion chambers of power plants. Due to low generated voltage (several μV/K), it is necessary to add an amplifier to the measuring circuit.

To prevent creation of another thermocouple sensor in the circuit, it is necessary to connect the sensor with conductors made from the same material and to add a compensation resistor to the circuit.

image
Fig. 6.1: Diagram of a thermocouple thermometer

To obtain stable measurement, standard configuration of a thermocouple contains reference junction, which is in fact another thermocouple placed in the environment with reference temperature, e.g. 20 °C. Then the measured value is the difference of temperature between the measured and the reference location. The measurement circuit contains an amplifier and a voltmeter – see Fig. 6.

Dependent electrical resistance of semiconductor elements

This is again a measuring of electrical resistance dependent on the temperature, but in this case semiconductor materials are used. Measuring range is from -50 °C to 180 °C. These sensors are called thermistors. Thermistors are classified in two types – NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) thermistors. NTC thermistors have a negative trend characteristics, which means that the electrical resistance decreases with increasing temperature. PTC thermistors, in contrast, have a positive trend characteristics, which means that with increasing temperature the electrical resistance increases as well.

The advantage of these sensors is shorter response time and higher sensitivity (about 20 times) in comparison to RTD sensors.

  • small measurement range,
  • they don’t have a linear characteristic,
  • calibration curve is time-dependent.

The measured data can be substantially influenced by positioning of a sensor. The measured temperature depends also on type of object or media, in which the measurement is performed. First of all, the measurement should not be influenced by external factors, which can be of many types – for example, unwanted cooling of the thermometer, influencing of the measurement by direct sunlight, improper placement in the stream of a medium, etc.

Principles of pressure measurement

Instruments for measuring pressure are called manometers. Measurement of the pressure is not provided directly as it is necessary to convert the pressure to another physical quantity. Pressure can be converted to displacement of liquid in manometer tubes, or deformation of the measuring element causes the change of electrical or optical parameters that can be measured. The amount of pressure then corresponds to displacement or deformation of the measuring element.

Nowadays, the most common method of pressure measurement is to measure the deformation of a diaphragm, which is equipped with strain gauge.