NAVIGATION
Sensors
Explained
What is a sensor?
A sensor is a device that detects a physical quantity and converts it into a signal that can be measured and analyzed. Sensors are used in various applications, including scientific research, industrial process control, and consumer electronics.
There are many types of sensors:
- Temperature sensors measure temperature and are used in thermostats, temperature control systems, and medical devices.
- Pressure sensors measure pressure and are used in weather stations, tire pressure monitors, and process control systems.
- Motion sensors detect motion and are used in security systems, gaming controllers, and automotive safety systems.
- Light sensors detect light and are used in cameras, cell phones, and ambient light sensors.
- Sound sensors detect sound and are used in microphones, voice recognition systems, and noise level monitors.
Sensors play a crucial role in modern life, providing the data and information used to control and monitor various systems and processes.
Channel Types
Analog Input Channel
Analog Input Channel are used to measure analog sensor signals. Analog signals are typically one of two types: single-ended or differential. Single-ended is the most common type of signal, where the signal is transmitted on a single wire or line, with the signal referenced to ground. Differential signaling is a type of signal transmission where the signal is sent on two wires, with the signal being the difference between the two wires. Single-ended signaling is the simplest and most commonly used method of transmitting electrical signals over wires, whereas differential signaling is typically used in applications where noise immunity is a major concern, such as in higher-speed systems. Additionally, single-ended signaling transmits a varying voltage on a single wire, while differential signaling transmits two complementary voltage signals in order to transmit one information signal.
Digital Input Channel
Digital input channels are used to record digital status signals, which are signals that represent the status of a system or device. These signals are usually represented in binary form as either high (1) or low (0). They can be used to indicate the on/off state of a system or device, the presence or absence of a signal, or the success or failure of an operation. Digital status signals can also indicate a signal’s direction, such as up or down, as well as the level of the signal, such as high (1) or low (0).
Digital Output Channel
A digital output channel is the relay output of the module. Status signals can be set automatically by the module according to the values of other channels, or the state of the output can be set manually via a bus.
Arithmetic Channel
Arithmetic Channel allows you to perform calculations with the actual values of other channels and with constant values. The results of the calculations are assigned to the arithmetic channel; thus, arithmetic channels can also be used to make calculations with other arithmetic channels.
Alarm Channel
An Alarm Channel can monitor another channel and generate an alarm message if any of up to four definable thresholds is exceeded. The alarm message can then be read via bus.
Setpoint Channel
The Setpoint Channel’s value can be set via bus. This enables you to set a value via the bus that another arithmetic channel can use for further processing (e.g., to set a factor for measurement by the user).
Measurement Types
Voltage Measurement
Voltage measurement is the process of measuring the potential difference between two points in an electrical circuit. It is usually expressed in volts (V) and is a measure of the electrical potential energy per unit charge between the two points. Voltage measurement is an important part of electrical engineering, as it is used to troubleshoot electrical circuits, determine the power output of components, and detect faults in wiring.
With the single ended type of measurement the voltage to be measured is connected between an analog input and analog ground. The measurement voltage may not exceed the voltage range. Voltage at high potentials can be measured with High Isolation / High Voltage measurement modules, e.g., A121.
Current Measurement
For measurements of current, the source of electricity is connected to an analog input and the analog ground. For the measurement, the required load on the current source is regulated by an internal resistor with a value of 100Ω. The maximum power of this shunt is limited to 0.25W, resulting in a measuring range of up to 25mA maximum.
If higher currents need to be measured, an external resistor that is connected in parallel to the source of current should be used. Terminals are connected to the analog voltage input and analog ground. The power of the external shunt must be adapted to the source of current to be measured in order to limit the voltage at the analog input. The analog input is configured as the voltage input, and the voltage is divided by the external resistor. The precision of the current measurement with an external shunt depends on the accuracy of the resistor used.
For current measurements at high potentials, High Isolation / High Voltage measurement modules can be used, such as the A121.
Resistance Measurement
Resistance measurement is carried out by measuring the voltages across a current-carrying resistor. The voltage drop caused by the resistor is measured using a resistance sensor. The current required for the resistance measurement is provided by the internal supply of the module.
For this purpose, the sensor module internally connects a supply point to the analog measurement input via a reference resistor. The voltage drop across the resistor is then used as a reference for further signal processing by the module. The value of resistance of the sensor can be calculated from the input signals as a multiple of the reference resistor.
For resistance measurements at high voltage potentials, High Isolation / High Voltage measurement modules can be used, such as the A121.
2-Wire:
This method is the most commonly used due to its simplicity and method of operation compared to the 4-wire resistance measurement. Accurate measurements above 100 kΩ can be easily obtained. The main disadvantage of this method is that it is not able to correct for the lead resistance of the component being tested.
4-Wire:
For precise measurements below 100 kΩ, a 4-wire method is more reliable than a 2-wire method. It requires more cabling, but the trade-off with increased accuracy is necessary for certain applications. For example, when the resistance of a component we want to measure is located a distance from our measuring device, the wire used between the component and the measuring device can introduce unwanted resistance. Having a 4-wire setup eliminates the resistance created by the measurement wires, resulting in more accurate measurements. This method is referred to as the Kelvin method.
Potentiometer:
Also known as a pot, a potentiometer is a three-terminal resistor that incorporates a sliding contact that behaves as a voltage divider. A potentiometer is used to measure electric potential (voltage) by providing a voltage output that is less than the voltage input. The three-terminal resistors on a potentiometer are linear circuits that can provide smooth transitions of voltage levels, which can be either rotary or linear. Any device that requires a smooth variation in current can make use of the functionality of a potentiometer.
The construction of a potentiometer consists of a resistor body, terminals at the end of the body where electrical connections can be attached, and a wiper arm that makes electrical contact as it moves across the resistor body. The resistive body of the potentiometer is available in various values, and it can come as a fixed resistive body or as a variable resistance body.
Resistance Bridge:
Bridge connections consist of two arms, each with two resistors. The resistance bridge is powered by the voltage output. The quantity measured by the bridge is the ratio of the bridge voltage to the voltage between the two resistance arms. Various measuring ranges are possible, and most bridges have four adjustable resistors, so that the bridge can easily be balanced using the controllable resistor. Changes in the sensor signal will affect the fourth resistor, causing a change in the measured quantity.
Strain:
Strain is the amount of deformation on a body caused by an applied force. Strain is a fractional change in the length of the material, and is usually expressed as a dimensionless unit such as micro-strain (μstrain). Strain can be measured either in terms of positive or negative strain. The magnitude of change is usually small in practical applications.
Strain Gage:
Using strain gages is one of the most common methods of measuring strain on materials. The electrical resistance of the strain gage varies proportionally to the amount of strain on the device; the strain is transferred directly onto the gage, and the strain is measured as a linear change in electrical resistance. The gage factor of a strain gage is a measure of its sensitivity to strain; it is calculated as the relative change in electrical resistance divided by the relative change in length (mechanical strain). Gantner measurement modules take the gage factor into account when calculating strain. Strain measurements are usually measured in millistrain, so accurate measurement of small changes in resistance is necessary. To measure these small changes, strain gages are usually used in a bridge configuration with an excitation voltage.
Temperature Measurement
Different Temperature Sensors:
- Thermocouples are two dissimilar metals that produce a voltage when they are at different temperatures, which is then measured to determine the temperature.
- Resistance temperature detectors (RTDs) consist of a fine wire coil wrapped around a ceramic or glass core, whose resistance changes with temperature. The resistance is measured to determine the temperature.
- Thermistors are resistors made of a special material with high resistance at low temperatures and low resistance at high temperatures. The resistance is measured to determine the temperature.
Temperature Measurement with Thermocouples:
Thermocouples consist of two thermoelectric wires made from different materials (such as platinum and platinum-rhodium) that are welded together at one end. When the contact point and the other ends of the thermoelectric wires have different temperatures, a thermoelectric voltage is produced at the contact point. This voltage is proportional to the temperature difference between the contact point and the ends of the wires. This voltage can be measured and used for temperature measurement purposes. In order to measure the actual temperature, a reference temperature of a known temperature must also be determined. This is either done through internal cold junction compensation (where the reference temperature is measured at the same location as the temperature being measured) or external cold junction compensation (where the reference temperature is measured in a different location). High voltage potentials can be measured with high isolation/high voltage measurement modules, such as the A124.
To measure temperature with internal cold junction compensation, an additional temperature probe is used to measure the reference temperature. For the Q.series X modules, a cold junction compensation terminal block with an integrated Pt1000 temperature probe is used. The temperature at the transition point is then determined and the voltage produced by the thermocouple is corrected depending on the thermocouple type.
For external cold junction compensation, a second thermocouple of the same type is needed and connected in series with the first thermocouple. The polarity is chosen so that the thermoelectric voltages subtract. The second thermocouple is located at a fixed reference point. The Q.series X module then calculates the temperature at the measuring point based on the linearization curve. The Q.series X module requires the reference temperature being used in order to do this calculation.
Temperature Measurement with Pt100 and Pt1000:
Pt100 and Pt1000 measurements are possible in 2, 3, and 4 wire configurations. With Pt100/Pt1000 measurements in 2-wire form, the supply lines cause an additional drop of voltage, which may distort the measuring result and influence the accuracy of the measurement. Therefore, it is important to use low impedance leads as much as possible when making Pt100/Pt1000 measurements in 2-wire form, and to ensure that the leads are properly connected to the sensor module and the sensor itself. With Pt100/Pt1000 measurements in 3 or 4-wire forms, the voltage drop is picked up directly at the sensor, so the supply lines do not affect the measuring results. The 4-wire form also compensates for the effects of non-symmetrical cable resistances. High Isolation / High Voltage measurement modules, such as A121, can be used to measure temperatures at high voltage potentials.
Charge Amplifiers
A charge amplifier is an electronic device that amplifies a charge signal. It is commonly used to amplify the charge produced by a piezoelectric sensor, which generates a charge when it is subjected to mechanical stress or pressure. The amplified charge signal can then be measured and analyzed to determine the magnitude of the mechanical stress or pressure applied to the sensor. Charge amplifiers are used in various applications, such as vibration monitoring, structural health monitoring, and load measurement.
Vibration Measurement (Accelerometers)
There are several types of vibration sensors:
- Piezoelectric accelerometers use a piezoelectric crystal that generates a voltage when subjected to mechanical stress or pressure.
- Capacitive accelerometers use a flexible membrane that is positioned between two stationary electrodes. As vibration moves the membrane, it changes the capacitance between the electrodes, which can be measured and used to determine the magnitude of the vibration.
- MEMS accelerometers use microelectromechanical systems (MEMS) technology to measure acceleration. They are small, lightweight, and inexpensive, making them popular in consumer electronics.
Vibration sensors, also known as accelerometers, measure vibration or motion. They are commonly used to monitor the condition of mechanical systems, such as engines, bearings, and gears, and detect earthquakes and other types of structural movement. Vibration sensors work by detecting changes in acceleration, which is a measure of how quickly the velocity of an object changes.
Vibration sensors are used in various applications, including predictive maintenance, structural health monitoring, and vibration analysis. They are an important tool for understanding the condition and performance of mechanical systems and can help identify problems before they lead to failure.
An accelerometer measures acceleration forces, both static and dynamic. An accelerometer can have either analog or digital outputs. An analog output accelerometer typically has a continuous output voltage directly proportional to the acceleration, while a digital output will typically be in the form of a PWM (a square wave determines the frequency, and the amount of time the voltage is on the high level is proportional to the acceleration).
Accelerometers are highly used in the automotive industry to measure a vehicle’s acceleration and provide engine performance numbers that can be used in comparison matrices. They can also measure the amount of vibration within a system, which is an important variable that determines a system’s health and safety standards.
Fiber Optics
Fiber optic sensors use fiber optic cables as the sensing element. They operate by measuring changes in the properties of light as it travels through the fiber optic cable.
There are several types of fiber optic sensors:
- Intensity-based fiber optic sensors measure the intensity of light transmitted through the fiber optic cable and are used to detect changes in temperature, strain, pressure, and other physical quantities.
- Interferometric fiber optic sensors use the interference of light waves to measure changes in the distance between two points. They are often used to measure displacement, strain, and temperature.
- Time-of-flight fiber optic sensors measure the time it takes for a pulse of light to travel through the fiber optic cable and are used to measure distance, velocity, and acceleration.
4. Polarization-maintaining fiber optic sensors use specialized fiber optic cables that maintain the polarization of light as it travels through the cable. They are used to measure temperature, strain, and other physical quantities.
Fiber optic sensors are used in various applications, including structural health monitoring, oil and gas exploration, and medical diagnostics. They are known for their high sensitivity, fast response time, and ability to operate over long distances without signal loss.
Advantages of fiber optic sensors include: high-voltage isolation, immunity to electromagnetic and radiation interference, intrinsic safety, resistance to lightning strikes, and the ability to operate in extreme temperature conditions (both high and low).