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Sensors Explained


Channel Types

Analog Input Channel – Used to measure analog sensor signals. In the column Sensor the type of connected sensor is selected (voltage, current, resistance, temperature) and in the next column the corresponding Type of Measurement is set.
Digital Input Channel – Used to record digital status signals. In the column Type of Measurement it is only possible to select the measurement function state recording.
Digital Output Channel – This is the relay output of the module. Status signals can be output automatically by the module according to values of other channels or it is possible to set the state of the output via bus.
Arithmetic Channel – With this channel it is possible 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, therefore arithmetic channels can also be used by other arithmetic channels for calculations.
Alarm Channel – An Alarm Channel can be used to monitor another channel and to generate an alarm message if one of up to 4 defineable thresholds are exceeded. The alarm message can be read via bus.
Setpoint Channel – The value of this channel can be set via bus. This way it is possible to set a value via bus which can be used by another arithmetic channel for further processing (e.g. to set a factor for measurement by the user).


 

Introduction

Voltage Measurement

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.

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 100 Ω. The maximum power of this shunt is limited to 0.25 W, resulting in a measuring range of up to 25 mA 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 th analog voltage input and analog ground. The power of the external shunt has to 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. The voltage has to be divided by the external resistor.

The precision of the current measurement with external shunt depends on the accuracy of the resistor that is used.

Resistance Measurement

Resistance measurement is carried out by means of measurements of voltages at a current carrying resistor. In this case the occuring voltage drop is measured via the resistance sensor. The current feed required for the resistance measurements provides the internal supply of the module.

For this purpose the sensor module connects a supply point internally with the analog measurement input via a reference resistor. The drop of voltage via the resistor is required 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.

Resistance Bridge

Bridge connections consist of 2 arms with two resistors each. The resistance bridge is supplied by the voltage output.

The quantity to be measured with resistance bridges is the relation between bridge voltage and the voltage between the two resistance arms. Various measuring ranges are possible.

Mostly there are four variable resistors in resistance bridges, so that the resistance bridge can easily be balanced via the controllable resistor. Variations of the sensor signal characteristically influence the fourth resistor and cause a change in the quantity to be measured.

Potentiometer Measurement

Potentiometer measurements are measurements with voltage relations, the division ratio of which can be adjusted. The quantity to be measured is the relation between the adjusted resistance and the combined resistance of such potentiometer.

Temperature Measurement with Thermocouples

Thermocouples consist of two thermoelectric wires made of different materials that are welded to each other at one end. If the contact position and the other ends of the thermoelectric wires have different temperatures, a thermoelectric voltage appears at the contact position of both thermoelectric wires. This voltage is largely proportional to the temperature difference. It can be measured and can be used for temperature measurement purposes.

Since thermocouples can only measure a temperature difference, a terminal temperature of known temperature reference also has to be determined. In the first case this is called internal cold junction compensation, in the second case external cold junction compensation.

Temperature Measurement with Pt100 and Pt1000

Pt100 and Pt1000 measurements in 2, 3, and 4 wire configurations are possible. With Pt100/Pt1000 measurements in 2-wire form, the supply lines cause an additional drop of voltage, thus distorting the measuring result and influenceing the measuring accuracy. Thefore it is necessary to pay attention especially with Pt100/Pt1000 measurements in 2-wire form to use low impedance leads as possible to the sensors and to make sure that the leads are well connected with the sensor module and the sensor. With Pt100/Pt1000 measurements in 3 or 4-wire forms the voltage drop is picked up directly at the sensor, so that te supply lines do not influence the measuring results any longer. The 4-wire form compensates for the influence of non-symmetric cable resistances.


 

Details

Accelerometers

An accelerometer is a device that will measure acceleration forces, static or dynamic. Knowing the amount of static acceleration helps determine the angle an object is at relative to its position on earth. Knowing the dynamic acceleration of an object can help analyze how the object is moving. An accelerometer has either analog or digital outputs. An analog output accelerometer typically has a continuous output voltage that is directly proportional to the acceleration. 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, providing performance numbers of the engine that can be used in comparison matrices. This type of sensor can also measure the amount of vibration within a system, which is an important variable that determines a system’s health and safety standards.

 

Current Measurements

Measuring current with an External Shunt: Current measurements are carried out by measuring the voltage drop across a resistance of known value (shunt resistance). In the Q.bloxx modules that are capable of direct current measurement, this is a resistor with a value of 50 Ω. Currents up to 25 mA are possible (max. shunt power dissipation is limited to 0.25 W). Higher currents would need an external shunt which is looped into the line that needs to be measured. The permissible power dissipation of the external shunt has to be higher than the power dissipation occuring at the shunt where the current is being measured. Also, the voltage drop across the resistor must not exceed the permissible input voltage that is rated for the analog input. Configure the analog input channel as a voltage input and divide the measured voltage by the external resistance.

NOTE: the error in the current measurement using an external shunt depends on the accuracy of the resistor being used.

 

Strain Gage Measurements

Strain: The amount of deformation on a body due to an applied force is known as strain. More specifically, strain is the fractional change in length of the material. Strain can be measured in terms of positive and negative strain. The measurement of strain is dimensionless typically expressed as micro-strain (μstrain); since the magnitude of change is very small in practical applications.

Strain Gage: Using strain gages are one of the most common methods of measuring strain on materials. The electrical resistance of this device varies in proportion to the amount of strain on the device. The strain gage is attached directly onto the test material; therefore the resulting strain on the material is transferred directly onto the strain gage. The strain measured corresponds to a linear change in electrical resistance.An important parameter when performing strain measurements is the gage factor of the strain gage being used. The data acquisition system takes this into consideration when calculating strain. The gage factor of a strain gage is a measurement of its sensitivity to strain. Gage factor = relative change in electrical resistance / relative change in length (mechanical strain). The Gantner measurement modules takes into consideration the gage factor of the strain gage when calculating strain.

Practical Application: Strain measurements are typically in the magnitude of millistrain, therefore accurate measurement of very small changes in resistance is required. In order to measure very small changes in resistance, strain gages are almost always used in a bridge configuration in combination with a excitation voltage.

Scaling of Strain Gauges in test.commander:
1. Click on Strain gauge calculator; the unit is automatically changed to μm/m.
2. Enter the gauge factor for your strain gauge in the left field. The gauge factor (k) is a measure of strain gauge sensitivity and is stated on the strain gauge spec sheet. It usually has a value between 1.8 to 2.2.
3. If you are using more than one active strain gauge in your bridge circuit, you must also state the resulting bridge factor in the right field. The factor depends on orientation of the strain gauge on the measurement object.

 

Resistance

2-Wire: This method is the most commonly used method because of it’s simplicity and method of operation in comparison to a 4-wire resistance measurement. Accurate measurements above 100 kΩ are easy to obtain. A fall back to this method is the inability to correct for lead resistance of the component being tested.

4-Wire: For precision measurements such as those below 100 kΩ, a 4-wire measurement is more reliable compared to a 2-wire method. A 4-wire method requires more cabling however, the trade off with increased accuracy is necessary in certain applications. One such scenario is when the resistance of a component we want to measure is located a distance away from our measuring device. The amount of wire being used between the component and measuring device can introduce unwanted resistance of the wires. Having a 4-wire setup negates the resistance created by the measurement wires. This method is called the Kelvin method.

 

Potentiometer

Also known as a pot, it is a three terminal resistor that incorporates and sliding contact that behaves as a voltage divider. Essentially, a potentiometer is a voltage divider used to measure electric potential (voltage). Potentiometers can be found in electrical devices; such as audio equipment to control volume, displacement transducers, motion control, and more.

The three terminal resistors on a potentiometer have voltage dividers (linear circuits), which provides a voltage output less than the voltage input. The smooth transitions of voltage levels can be rotary or linear. Any device that requires smooth variation in current can utilize 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 to, and a wiper arm that makes electrical contact as it moves across the resistor body. The potentiometer’s resistive body are available in various values, the resistive body comes as a fixed resistive body.

 

Thermocouples

Introduction: Thermocouples consist of two thermoelectric wires which are made from different materials, (i.e. platinum, platinum/rhodium) and joined together at one end (usually be welding). If this contact point and the other ends of the thermoelectric wires have different temperatures, a thermoelectric voltage is produced at the contact point. This measureable voltage is proportional to the temperature difference between the contact point and the ends of the cables.

Methods of Measurement: Since thermocouples only measure a temperature difference, the terminal temperature or the transition from the thermocouple cable or compensating cable to the copper cable must occur at a known temperature. The first is known as internal cold juction compenstaion and the second is known as external cold junction compensation.

Measuring Temperature: To acquire the temperature with internal cold junction compensation an additional temperature probe is used to measure the reference temperature. For the Q.bloxx modules a cold junction compensation terminal block with an integrated Pt1000 temperature probe is used. Using this method, the temperature a the transition point is determined and the voltage produced by the thermocouple is corrected depending on the type of thermocouple.

To measure the temperature using external cold juction compensation, a second thermocouple of the same type is needed which is connected in series with the first thermocouple. The polarity is chosen such that the thermoelectric voltages subtract. The second thermocouple is located at a fixed reference point. The Q.bloxx module calculates the temperature at the measuring point based on the linearization curve. The Q.bloxx module needs the reference temperature being used (provide value in ICP-100/channel configuration).

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