1. How do strain gauge position sensors work?
A Strain gauge position sensor is an absolute position sensor that utilizes strain gauges to indirectly measure linear motion. A strain gauge is a thin laminated foil pattern used to measure mechanical strain. Stretching the strain gauge causes its length to increase and its cross sectional area to decrease, thereby increasing its electrical resistance. By measuring the change in electrical resistance, it is possible to determine the amount by which the strain gauge has been stretched.
A typical configuration for measuring linear position is illustrated above. Two cantilever beams, to which strain gauges are attached, each have a roller bearing mounted on one end. A wedge, that is free to move in the axial direction, is inserted between the two roller bearings, causing the beams to deflect outwards. The strain gauges, which are stretched by the deflection of the cantilever beams, undergo increases in electrical resistance. The built-in electronics can determine the position of the wedge from the change in electrical resistance of the strain gauges.
Both the wedge and cantilever beams are contained within the sensor casing (not shown). The wedge is attached to a rod which protrudes from the casing and is connected to the target object whose position is being tracked. In this manner, any motion of the target, results in an equal motion of the wedge. Although the diagram shows only one strain gauge, real strain gauge position sensors utilize a full Wheatstone bridge arrangement which improves the signal to noise ratio and provides temperature compensation.
2. General characteristics of strain gauges position sensors
Strain gauge position sensors are highly accurate, medium cost sensors capable of measuring displacements of up to about 10 cm, which is a smaller measurement range than other rod type position sensors. Their measurement accuracy is similar to that of LVDT and LVIT sensors. There is no sliding contact within the sensor, only rolling contact through bearings and so friction is negligible. However, mechanical stress caused by the bending of the cantilever beams does limit their lifetime. As a result, the lifetime of strain gauge position sensors is somewhere in between that of a linear potentiometer (which has a short lifetime) and a LVDT sensor (which has a long lifetime).
There are few obvious circumstances under which strain gauge position sensors have any advantage over LVDT or LVIT sensors. They do however standout as the only rod type position sensor to provide a mV/mm/V output signal. This output may be an advantage if you are already using other sensors that output a mv/V signal (e.g. load cells) as they can be connected to the same data capture instrumentation.
Important note: Strain gauge sensors are also used as gauge probes; these are miniature sensors with a very small measurement range and extremely high accuracy. Such strain gauge sensors have sub-micron resolution and are typically used in quality control applications e.g. as part of coordinate measurement machines. Although they operate using the same principal, they are built somewhat differently to the strain gauge position sensors discussed in this article and are not part of the current discussion.
3. Input and output signals
Strain gauge position sensors require fixed 5-48 Vdc supply voltages. The strain gauge elements are arranged in a full bridge Wheatstone bridge formation, providing temperature compensation, common noise cancellation and a high signal to noise ratio. The sensors output signal takes the form of millivolts per volt (of supply voltage) per mm traveled. Most sensors have outputs in the range of 1-10 mv/V/mm, depending on their full scale range. Before being read by the controller, the output signal must be amplified. Many amplifiers also convert the signal to a standard output signal such as 0-10 V or 4-20 mA. Furthermore, many process controllers contain built in amplifiers so that no additional electronics are required between the sensor and controller.
4. Applications of strain gauges position sensors
Strain gauge position sensors are relatively uncommon because whilst they have a similar cost and accuracy to LVDT and LVIT sensors, they have a limited measurement range and a shorter lifetime. They can be used in many of the same applications as other rod type position sensors and become an attractive choice where there is a preference for a mv/V output signal, in low power applications (i.e. remote sensing) and in applications where there is concern that the electric field produced by LVDT and LVIT sensors could cause a disturbance to other instruments.
5. Typical specification
|Measurement range||0-10 cm|
|Repeatability||0.1 % F.S.|
|Linearity||0.1 % F.S.|
|Lifetime (cycles to failure)||50,000 to 50,000,000 cycles|
|Ambient temperature||-10 to 60°C|
|Supply voltage||5-48 Vdc|
|Output signal||0-10 V, 4-20 mA, mV/V/mm|
|Ingress protection||IP54 typical|
|Passive / active||Active|
|Contact / non-contact||Rolling contact|
6. Purchasing tips
- Spring-loaded: All the strain gauges we have come across had spring loaded rods which pressed against the tracked object, without the need for mechanical coupling. In contrast, most other types of rod type position sensors (e.g. potentiometer, LVDT and LVIT sensors) are also available with screw threads or rod end bearings for mechanical coupling.
- Cycles to failure: The number of cycles to failure of a strain gauge position sensors is strongly dependent on the proportion of its full scale range that is utilized in practice. Counter to normal advice which is to select a sensor with the smallest measurement range that covers the maximum range of the process variable, consider purchasing a strain gauge position sensor with a larger measurement range than required to increase its lifetime.
7. Advantages of strain gauge sensors
Strain gauge position sensors:
- Have a higher accuracy and longer lifetime than linear potentiometer position sensors
- Are the only rod type position sensor to provide a mV/V output signal, which can be an advantage if you already own electronics to read mV/V output signals.
- Use less power than LVDT and LVIT sensors, giving them an advantage in remote sensing applications.
8. Disadvantages of strain gauge sensors
Strain gauge position sensors:
- Have a shorter lifetime than LVDT and LVIT position sensors
- Are more expensive to purchase than linear potentiometer position sensors
- Have a shorter measurement range than all other rod type position sensors
9. Application tips
- Spring-loaded: All the strain gauge position sensors we have come across had spring-loaded rods. Furthermore, the cantilever beams themselves acts as springs, preventing the wedge from remaining in its last measured position once the actuation force is removed.
- Wheatstone bridge: Strain gauge position sensors contain built in full-bridge Wheatstone bridge circuits. This means that even if the sensor has a mV/V output (i.e. does not have a built in amplifier), it is not necessary to provide Wheatstone bridge circuitry.
- Lifetime: The lifetime of strain gauge position sensors varies from 50,000, to 100,000,000 cycles, depending on the proportion of the sensor’s full scale measurement range being utilized.