Position Sensor Technology Comparison
Linear, rotary and tilt position sensors are available in a variety of styles and using several different technologies. There is no single technology that is widely regarded as “better than the rest” – each application requires different attributes, so some technologies will suit certain environments better than others.
A potentiometer for example is usually cheaper than its alternatives. However, when used in a high-vibration application, a potentiometer can become worn out in a matter of hours.
This article will briefly explain the basic principles and pros and cons of the main sensing technologies available to purchase today. So, if you are looking for a position sensor, but you’re not sure which type of sensor will suit you best, read on.
The simplest of measuring devices is the potentiometer. It works by having an electrical contact attached to the moving part which makes contact with a restrictive track.
The potentiometer is a passive device and therefore does not suffer with complicated electronics or coils which can prove problematic. They are cheap and readily available from many manufacturers and wholesalers, usually costing anywhere from a couple of pounds for a very simple device to several hundred for a higher quality, longer-life device.
The down-side to most potentiometers is that their restrictive tracks wear over time and cause poor performance or failure of the sensors. In applications where there is very little vibration, this may not be a problem. However, in high-vibration applications, the restrictive track can wear out in a matter of hours. If a potentiometer with a 10 million cycle life is installed in an application with vibration at 50hz, the track could be work through in just 50 hours!
Inductive sensors are available in several types, the most common being the LVDT mentioned below. Other devices use a single coil which reduces the overall length to a little more than the measuring range.
Positek Inductive Sensors
Our inductive sensors operates at a high frequency. The benefit of this is that they use a very simple single layer coil wound in a robust wire size which is easy and quick to manufacture. The integrated electronics are inherently temperature stable over a wide range, resulting in small linear temperature coefficients. They provide a complete DC/DC solution in a convenient package and use a simple non-magnetic target tube to change the inductance of the sensing coil. This means they are simple to install as there are no magnets to isolate from steel components, as in magnetostrictive sensors.
Magnetostrictive sensors are highly accurate and have good overall length to stroke ratios. They can however suffer from poor EMI immunity, and don’t cope well in high vibration/shock environments. A magnetostrictive sensor has a signal conditioner and a sensor element. The signal conditioner sends an electrical pulse and starts a timer. The electrical pulse generates a magnetic field around the sensing element and this creates a mechanical pulse. When the signal conditioner detects the mechanical pulse, it stops the clock and calculates the displacement. This happens many times per second.
The LVDT has been the industry standard or “go-to” device for linear position measurement in high-vibration applications. The most basic models are AC/AC and require external signal conditioning, however most manufacturers offer a DC/DC LVDT which offers internal signal conditioning. While the LVDT can be a reliable choice, if using the AC/AC version, the sensing element can often be exposed to high temperatures. The LVDT element is made up of many spools of fine wire which can cause long-term reliability issues as the coils can get damaged fairly easily. And because the LVDT has two main secondary coils that are balanced when the core covers equal amounts of each coil, the sensing element is usually well over twice the measurement range. This especially becomes a problem with longer measurement ranges.
What Is a Linear Variable Differential Transformer ( LVDT)?
LVDTs, or linear variable differential transformers, are a type of sensor used by engineers in a huge variety of industries, from manufacturing to advanced aerospace. Considering it is such a versatile component, how many of us can actually answer the question of what is an LVDT? This article aims to answer that question as a starting point, as well as outline some of the most common uses for LVDTs, give a basic overview of how LVDTs work, and describe the construction and function of specialist submersible LVDT sensors.
AKA as Linear Variable Differential Transformers
LVDTs, also known as linear variable differential transformers, are electric transformers which convert rectilinear or linear movement into an electrical signal. At a basic level, linear variable displacement transformers are composed of a movable core housed inside a stainless steel, or similarly robust, assembly, surrounded by coils which measure its position. The LVDT is connected to a known fixed point at one end and the object to be measured at the other. As the measured object moves, the core within the LVDT is shifted, allowing it to send an electric signal (based on a voltage change) based on the position of the object.
An Explanation of the LVDT Sensor Operation / Output
LVDT’s basically provide high quality signal data on the position / linear displacement from a mechanical reference point into a variable electrical signal containing phase (for direction) and amplitude (for distance) data. Importantly this system does not need any electrical contact between the moving parts ( the core assembly) and the static parts (the coil assembly), as the process utilises electromagnetic coupling. This factor enables LVDT’s to operate in extreme environments.
LVDTs can be produced to measure a wide variety of potential rectilinear ranges, from under 1mm to those in excess of 100cm, limited primarily by the measuring systems the LVDT is connected to rather than the LVDT itself, making them invaluable to an incredibly wide range of applications.
Inductive sensors used as an Absolute Position Sensor
One of the advantages of the LVDT/Inductive sensor is that even if the power is switched off, when it is restarted the unit provides the same measurement, therefore ‘no positional information is lost’. Another huge advantage is the fact that once properly configured, these units will provide the same level of information time after time. Another important fact is that aside from the uni-axial linear motion of the core, all other movements, such as the rotation of the core around the axis, will not affect the data provided.
The three primary types of inductive sensor are captive, unguided and force-extended armatures.
Captive armatures are used for applications requiring larger working ranges, as they are constructed (using low-friction components) to prevent any misalignment due to being restrained (and guided) by the assemblies to which they are attached (and that hence make them ‘captive’).
Unguided armatures are used where precision trumps all else, as the lack of any guidance means the resolution of the measurement is only limited by the measuring instrument, not the LVDT. Unguided armatures are attached only to the measured object, with the linear transducer tube connected to a separate secure object. This is less common with LVDT style inductive sensors due to the fragility of the windings.
Lastly, force-extended armatures use a type of mechanism, be it mechanical, pneumatic or electric, to continuously push the armature to its maximum range, meaning the LVDT does not need to be physically connected to the object it is measuring - the maximum range of the armature will be defined by the position of the object.
Common Uses of Inductive sensors
As would be expected based on the function, the most common uses of inductive sensors such as LVDTs all involve measuring the relative position of objects from a known point. Practical examples of the uses of LVDTs (in the automotive industry) include balancing crankshafts or similar machinery, where an LVDT is connected to the static external housing of the machinery, and to the shaft, so that when the motor is at speed, any movement caused by the oscillation and vibration of the machinery can be measured, allowing the machine to be accurately calibrated with minimal cost, ensuring a long working life.
Another industry in which absolute linear position/displacement transducers are used is aerospace. In the aerospace industry, lifespan, reliability and precision are incredibly important when it comes to all aspects of construction, especially flight control surfaces. With flight control surfaces, absolute linear position/displacement transducers are used to accurately measure the position of the control surface down to the smallest fraction, as any inaccuracy could lead to loss of life.
Inductive sensors are used due to their reliability over a long operating lifespan therefore making them a low cost long term option, as well as their ability to act as an absolute position sensor. What this means is that once an inductive sensor is properly configured, a loss of system power or similar failure will not change the measurement - the sensor will give the same signal once power is restored, meaning that minor failures are entirely recoverable rather than ending up with erroneous readings. In the aerospace industry, it is most common to use force-extended armatures, as their ability to be secured at only one end makes them more robust and less prone to failures. Another important fact is that apart from the uni-axial linear motion of the core, all other movements, like the rotation of the core around the axis will not affect the data provided. The core/target tube for a Positek sensor is made from non magnetic stainless steel where as the LVDT core is magnetic. This can have an effect on the suitability of the sensor type relative to the application.
Submersible Displacement Sensors
More specialized versions of the same sensor are Submersible inductive Displacement Sensors such as the Positek deep sea range or more generic IP68 LVDT’s, which are constructed either to be watertight or to allow the armature tube to be flooded without affecting the operation of the sensor, and they are constructed using corrosion-resistant materials. An application of Submersible Displacement Sensors is leak detection within an engine or larger machine. In this case, an LVDT or other inductive technology would be connected to a float in the fluid, giving precise real-time measurement of levels, so if there were a sudden drop beyond a specified point which could indicate a leak, a shutdown could be triggered to prevent damage. The LVDT is more difficult to design in to a submersible package than the Positek inductive technology due to the materials used for the LVDT core.
Submersible sensors are also used regularly to detect and monitor erosion and soil slippage. In such a case, a sensor is connected to a secured object as well as to smaller objects immersed in the ground to be measured. Erosion or slippage will cause the connected objects to shift, giving a consistent measurement of the soil progression and allowing remedial action to be taken in the places where particularly necessary.
Inductive sensors Suitable for Monitoring Medical Installations
While all of the other applications have been relatively industrial, LVDT are also used in advanced medical equipment, including use in probes developed for brain surgery. LVDTs are used in part for their small size and extremely accurate real-time measurements, but that could also be accomplished with other sensors. What LVDTs offer is more reminiscent of their industrial applications, which is incredible robustness.
One advantage of LVDT over other inductive technologies is the high temperature survivability of the sensing element. This is especially useful in high temperature medical applications. The LVDT is often supplied as just a sensing element with external signal conditioning which meand the LVDT probe is able to withstand temperatures in excess of 150°C. Other inductive technologies such as the inductive principle that Positek uses have integrated signal conditioning and therefor a slightly lower temperature range than an LVDT. Positek sensors can be used up to 125°C (dependant on model).
Surgical instruments are sterilized in steam autoclaves, with temperatures in excess of 135 C, so many sensor components simply would not survive. However, a specialist LVDT, reminiscent of a submersible LVDT but specially constructed to resist the extreme temperatures, can be fully sterilized with the rest of the equipment without losing accuracy or degrading its working life.
Inductive sensors are used in a huge variety of circumstances, from the mundane to the exceptional, so it is almost surprising that so few people could confidently answer the question ‘what is an LVDT'. Hopefully, though, after reading through this, you will have a better understanding of what a position sensor is, how they are used and the pros and cons of the different technologies relative to each other.
Positek use a novel technology that offers the benefits of a LVDT or magnetostrictive sensor whilst also eliminating the negatives. Key benefits such as long life, accuracy and temperature stability all a given while successfully overcoming the weaknesses of the other technologies such as poor stroke to length, vulnerability to shock and vibration and poor temperature stability.
Contact Us for More Information
For help and assistance in this area, please do contact us, our technical staff and engineers we will be pleased to advise you on the installation of these position sensors and how they can be integrated into your products.