1. How do LVDT position sensors work?
A LVDT (Linear variable differential transformer) is an absolute linear position sensor that operates on the same principal as an AC transformer. It consists of a primary electrical coil and two secondary electrical coils all wound around a common non-metal hollow cylinder. The secondary coils are wound in opposite directions to one another and connected in series. An iron core mounted to a rod (the dynamic element whose position is tracked) passes through the hollow cylinder.
The primary coil is excited by an AC voltage, Vp. The electricity flowing through the primary coil induces an electrical field in the iron rod. In turn, the electric field induces an AC voltages in each of the secondary coils due to Faraday’s law. The voltage magnitude in each secondary coil is proportional to the extent of overlap between the respective coil and the iron rod. Owing to the two secondary coils being wound in opposite directions, the induced AC voltages are 180 degrees out of phase (the AC equivalent to opposite polarity of DC voltages). As a result, the sum of the secondary coil voltages, Vs, tells us the distance of the rod from its center position (at its center position the output voltage is 0 V because the iron core overlaps each secondary coil by an equal amount). The phase of the output voltage (which will be either in phase or anti-phase to the primary coil voltage) tells us the direction of movement of the rod.
2. General characteristics of LVDT position sensors
LVDT sensors are high accuracy sensors with measurement ranges up to 100 cm and the ability to withstand strong vibrations and harsh environments (typically rated to IP67 ingress protection). They are medium cost sensors and although they require relatively complicated electronics, the circuitry is built into the sensor. The LVDT rods are guided by either low friction sliders, ball bearings or are unguided. Rods that are bearing guided or unguided have little or no friction, making them an excellent choice for applications with high velocity and high cycle rates. In fact, the velocity of unguided rod type LVDTs is limited only by their sampling rate (as velocity increases, the distance travelled between successive sensor readings increases).
Important note: LVDT sensors are also used as gauge probes, these are miniature sensors with very small measurement ranges and extremely high accuracy. They are typically used in quality control and are not part of this discussion.
3. Input and output signals
LVDT sensors require an AC supply voltage (typically 5 Vac at 0.1-5 kHz) to power their primary coil. Many LVDTs include built in power inverters (DC to AC converters), enabling them to accept a DC supply voltage. The unconditioned output signal of a LVDT sensor is an AC voltage that is the sum of the two secondary coil voltages. The output signal phase (which is either in phase or anti-phase with the primary coil phase) is measured by the built in electronics to determine the direction of movement. The output signal is then rectified (i.e. converted to DC) and converted to 0-10 V or 4-20 mA. Alternatively, mV/V/mm outputs are also available among LVDTs with AC supply voltages.
4. Applications of LVDT position sensors
LVDT sensors are widely used in industry and are, along with linear potentiometers, one of the most purchased rod type position sensors. LDVTs are more expensive than linear potentiometers so are typically purchased for applications where the accuracy, robustness and lifetime (cycles to failure) of potentiometers are insufficient. These applications include use within industrial machines, agricultural machines and civil engineering.
5. Typical specification
|Measurement range||0 to 100 cm|
|Velocity||Limited primarily by spatial resolution|
|Lifetime (cycles)||High up to infinite (unguided rods)|
|Ambient temperature||-40 to 120°C, (200 °C available)|
|Supply voltage||5 Vac, 0.1-5 kHz or 10-24 Vdc|
|Output signal||0-10 V, 4-20 mA, mV/V/mm|
|Vibration resistance||10-20 g|
|Shock resistance||1,000 g|
|Ingress protection||IP67 typical, IP68 available|
|Passive / active||Active|
|Contact / non-contact||Minimal or no contact|
6. Purchasing tips
- Rod Configurations: There are 3 common LVDT rod configurations; slider guided, bearing guided and unguided. Slider and bearing guided rods, as their names imply, are guided by either low friction plastic sliders or by ball bearings so that they can only move axially. Unguided rods do not make any contact with the rest of the sensor. Instead, they rely on being connected to and guided by the object whose position is being tracked by the LVDT.
- Supply voltage: Many LVDTs can accept a DC supply voltage. These are particularly useful in applications where a high sampling rate (and therefore high supply frequency) is required.
- Direction of movement: If knowing the direction of movement is important, be sure to purchase an LVDT with built in electronics to extract the phase information, required for determining the direction of motion.
- Spring loaded: Many LVDT sensors have spring loaded rods which do not need to be mechanically coupled to the target object because the spring ensures contact is maintained. Other LVDT models include mechanical joints.
7. Advantages of LVDT sensors
- Have a high measurement accuracy, typically better than 0.5% of their full scale range, in part due to built in temperature compensation and noise reduction.
- Can operate in harsh environments. Most can withstand vibrations of 10-20 g, shocks of 1000 g and environments of IP67. Models are available with IP68 ingress protection.
- Have a very long lifetime. Unguided LVDTs are capable of undergoing an unlimited number of cycles due to the lack of mechanical friction. Furthermore, LVDTs can operate at high velocities, the main obstacle being the deterioration of spatial resolution.
8. Disadvantages of LVDT sensors
- Are typically cost more than potentiometer linear position and displacement sensors
- Require more complex signal processing because phase information must be extracted and the output signals must be rectified (though this is normally built into the sensor).
- A small output voltage is present when the LVDT rod is at its center position (which should ideally result in a 0 V output). This can result in oscillations about the center position in machines with closed loop feedback control systems.
- Are significantly longer than their full measurement stroke. In contrast, most other rod type position sensors are only slightly longer than their full measurement strokes.
9. Application tips
- Output linearity: LVDT sensors posses high output linearity. Linearity is highest near to the rods center position (i.e. where output voltage is near to 0 V) and deteriorates as the rod moves away from its center position.
- Maximum travel: Unguided rods can be pulled out of the sensor without causing damage. This is useful when there is a risk of exceeding the sensors maximum travel.
- Non-axial forces: Take care not to apply non-axial forces to the rod as this could damage the LVDT. Rather than causing catastrophic failure (which would be easy to detect), the damage may result in a deterioration of accuracy.
- High accelerations: If relying on a spring for maintaining contact with the target object, high accelerations could cause temporary separation of the rod from the target. Better contact can be obtained by using stiffer springs.