Frequently Asked Questions (FAQs) – Position transducers
Sensors and Position Transducers
What is a sensor and what is a position transducer?
A sensor is a technical component that determines various physical parameters, e.g. temperature and pressure, and converts them into electrical signals. A position measuring system, also known as a position transducer, can use sensors to calculate the distance between two points – for example, the distance covered by the cylinder piston. The system translates the calculated distance into a signal and transmits it to a control system, for example. Position transducers are used in various industries, e.g. for the handling of materials, agriculture and forestry, road construction, construction machinery and automation technology. One concrete application for position transducers is in forklift trucks. They can measure the lifting height of the load and thus help to optimise the travel speed.
LiView Position Transducer
Can I seamlessly integrate LiView into my cylinder?
LiView is suitable for any differential cylinder with a plugged or screwed piston rod bearing as well as for plunger cylinders. Before it can be integrated, the cylinder only needs to be modified according to the specifications of Liebherr Components. Only minor modifications are required, such as ensuring that the probes and the RF termination element are correctly positioned.
Before installing LiView, you will need to prepare your installation and your cylinder. We will therefore carry out an electrical simulation of your cylinder before you start production. We will also support you in the mechanical modification of the piston rod bearing.
What is the optimal distance between the electronics module and the cylinder?
The cable length between the module and cylinder can be up to 1.5 m. You can order the cable in 10 cm units.
Hydraulic oil can age, and its temperature and pressure can change. Will this influence the measurement result?
No, it will not. These factors do influence the electricity constant and thus the polarity as well as the conductivity of the oil. However, because our system provides for compensation, the measuring accuracy of LiView remains permanently stable and accurate.
Can the probes wear out over time?
No, because the contact element of the probes is floating, it cannot wear out. In our tests, there was no wear and tear, even after a very long distance.
For which kinds of oil can LiView be used?
LiView is ideally suited for mineral oil-based hydraulic oils, including bio-oils by Liebherr (Liebherr Hydraulic Plus and Liebherr Hydraulic Plus Arctic). One special type is the bio-oil by PANOLIN for which we can currently only measure a cylinder stroke of 1 m with LiView. You use a different oil for your cylinders? We would be happy to test whether LiView can be used according to your requirements.
Is calibration of LiView fast and easy?
Yes! Fine calibration is carried out once the electronic module has been installed on the cylinder. For this, the cylinder must be moved from stop to stop to achieve the full performance of LiView. You can easily carry out the calibration yourself – with our proprietary Liebherr software tool, which we will be happy to provide you with free of charge. The software tool also enables you to
- configure the CAN interfaces
- set the calibration parameters
- read the saved errors
Overview: Measurement types, methods and principles
|High-frequency measuring systems||Draw-wire sensors||Magnetostrictive sensors|
|Type of measurement||linear||rotary||linear|
|Measurement method||absolute||absolute and incremental||absolute|
What are the different measurement types, methods and principles and how do they differ?
- Types: There are two types of position measurement: linear and rotary. Linear position measuring systems capture a distance which is covered in a linear way. Rotary systems detect rotating movements, which are then emitted as analogue or digital signals.
Measurement methods: Position measurement can be performed with an absolute or incremental scale.
- Absolute: Here, the sensor measures directly between a defined starting point and a reference point. Upon activation of the power supply, an absolute signal with the measured value is emitted immediately. A good example of the absolute measuring principle is a clock: the time is calculated from the reference value 0.
- Incremental: This measuring method uses a relative instead of an absolute scale. This means that only the difference between two points is measured. An example: a stopwatch that counts the seconds since the start of the measurement. To obtain absolute values, a reference point to the measured object must first be defined via a reference run. This reference point to the measured object is also called the zero position. The distance between the measurement object and the zero position is broken down into individual units, known as increments. These are counted and together form the measuring distance.
- Measurement principles: Based on these types and methods, there are several principles for measuring the piston position in the hydraulic cylinder. Some examples are high-frequency (RF) measuring transducers, draw-wire sensors and magnetostrictive position sensors.
High-frequency measuring systems
- The principle of high-frequency measurement used in LiView measures linear movements and in absolute terms. It uses the cylinder itself as the measuring distance. With draw-wire and magnetostrictive sensors, a measured value is available on the CAN bus approx. every 5-40 ms. However, with the LiView measuring system a new measured value is displayed with an internal sampling rate of approx. 330 µs - about 10 times as often.
- A scattering parameter measurement determines piston position and speed. A probe excites the cylinder with a high-frequency signal at different frequencies. The signal moves in the cylinder, from the piston rod bearing to the piston, and is reflected there. Another probe reads this response signal. The electronic module checks the response signal for changes in phase and amplitude. LiView uses these changes to calculate the position and speed data of the cylinder. And that's it - finally the results are put on the CAN-Bus interface and can be transferred. The high frequency ensures that the measurement of piston position and speed is very accurate. Furthermore, the system is highly robust because it uses the cylinder itself as the measuring distance.
- A draw-wire sensor is also called a string potentiometer. It is a rotary displacement encoder. The position can be measured either absolutely or incrementally. The measuring cable is clamped between the two eyes of the hydraulic cylinder. When the cylinder extends, the spool rotates and the measured value is emitted as a signal. This can happen via a rotary encoder or an angle sensor.
- One of the advantages of a draw-wire system is that installation is very flexible. One disadvantage, however, is its lack of robustness, as the rope can break and get damaged. This can lead to machine downtime. It is important for the accuracy of the system that the rope always rolls up correctly. Therefore, there is a risk of low measuring accuracy in the case of high mechanical shock loads, for example if the machine is subjected to a shock.
- Magnetostrictive displacement transducers are also called magnetostrictive sensors. They are linear and absolute displacement measuring systems with their own measuring distance. Their mode of operation is based on the physical principle of magnetostriction: if ferromagnetic material, such as iron, is in the vicinity of a magnetic field, it deforms and changes its volume. The sensors contain three components: a waveguide – the measuring element – made of ferromagnetic material, a permanent magnet and a pulse converter. The permanent magnet is typically located on the moving component of the machine, for example the cylinder piston.
- An electrical impulse conducted into the waveguide triggers the measuring process. This current pulse builds up a magnetic field that reacts with the field of the permanent magnet. Magnetostriction generates a wave that propagates at ultrasonic speed in the conductor. As soon as the wave reaches the end of the conductor, a pulse or signal converter generates an electrical signal. The time difference between the triggering and reception of the pulse indicates the absolute position of the permanent magnet and thus that of the hydraulic cylinder.
- The sensors are integrated into the cylinder by means of a deep-hole bore. This mathematical principle of operation allows for very precise measurements. However, only certain cylinders can be used for measurement. Due to the internal installation of the system, any damage requires total removal and disassembly of the cylinder. Another disadvantage is the high mechanical integration effort: to install the system, a deep hole has to be drilled in the piston rod.