Position Measurement on Hydraulic Cylinders

Hydraulic cylinders extend or retract as soon as pressure or vacuum is applied to them. The movement is limited only by the technical design of the cylinder or the length of its piston rod. Without automatic or manual control, however, the pressure in the cylinder is maintained. This can be intentional, for example, to keep a load at the same height. However, with the help of position measurement, the movement of a linear cylinder can be made significantly more convenient.

Benefits of Position Measurement in Hydraulic Cylinders

Position measurement is a sensory detection of the extended or retracted piston over a specific path. It serves to communicate the piston’s end position to a control unit, which can then trigger any desired actions. Position measurement is useful for various applications:

• Limiting the piston’s extension path
• Repeatable movement of the piston along a defined path
• Triggering an action chain after reaching a defined path
• Safety functions

Fundamentally, two options are available for implementing position measurement in the design: integration during the planning of the hydraulic system or as a subsequent upgrade. However, proactive planning of hydraulic pistons with position measurement is considerably simpler and more convenient than retrofitting them.

Technical Principles for Position Measurement

Position measurement records the distance covered by the piston tip of a hydraulic linear motor. In the past, simple limit switches were used for this purpose, which opened or closed an electrical circuit with the help of a pin. However, these had the disadvantage of a limited lifespan due to mechanical wear. Therefore, today, practically only non-contact sensors are used in hydraulic cylinders with integrated end position detection. For retrofitted end position detection, however, mechanical systems are still frequently used today. These usually work with cable pulls.

Advantages of Integrated End Position Sensors for Position Measurement

Practically all manufacturers of hydraulic cylinders offer integrated end position detection or position measurement as an option for their linear motors. To ensure repeatable and wear-free measurement, magnetic or eddy current sensors are used. Optical sensors are not suitable for this purpose due to their limited reliability in dirty environments. The hydraulic system does not even have to be installed in a particularly dirty location. Even oil mist escaping from the cylinder can render a photosensor unusable within a very short time.

Magnetic sensors work with a strong permanent magnet built into the piston rod. This triggers the signal as it passes the sensor. The control unit registers the signal and can trigger any action chain. This ranges from stopping and retracting the cylinder to activating further actuators, signals, and commands. Eddy current sensors work similarly. Their essential difference is that they require less strong magnetic fields. For these sensors, an aluminum sleeve on the piston rod is sufficient to trigger the desired change in the measured value at the sensor. High-quality induction sensors, in turn, work with two electromagnetic coils that mutually induce a measuring current when approaching each other.

These principles differ primarily in price, lifespan, size, and precision. What they have in common is that they can be practically integrated into the piston rod. However, this is only possible ex-works. Retrofitting a hydraulic or pneumatic cylinder with an induction, eddy current, or magnetic sensor is not possible.

Retrofittable Position Measurement with Cable Sensors

Cable sensors are technically coarser than integrated, non-contact position measurement sensors. However, they are more suitable for demanding environments. A common application area for cable sensors is construction vehicles. They offer a high degree of customization and cope well with temperature fluctuations. However, their inspection must be integrated into the daily maintenance protocol.