The term “sensor” takes on significant meaning when applied to industrial machinery and electronic devices, especially when we anthropomorphize the concept of “sensing.” Humans perceive their surroundings through various sensory systems, and some believe the existence of these systems extends beyond the realm of traditional science. While we have all likely had inexplicable sensory experiences, this article will focus on electrically-based sensing technologies.

For equipment like hydraulic presses (such as the model being tested in the image), both safety and precision are ensured by sensor technology.

A sensor is essentially a device capable of detecting environmental characteristics, with measurable parameters including temperature, pressure, or spatial position. In hydraulic cylinder applications, we primarily focus on the linear spatial position of the piston rod, piston, or connecting parts. Designers can choose between digital or analog position sensing methods; the digital sensing referred to in this article refers to the detection of “in position” or “not in position” states.

Digital position sensing

Digital position sensing is typically used to detect the end of the stroke by means of devices installed inside or outside the hydraulic cylinder. Designers may need to confirm that the hydraulic cylinder is fully extended to verify that the first step of a sequential operation is complete. For example, if a programmable logic controller (PLC) does not detect the signal that the piston has reached the end of its stroke, the system will trigger a fault state, preventing subsequent operations from starting, which could otherwise lead to serious damage or safety hazards. In the case of a series of presses, where a sequence of presses gradually stamps metal parts into complex shapes, if the position confirmation of an upstream press is not completed, the entire production line must be shut down to prevent subsequent errors from escalating or causing equipment damage.

Conversely, for safety reasons (especially in scenarios involving the risk of human contact), hydraulic designers may need to verify that the hydraulic cylinder is fully retracted. If jamming or malfunction prevents the hydraulic cylinder from completing its full cycle, materials or personnel may be in a dangerous position, and restarting the cycle could cause damage or injury. In this case, the PLC must confirm that the hydraulic cylinder is fully retracted before starting the next cycle.

Unlike pneumatic cylinders (with the exception of special products from some manufacturers), hydraulic cylinders cannot utilize reed switches mounted on the piston rod to detect a magnetic piston. Reed switches are magnetically controlled switches sensitive enough to operate through the aluminum cylinder barrel of a pneumatic cylinder. Pneumatic cylinder manufacturers typically only need to replace the piston’s wear strip with a magnetic strip; other magnetic rings can also be used. However, the steel cylinder barrels used in hydraulic cylinders have magnetic permeability characteristics that prevent the magnetic field from effectively penetrating the cylinder barrel, making reed switches unsuitable.

Currently, the standard solution for detecting the end of the stroke in hydraulic cylinders is the inductive sensor. This sensor uses the principle of electromagnetic induction to detect the presence of nearby metal objects and is also known as a proximity switch. Unlike reed switches, which require no power, inductive sensors require power to operate, typically 24V DC or 120V AC depending on the hardware selection.

The inductive proximity switch’s probe is mounted on the cylinder head and/or cylinder base of the hydraulic cylinder and secured by a flange. The sensor is sealed with an O-ring to prevent hydraulic oil leakage, and the flange is typically fastened with two bolts. The distance between the probe and the metal detection target must be controlled to within approximately 2 mm. The detection target is primarily the hydraulic cylinder’s cushion spear or cushion sleeve, unless the manufacturer provides a dedicated detection component. In most cases, because manufacturers’ hydraulic cylinders are commonly equipped with cushioning devices, and the cushion sleeve (located on the piston rod side) or cushion spear (extending from the piston side) is easy to manufacture and install, they become the commonly used detection targets.

When selecting inductive proximity switches, it’s important to note that only the external dimensions of hydraulic cylinders (such as those conforming to the NFPA standard) are standardized; the cushioning method and sensing design can vary significantly. Because the depth of the cushioning rod or sleeve relative to the cylinder head/base surface may differ between different hydraulic cylinders, shims are often required to precisely adjust the position of the sensor tip, ensuring it is within approximately 2 mm of the target.

Linear displacement sensor

Inductive proximity switches are suitable for end-of-stroke detection, but if you need to obtain information about the intermediate position, acceleration, or velocity of a hydraulic cylinder, other sensing solutions are required. Linear displacement sensors can provide real-time transmission of these parameters in several ways, the three most commonly used types being:

• Magnetostrictive sensors,
• linear displacement transducers (LDTs),
• linear potentiometers.

In addition, there are other types such as optical sensors and Hall effect sensors.

However, the first three types are the most widely used in hydraulic cylinder applications. This article will first introduce the working principles of each type of sensor, and then analyze their performance advantages and disadvantages.

Working principles of various types of sensors

• Magnetostostrictive sensors: These sensors utilize the magnetostrictive effect to calculate position by measuring the propagation time of a magnetic pulse along a rod. The built-in controller is mounted at the bottom of the cylinder (typically end-mounted, but side-mounting options are also available), and the measuring element is located inside a gun-drilled rod. A non-contact magnet is mounted on the piston, and the sensor reads the magnet’s position, outputting an analog or digital signal to the PLC (multiple output options are available).

• Linear Displacement Transducers (LDTs): Based on the principle of electromagnetic induction, these sensors measure position by changing the induced voltage in the secondary coil through the movement of a magnetic core within the primary coil assembly. Their appearance and installation methods are similar to magnetostrictive sensors.
• Linear Potentiometers: These are resistive sensors consisting of a resistive element (in this case, a conductive track) and a sliding contact (brush). The brush is also mounted on the piston, but unlike the previous two sensors, it uses a contact-based design. As the brush moves along the resistive element, the resistance value changes, and this change can be used to calculate position information. Linear potentiometers offer greater installation flexibility, requiring only a wire connection to a connector at any convenient location on the cylinder base. Similar to the previous two sensors, the conductive track must also be placed inside the through-rod bore.

Performance parameter comparison

Output methods and protocols

After selecting a linear displacement sensor, various analog or digital output options can be chosen based on the requirements of the PLC or controller. Standard options include 0-5V and 0-10V voltage signals and 4-20mA current signals.

The first two voltage signals are commonly used in industrial automation, while the 4-20mA current signal offers superior resistance to electromagnetic interference and fault detection capabilities. The 4mA zero-point calibration design clearly distinguishes the zero state, preventing misreadings caused by short circuits resulting in a current slightly above zero.

Some hydraulic cylinder-specific control options support reverse output (20-4mA current or 10-0V voltage). For hydraulic cylinder applications where the natural starting position is fully extended, the retraction movement will cause the current or voltage value to decrease, and this change can be used to indicate the retracted position.

LDTs and magnetostrictive sensors often support feedback protocols such as PWM (Pulse Width Modulation) and SSI (Synchronous Serial Interface), providing high-precision feedback to PLCs or controllers. SSI is a digital communication protocol that enables absolute position measurement with high resolution and strong noise immunity, but its wiring is more complex than PWM. PWM is an analog output method that provides a continuous signal for relative position measurement, reflecting only position changes rather than absolute position information. However, it is simpler to deploy and provides good dynamic feedback for motion state and speed detection.

Technological development trends

Although magnetostrictive sensors, LVDTs, and linear potentiometers have dominated the field of linear displacement detection in hydraulic cylinders for many years, they are expected to be replaced by newer solutions such as fiber optic sensing, piezoelectric sensors, or laser displacement technology in the future. The next generation of dominant technologies will need faster response times, lower costs, and higher accuracy to surpass existing technologies. Based on the current pace of technological development, these new types of sensors are expected to quickly gain market dominance.

★★★Review of previous selected articles:

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• What to do if the fastening bolts of a hydraulic cylinder become loose after a pressure test?
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Contact: Nancy Zhu, Sales manager, JW GROUP.

Email: nancy@jwgroup.cc

Web: https://jwcylinder.com

Mobile/Whatsapp:+86 15902166721

HYDRAULIC CYLINDER and CNC parts specialist

JW GROUP is an integrated steel product group, products include: hydraulic cylinder, hydraulic cylinder spare parts, drilling forging parts, pneumatic actuator-scotch yoke, CNC machinery parts etc.