What is a Pneumatic Jack Sensor?
Sensors are used to provide position feedback to control systems in automated machinery and equipment. Pneumatic jacks use sensors to detect the linear position of the piston for applications where position feedback is critical. The most common type of sensor used for pneumatic cylinders are magnetic proximity sensors that detect the magnetic field of a magnet in a piston jack. The sensor is mounted on the body of the pneumatic jack and indicates “disconnected” or “connected” depending on the proximity of the magnet. Depending on the application, different magnetic proximity sensor technologies can be used to maximize performance, space and reliability.
Reed sensors are the most common type of pneumatic jack sensor. They have been used for years and have proven technology. The two main reasons for the reed sensor’s concern over the other sensors discussed below are concerns about life and motion / vibration course. The life cycle of reed sensors is over 10 million, and the reed sensor is usually not the first to operate in the event of a shock or high vibration. For these reasons, reed sensors have been and are the most popular pneumatic jack sensor.
Why use a sensor for a pneumatic jack?
Linear Position Sensors Pneumatic jacks are used to detect the linear position of the piston during operation. Pneumatic cylinders are usually made with a magnet already attached to the piston, so magnetic proximity sensors can be used if desired. Depending on where the sensor is installed, it can detect stretches, retreats, or individual positions along the jack body. Multiple sensors can be connected to a pneumatic jack for multi-position feedback locations. Pneumatic cylinders with position sensors provide greater safety and feedback to ensure the location of the piston for important applications.
Installation of pneumatic jack sensor
Two common types of pneumatic cylinder bodies are profiles, such as ISO 15552 or round, such as ISO 6432. Depending on the body type, there are different methods for installation. Installation methods can also vary with different types of sensors, so it is important to understand the type of jack body with the type of sensor.
Profile jacks are rectangular and have two easy ways to install sensors on the jack body. For pneumatic cylinders that comply with ISO 15552, there are grooves along the body that insert the sensor as shown below. The sensor (1) is then secured in place by a screw (3) with an adjusting screw (2). Other cylinders have a tie rod that runs the length of the cylinder body at all four corners. The sensors can be mounted on a tie rod and pulled to the proper position along the jack.
Figure: ISO 15552 pneumatic jack with sensor (C) mounted via screw (B) with screwdriver (A).
Round cylinders are usually smaller, such as ISO 6432. But the sensors can still be mounted on them using a circular strip around the jack housing. The bar should be marked according to the diameter of the jack. Once installed, the sensor and tape can slide along the jack and then fasten in place. The figure below shows an ISO 6432 pneumatic jack with a sensor mounted on it.
ISO 6432 pneumatic jack with sensor installed
Magnetic proximity sensor options
All sensors used for pneumatic jacks to feedback the linear position of the piston use a magnetic field. Therefore, typically all pneumatic cylinders have magnets mounted internally on the piston. However, if you need linear positional feedback, it is important to check these design specifications for your pneumatic jack.
The reed switch sensor is a magnetic proximity sensor that lights up “ON” when a magnetic field of axial alignment is applied to it. The magnetic poles of an axial alignment magnet are located next to each other on the axial plane. As the axial magnet approaches the reed sensor, a magnetic field is generated parallel to the reed switch. A reed switch consists of a pair of ferromagnetic metal reeds enclosed in a sealed glass tube. Without the magnetic field (pictured above), the metal reeds are disconnected and the “OFF” sensor is turned off. When the cylindrical piston passes through the switch and exerts a magnetic field strong enough to absorb the reeds together (middle image in the figure below), the “ON” sensor turns on (image in the figure below).
Reed switch function
Compared to other sensor options, reed switches are cost-effective and can operate at AC or DC voltages. In addition, reed switch sensors have low power consumption, which makes them suitable for applications with high power consumption. Due to the mechanical nature of replacing reed contacts, reed switch sensors have limitations. First, switching contacts have a limited number of switching cycles and require maintenance throughout the life of the device. Next, reed switches are not suitable for applications that are subject to vibration or shock. Excessive shock and vibration can cause the Reed contacts to call, causing incorrect signaling. Switch Features A single switch can also cause unwanted dual switching. The dual switch is when the sensor output turns on “ON” and “OFF” twice, while the jack magnet passes the reed switch once. The false dual switch of the sensor output is due to the uniform strength of the magnetic force field. The strength of the magnetic field is strongest at each pole of the magnet and weakest at the center between each pole. If the piston magnet is not strong enough, the output of the switch may double as it passes through the sensor. Finally, compared to solid-state sensors, it is relatively late to activate reed switches, making them unsuitable for applications that require fast response times. However, switch reed sensors for pneumatic cylinders are widely used because they are cheaper than other sensors, do not require standby power, can work with DC or AC loads, and are a well-known solution. Is. If the piston magnet is not strong enough, the output of the switch may double as it passes through the sensor. Finally, compared to solid-state sensors, it is relatively late to activate reed switches, making them unsuitable for applications that require fast response times. However, switch reed sensors for pneumatic cylinders are widely used because they are cheaper than other sensors, do not require standby power, can work with DC or AC loads, and are a well-known solution. Is. If the piston magnet is not strong enough, the output of the switch may double as it passes through the sensor. Finally, compared to solid-state sensors, it is relatively late to activate reed switches, making them unsuitable for applications that require fast response times. However, switch reed sensors for pneumatic cylinders are widely used because they are cheaper than other sensors, do not require standby power, can work with DC or AC loads, and are a well-known solution. Is.
Hall effect sensor
The Hall effect sensor is a magnetic proximity sensor that lights up “ON” when a radial alignment magnetic field is applied to it. A magnet with a radius of alignment produces a magnetic field perpendicular to the magnetic field of the Hall effect sensor, as shown in the figure below. Unlike reed switches, Hall effect sensors are solid-state devices and are designed with different components. Reed switches depend on the movement of mechanical contacts to provide sensor output. Solid state devices provide sensor output using electrical circuits without moving components. The Hall effect sensor consists of a semiconductor with a continuous current flowing through it, which can be seen in the image above in the figure below. When a magnetic field is applied radially (2) to a current (1), as shown in the figure below, the charged electrons are separated on both sides of the semiconductor by polarity. Isolation of charged electrons causes a voltage in the Hall effect circuit (4). When the output voltage in the circuit is higher than the reed threshold, the output of the “ON” sensor lights up, as shown in the figure below.
Hall effect sensor function: current (1), magnetic field (2), voltage = 0 (3 and 4)
Unlike reed switch sensors, Hall effect sensors do not include moving components and will have a smaller installation footprint. The solid state design increases the life of the sensors due to abrasion-free operation and also makes them resistant to shock and vibration. Without having to overcome the inertia of mechanical components, Hall effect sensors are also suitable for important time applications that require rapid replacement. Similar to reed switch sensors, the magnetic direction is important for proper operation. In addition, Hall effect sensors are low sensitivity. Depending on the diameter and thickness of the jack body, the switching output may not be activated properly. Similar to reed switches, dual switch points are possible due to the low sensitivity of the sensors.
Anisotropic magnetoresistive sensors
Magnetic Resistor (AMR) sensors are solid-state magnetic proximity sensors that turn on “ON” when a radial or axial magnetic field is applied to them. An AMR circuit includes a Wheatstone bridge circuit (Figure below) to measure resistance. The resistance of an AMR sensor decreases with the strength of the magnetic field, which leads to an increase in the voltage gradient in the AMR circuit. When the voltage across the circuit exceeds the switching threshold (Figure 3 below), the sensor output illuminates “ON” as shown in Figure 4 below.
Figure above: AMR sensor performance: magnetic field (1), applied magnetic field (2), bias voltage (3) and voltage = 0 (4)
AMR sensors, like Hall effect sensors, are fast, wear-free, and shock and vibration resistant. The advantage of AMR sensors is that they are less sensitive than indoor sensors and respond well to changes in magnetic field strength. This improves piston detection at longer distances due to its ability to detect weaker magnetic fields. Due to the greater sensitivity, the possibility of changing two point switches is eliminated. In addition, the sensors detect axial and radial magnetic magnets. AMR sensors are more compact than switch reed sensors and are also competitive in cost. The disadvantage of AMR sensors is that they usually draw current continuously. For applications with low power consumption, a reed switch sensor may be a more appropriate sensor choice.
Giant magnetoresistive sensor
The Magnetic Giant Sensor (GMR) is a solid-state magnetic proximity sensor that illuminates “ON” when a radial or axial magnetic field is applied to it. A GMR sensor consists of alternating layers of magnetic and non-magnetic conductive layers as shown in Figures 2 and 3 below. Similar to the AMR sensor, the resistance properties of the circuit change when a magnetic field is applied to the sensor. As the magnetic field increases, there is a higher voltage gradient in the circuit. For example, in the presence of a magnetic field (Figure 1 below), the resistance of the circuit decreases to allow current to flow (Figure 4 below) and the voltage across the circuit increases. When the voltage in the circuit is higher than the switch threshold, the output of the “ON” sensor turns on.
Figure above: GMR sensor performance: application of external magnetic field (1), ferromagnetic layer (2), non-conductive magnetic layer (3), low resistance to applied magnetic field (4) and high resistance without external magnetic field (5)
GMR sensors offer similar advantages to AMR sensors, however, they are even more sensitive to the presence of a magnetic field. High sensitivity also allows for a very compact sensor that is suitable for smaller and shorter cylinders. Although high sensitivity is useful for applications that require immediate sensor feedback, it may cause an unwanted output signal if disturbed by the surrounding magnetic fields. For example, near-high power environments (AC motors or AC input power) may interfere with the sensor signal and cause an unwanted error.
Choosing the right sensors
For most applications, the reed sensor is usually selected. They are a proven technology and have a long life cycle and vibration resistance to handle routine applications. However, other criteria that should be considered for special programs are:
- Environment : Is the jack exposed to a lot of vibration or shock? In this case, a solid state sensor operates reliably without output sound. Common solid state sensors include Hall effect sensors, AMR and GMR. In addition, whether the sensor is housed in a clean environment or, like IP67, requires a high-protection enclosure. Temperature considerations must also be taken into account.
- Switching speed : How important is the speed of output change to your application? Solid state sensors offer faster switching times. Common solid state sensors include Hall effect sensors, AMR and GMR.
- Output Type : What type of output signal is required for the control system? PNP and NPN output signals are available for solid state devices.
PNP: The PNP output provides a path to supply positive output power. This is also commonly known as “sourcing sensors”. PNP is more common in North America and Europe.
NPN: The NPN output provides a path to ground supply. This is also commonly known as a “sinking sensor”. NPN is more popular in Asia.
- Switching signal characteristics : What is the switching power and current needs of the control system? The selected sensor must be compatible for proper operation.
- Installation : What installation options are available for each type of jack? Depending on whether you have a profile jack with a groove or tie rod or a round jack, the installation types will change.
- Magnetic orientation : Reed switches and Hall effect sensors require the correct orientation of the applied magnetic field for proper operation. Therefore, the sensor must be installed in the proper direction of the piston.
- Circuit protection : Sensors can use circuit protection such as short circuit, reverse polarity and impact protection if necessary.
- Wiring : The sensor power supply wiring varies depending on the solid state sensor (ie AMR, GMR, Hall effect) or the reed sensor. An LED to indicate proper wiring is often available for each sensor. For example, if the polarity of the reed switch power supply is reversed, the LED on the sensor will not light up. Reed switch sensors usually have a 2-wire configuration, while solid-state sensors have a 3-wire configuration. In addition to the positive and negative wires, a third wire is used to connect to the load. Proper wiring The load wire should always be checked before using power because incorrect wiring can permanently damage the sensor.
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