By Josh Cosford, Contributing Editor
“Sensor” is an interesting word as it applies to industrial machinery and electronics, especially when we personify what it means to be sensing something. We can sense our environment through one of our various sensory systems, and if you ask some, sensory systems exist outside the realm of traditional science. I’m sure we’ve all sensed something we couldn’t explain, but we’ll keep the conversation related to electricity in some form for this discussion.
A sensor is simply a device that can detect the properties of its environment, which could be temperature, pressure or a position in space. We primarily seek to observe hydraulic cylinders’ rods, pistons or attachment’s position in linear space. Designers can choose between digital or analog position sensing, and by digital, in this case, I mean here or not here.
Digital position sensing typically entails a device installed into or on the cylinder to observe end-of-stroke measurement. Designers may wish to know if a cylinder has fully stroked to confirm the first step in a sequential operation. If the PLC, for example, does not read the piston to confirm the end of stroke, an error condition prevents the subsequent operation from beginning, which may otherwise cause severe damage or harm. Consider a tandem press, for example, where a line of presses stamps progressively complex shapes into metal parts. If position cannot be confirmed achieved by an upstream press, the entire line must shut down to prevent further errors or even damage.
Conversely, a hydraulic designer may need to sense the cylinder fully retracted due to safety concerns, especially with the potential for human interaction and harm. If a jam or failure prevents the complete cycle of a cylinder, material or personnel may be in a position where restarting the cycle would cause damage or harm. In this case, the PLC must know that the cylinder is fully retracted before initiating the next cycle.
Unlike pneumatic cylinders, and with a few exceptions from some manufacturers, hydraulic cylinders cannot take advantage of reed switches attached to the tie rod to sense a magnetic piston. A reed switch is a magnetically activated switch that is sensitive enough to operate through the aluminum barrel of a pneumatic cylinder. Most often, cylinder manufacturers simply replace the piston’s wear strip with a magnetic strip, although other magnetic rings may also work. However, the magnetic permeability of steel barrels used in hydraulic cylinders prevents the magnetic field from transmitting strongly enough through the barrel.
Instead of reed switches, the current end-of-stroke switching standard is the inductive sensor. An inductive sensor is a type of switch that uses electromagnetic induction to detect the presence of metal nearby. Also called proximity switches, appropriately enough, you may even find them casually referred to as “proxy” or “prox” switches. Regardless of the name, they are powered devices (unlike unpowered reed switches), so you need electrical input such as a 24 V power supply or a 120 Vac input, depending on your choice of hardware.
The inductive proximity switch has a probe that inserts into your hydraulic cylinder’s head and/or cap and fixes in place with a flange. O-rings seal the sensor to prevent hydraulic oil from leaking out, and typically, two bolts to fasten the flange down. The probe must sit less than 2 mm or so away from the metal of the sensing target. That target is predominantly the cushion spear or sleeve of the cylinder unless the cylinder manufacturer offers a switch-specific item. In most cases, the sleeve (which resides on the rod side) or the spear (protruding from the piston side) are used because manufacturers already offer their cylinders with cushion options, they’re easy to manufacture and install.
One point of note for selecting these inductive proximity switches is that only the external dimensions of cylinders, such as NFPA are standardized; the method of cushioning or sensing could vary widely. You often must select a spacer to help precisely position the probe end within about 2 mm because the spear or sleeve on any given cylinder may be different depths from the head or cap surface.
The inductive proxy switch is excellent for end-of-stroke measurement, but what if you want to know intermediate cylinder position, acceleration or velocity? This opens the door for many more options, although each functions roughly the same way. Linear displacement sensors offer a few methods to achieve the same results, such as an electronic method to relay cylinder stroke position and/or velocity in real-time.
The three most common linear displacement sensors are magnetostrictive sensors, linear displacement transducers and linear potentiometers. There are many other ways to measure linear displacement, such as optical and hall effect sensors, but the prior three are the most commonly used in hydraulic cylinders. I’ll describe each operation before breaking down the performance advantages and disadvantages of each.
Magnetostrictive sensors use the magnetostrictive effect to measure position based on the time it takes for a magnetic pulse to travel along a rod. A built-in controller mounts to the cap (usually at the end, but they’re available as a side mount), and a measurement element resides inside a stainless tube inserted into the gun-drilled rod. A contactless magnet mounts to the piston where the sensor can read its position, providing the PLC with one of many analog or digital output options.
Linear Displacement Transducers (LDTs for short) use electromagnetic induction principles to measure position by varying the voltage induced in secondary coils as a magnetic core moves within a primary coil assembly. Otherwise, they are similar to magnetostrictive sensors in both appearance and installation.
A linear potentiometer is a resistive sensor consisting of a resistive element (a conductive track in this case) and a sliding contact (wiper). This wiper also installs on the piston but is not contactless as the other two options. As the wiper moves along the resistive element, it changes the resistance, which is used to determine position. The mounting of the linear potentiometer is much more flexible since it’s just a few wires attached to a connector you can mount in any convenient location on the cap. As with the prior two, the track requires a gun-drilled cavity within the rod.
Comparing technical considerations of Magnetostrictive, LDTs and linear potentiometers
- Magnetostrictive sensors: Suitable for longer measurement ranges, often several meters.
- LDTs: Can be used for both short and long measurement ranges, but the maximum range may be limited.
- Linear potentiometer: Typically used for relatively shorter measurement ranges, usually a few inches to a few feet, although longer versions are available.
- Magnetostrictive sensors: Offer high-resolution measurements with fine position accuracy, sometimes as fine as one micron.
- LDTs: Provide good resolution, although they may be slightly lower than magnetostrictive sensors in some cases.
- Linear potentiometer: Offer moderate to good resolution depending on the design and length, but the resolution may not be as high as some magnetostrictive sensors.
- Magnetostrictive sensors: Known for high accuracy and repeatability.
- LDTs: Provides accurate measurements still down to less than a thousandth of an inch.
- Linear potentiometer: Can offer good accuracy but may be more susceptible to wear and mechanical factors affecting accuracy over time.
- Magnetostrictive sensors: Typically have a fast response time.
- LDTs: Offer relatively fast response times.
- Linear potentiometer: Response time is generally fast since there are no onboard electronics, which is suitable for dynamic applications.
- Magnetostrictive sensors: Robust and suitable for harsh environments.
- LDTs: Also robust and can handle environmental challenges.
- Linear potentiometer: Susceptible to wear and environmental conditions may impact performance, especially in extreme environments, since it requires electrical conductivity across physical connections. And since temperature changes the resistance of metals, extreme heat or cold will affect precision.
- Magnetostrictive sensors: Tend to be more expensive.
- LDTs: Can be cost-effective.
- Linear potentiometer: The most cost-effective solution.
Once you’ve selected your linear displacement sensor, you are given myriad choices in analog or digital outputs to suit your PLC or controller. The standard 0-5 V, 0-10 V and 4-20 mA may be selected. The first two voltage-based signals are commonly used for industrial automation, while 4-20 milliamps provide outstanding resistance to electrical noise and faults since the four mA zero-point calibration provides a clearly identifiable zero point not achieved accidentally where short circuits false readings might observe amperage just above zero.
Some cylinder-specific control options might include an inverted 20-4 mA or 10-0 V, which, when used on cylinder applications where the natural starting position is fully stroked, and the act of retraction decreases amperage or voltage to indicate a retracted position.
Feedback protocol options such as PWM and SSI are not uncommon on LDTs or Magnetostrictive position sensors, each providing a different method of precise feedback to your PLC or controller. SSI (Synchronous Serial Interface) is a digital communication protocol that provides absolute position measurement with high resolution and noise immunity but requires more complex wiring than PWM. At the same time, PWM is an analog output method that provides a continuous signal with relative position measurement. It can only offer a change of position but not absolute position detail. However, PWM is simpler to employ and provides excellent dynamic feedback for movement and velocity.
Although Magnetostrictive, LDT and potentiometers have dominated linear displacement technology for cylinders for years, they will be one day replaced with new technology involving perhaps fiber optics, piezoelectric sensors or perhaps laser displacement technology. The next leading technology will have to be faster, cheaper and more precise to beat the current technologies, and at the current rate of advancement, we’ll see one of them dominating the market soon.
Filed Under: Components Oil Coolers, Engineering Basics, Sensors, Sensors & Gauges, Technologies