Sensors and switches in fluid power use electronics to gauge critical system parameters, such as pressure, temperature, position, level and more.
By Josh Cosford, Contributing Editor
A sensor is any mechanical or electronic device designed to recognize a physical input property and then relay or transform that input in a form of an electric or electronic output. The fluid power realm enjoys the benefits of electronic sensing technology for the various benefits they offer machines and operators. For example, properties such as pressure and temperature are commonly measured with sensors, but even more obscure sensors may measure humidity or acidity.
Sensing and switching technology operates using either electro-mechanical or electronic form. An electro-mechanical sensor manifests most often as a switch, while electronic sensors use solid-state processors to interpret and send signals. Electronic sensors often go by the term transducers, which are any device that converts one energy form into another; the “other” in this case is an analog output usable by a PLC or controller.
One of the earliest sensing technologies wasn’t necessarily specific to fluid power. The liquid column manometer was just a U-shaped tube open at both ends. Differential pressure at either end of the tube would move a column of mercury or other fluid proportionately to the differential. In theory, this early pressure gauge was indeed a sensor, albeit with merely visual output.
The Bourdon tube pressure gauge, dating back to the mid-1800s, offered early readings of the pressure it sensed. The Bourdon tube is a bent tube that tries to straighten when the interior is exposed to pressure. A lever then rotates the needle proportionate to the bending caused by the pressure increase. Just as with the liquid column manometer, this sensing technology was none that could be interpreted by a PLC or machine (which, of course, did not exist nearly two centuries ago).
Modern sensing technology offers more than a visual indication of machine parameters, primarily because we use those outputs for automated functions or advanced machine control. We need physical properties to trigger sequential machine functions or provide feedback on important system health markers.
Simple and useful pressure switches
The switch offers the most basic feedback and opens or closes the electric contact when the sensor achieves its factory or user-designated set point. In hydraulics, the most popular switches are for pressure and temperature.
Mechanical pressure switches use a piston or diaphragm opposed to a spring. When pressure rises enough to push the piston or diaphragm up high enough, the assembly, in turn, pushes a lever or pin to close the electrical contact. The spring assembly most often has an adjustable perch that allows the user to vary the spring tension, adjusting the switching pressure.
Rising and falling switching values are an essential consideration for mechanical pressures switches. If your switch is designed to switch as pressure rises to its set point, it’s known as rising. However, once the rising pressure set point is achieved, the switch does not reset at the same setting when pressure decays. For example, if you set your switch point to 2,000 psi, the electric switch closes at that pressure and will remain closed until pressure again falls. However, the pressure at which the switch opens again will rarely occur at that same 2,000 psi. It’s more likely to reset at 1,950 psi or less, for example.
The difference between the rising and falling pressure is also known as hysteresis. In some cases, hysteresis is undesirable due to a poorly manufactured switch with much friction. When that is the case, the hysteresis may be extreme and unpredictable, unswitching at very low pressure compared to its switch pressure. Be sure to study the literature to prevent any unsafe operating conditions. With high hysteresis, there is a chance your operator or machine controller will believe a safe condition, such as clamping, is being met when it is not.
Understanding your switch’s hysteresis may offer benefits, even if the difference between rising and falling pressure is high. Being sure pressure has fallen to a safe, low pressure may be preferred in some circumstances. An accumulator circuit, for example, is not one where trapped pressure unknown to the machine or operator is desirable. Having the pressure switch shut off below the rising pressure rather than at it offers an extra level of safety, especially if the switch has poor repeatability.
Keeping an eye on temperature
Hydraulic systems employ more than pressure switches. Temperature switches installed into heating and cooling systems are the traffic lights directing temperature control of your sensitive hydraulic fluid. Hydraulic oil has a narrow thermal operating window. Depending on the quality and viscosity of the fluid, 100-140°F supplies the best mix of efficiency, lubricity and longevity for your hydraulic components.
In cold weather, hydraulic oil needs heating. With high ambient temperature, your machine requires cooling. Some machinery may require both on the same day. Temperature switches supply the signal to relays or controllers to turn on either an electric heating element or an electric liquid-to-air heat exchanger.
Simple temperature switches work by traditional hydraulic principles. A liquid-filled bulb placed into the hydraulic fluid increases volume as pressure rises due to the increased thermal energy. The pressure exerts a force on the diaphragm or piston like the mechanical pressure switch, and when a particular heat-induced pressure is achieved, the plunger closes the electrical contact.
Most switches used in hydraulic applications come optioned with both normally open and normally closed contacts. Normally open or closed expressed in fluid power terms differs from the electrical term. In hydraulics, a normally closed valve is non-flowing in its neutral state. In electrics, a normally closed switch is flowing in its neutral state.
A normally open switch prevents the electrical function from working until the pressure or temperature signal is met, at which point the switch closes, and electrons can flow. For example, the switching function, in terms of an electric cooling fan, will then turn on the fan to suck/blow air past the heat exchanger, thereby cooling the hydraulic fluid.
Many switches used in fluid power are bi-wireable so that they may be used as either normally open or normally closed contacts. The same switch, for example, may be used to simultaneously turn on and off two different electrical devices, such as turning on the cooling fan while turning a fluid condition monitoring lamp.
Just a step up in the switching technology, you’ll find we do without the mechanical switch in place of solid-state electronics. Solid-state electronics use various components that change properties when exposed to pressure or heat. A digital temperature sensor may use one of many electronic principles to measure temperature. For example, some use a measurement of voltage across diodes. That voltage changes proportionate to the temperature change, and the onboard electronics turn that voltage change into a digital signal. The processor may use the digital signal for its switching output or convert it into an analog output preferred by machine controllers.
Digital temperature switches combine the transducer with switching outputs and offer myriad benefits thanks to their onboard processors. The processor provides a powerful resource to add features above the switching function alone. Because their onboard transducers feed their signals to the onboard processor, another analog signal may also output to an external device or controller. For example, a manufacturer may offer a switch in one of the three more common analog outputs of 0-5 V, 0-10 V or 4-20 mA.
Advanced pressure measurement
More advanced pressure switches offer features to increase their practicality, such as a digital display with programmable functions. For example, instead of a mechanical spring to adjust the temperature switch range (or worse, factory set), buttons on the switch offer programmability not just in the switch point but also the hysteresis, display units, and desired analog output. This last feature reduces the stock numbers by manufacturers and distributors alike since they will output any of the 0-5 V, 0-10 V or 4-20 mA signals most often used in the fluid power industry.
The pressure switch realizes many of the same benefits when upgraded to a digital offering. A similar onboard processor with a display may give the user multiple programmable switching outputs, configurable display, switch time delays, etc. Once the processer receives its signal, much of the technology offered to the user is like the temperature switch. Where the pressure switch differs is the nature of its transducer.
Most often, the pressure transducer uses a strain gauge to convert the force of the pressure into a usable electrical signal. The “interlocked combs” construction of the strain gauge results in a change in its electrical resistance as it’s stretched or deformed. The processor observes the difference and outputs its digital signal proportional to the rise in pressure.
The strain gauge alone cannot offer a usable output for any controller, especially in such thermally variable environments where hydraulics make home. The strain gauge is used in a wheatstone bridge circuit, which is both complicated and simple at the same time. It uses two fixed and two variable resistors to balance voltage and net-zero compared to the incoming voltage.
One of the variable resistors is the strain gauge, while the other is a variable resistor used to set the baseline voltage to zero. Any change in the resistance of the strain gauge results in a change in the voltage across the wheatstone bridge, providing the controller with an accurate pressure reading. I should mention that temperature sensors may use a wheatstone bridge, but just with a temperature-sensitive resistor rather than a strain gauge.
With so little technological progression in hydraulics revolving around the hydraulic system itself, the way we control them offers a peek into the future of fluid power. Sensors, including transducers and switches, will play an integral role in that future, and more of our industry becomes digitized, automated and data-driven.
Filed Under: Engineering Basics, Sensors, Sensors & Gauges