Sensing technologies

Sensors are used heavily in fluid power applications, to measure critical functions such as position, flow, pressure, temperature, and more. The variety of devices that exist to gauge these functions are many, but here, we take an in-depth lookat some of the more commonly used devices — position and pressure sensors and transducers.

PRESSURE TRANSDUCERS

Pressure is defined as the force per given area required to stop a fluid from expanding. Pressure transducers, which are a subset of pressure sensors, can be any number of devices that sample and record the pressure in a system. A pressure transducer converts a pressure measurement into an analog electrical output signal, which can be used by sensing instrumentation such as microprocessors and computers. Most often, this is accomplished simply through physical deformation or mechanical deflection. Important criteria to consider when selecting a pressure transducer are the general mechanism type, input and output, and performance specifications.

The most common types of pressure transducers are strain gauge, and thick/thin film. Strain gauge transducers use the mechanical deformation under pressure of strain-sensitive variable resistors, which may be integrated into measurement circuits such as a wheatstone bridge. In a thick/thin film transducer, a titanium nitride or polysilicon film may be applied to sensing equipment to impart the circuit with piezoelectric sensitivity to pressure.

Image courtesy of Sigma-Netics

Almost all pressure transducers require a source of electrical input. The transducer input voltage can vary but typically falls under 10 V, while the output is typically in the hundreds of thousandths of volts. A change in the system’s pressure would cause a change in the transducer’s resistance on the electrical circuit and would result in a change to the output voltage. With the aid of an analog to digital converter (ADC), the transducer’s output signal can be used in systems that require digital signals. For example, a programmable logic controller (PLC) or a programmable automation controller (PAC) can use the digital signal to monitor the pressure and take action if needed.

Some pressure transducers output current rather than voltage, and are then often referred to as transmitters. These values typically fall within tens of thousandths of amps. When choosing the output of a pressure transducer, it is important to keep in mind the input requirements of the device that will be accepting the signal, the distance the signal must travel and possible interference that can be found in the environment around the system.

Important performance criteria to consider are the pressure transducer’s operating pressure range, maximum rated pressure, accuracy and operating temperature range. The operating pressure range demarcates the intended pressure bounds at which the transducer has been designed to perform optimally. The maximum rated pressure is the highest allowable pressure that the pressure transducer is rated to withstand. The accuracy of the transducer is usually represented by suppliers in terms of ASME B40.1 grades: 4A (0.1%), 3A (0.25%), 2A (0.5%), A (1%), B (2%), C (3%) and D (4%) deviance from the true pressure value.

A good pressure transducer is designed to operate independently of temperature; however, the operating temperature specifies a “safe” range; operating outside of this temperature may significantly affect the accuracy of pressure sensing.

For typical industrial applications, select a 0.5% accuracy class. This should be sufficient for most closed-loop systems. Higher accuracy will quickly increase the price. Before making that investment, determine if the rest of your system requires this higher accuracy.

Image courtesy of IMI Norgren

Accuracy is a constant value found on the data sheet. Unfortunately, most hydraulic systems start cold and get hot, so your actual pressure accuracy will depend on temperature change. The overall accuracy is accuracy class plus error due to temperature change.

The most common output for industrial transducers is 0 to 10 Vdc. Gaining popularity is 0.1 to 10 Vdc, because the control system can detect a transducer fault. If the pressure signal falls below 0.1 Vdc, either the cable has been disconnected or the transducer has failed.

For longer cable runs, a 4 to 20 mA output is preferable. Pressure transmitters reject electrical noise, so the analog signal is clean. The 4 mA offset helps the control system detect sensor faults. However, 4 to 20 mA transmitters have 20% lower resolution, because the 0 to 4 mA is not usable.

POSITIONING SENSING

Several technologies exist to provide position feedback or data of a cylinder’s position. The sensor converts the position intoa proportional analog or digital signal.

Hall-Effect Transducers use a magnet that communicates with the Hall chips, which then give an output to the internally built microprocessor. The output from the microprocessor is converted to a signal required by the user interface such as voltage, current, PWM or digital output. Using Hall-effect technology on linear position sensors allows manufacturers to make small, compact designs that can be mounted internally or externally to a cylinder. Hall-effect technology is well suited to mobile applications as it is highly resistive to shock and vibration. It is used commonly on steering and depth applications.

Image courtesy of Rota Engineering

LVDTs or Linear Variable-Distance Transducers are durable and resist shock and vibration while offering high repeatability. These absolute linear position/displacement transducers convert a linear displacement into an analog electrical signal. Their design includes transformer coils wound around non-magnetic coils.

LVITs — Linear Variable Inductive Transducers — are contactless position sensing devices, with sensing ranges up to 30 in. or more. Most designs feature an inductive probe surrounded by a conductive tube. This is attached to the moving object to make the reading. These contactless position sensing devices use eddy currents developed by an inductor in the surface of a conductive movable element to vary the resonant frequency of an L-C tank circuit. Modern electronics using microprocessors and small component size makes high performance possible, achieving linearity errors of less than ±0.1% and temperature coefficients of 50 ppm/°F, along with either analog or digital outputs.

Magnetostrictive Transducers measure the distance between a position magnet attached to the component in motion and the head-end of a sensing rod that is attached to the axis to be measured. The magnet does not touch the sensing rod, so no parts can wear out. The sensing rod mounts along the motion axis to be measured and the position magnet attaches to the member that moves. An electronics module sends an analog or digital position reading to a controller or other receiving device. Also within the electronics housing is the electrical connection interface, either an integral connector or cable and visual diagnostic LEDs to ensure proper wiring, power, and magnet positioning.

Encoders are used heavily on mobile machinery. Available with capacitive, optical or magnetic sensing and with incremental or absolute position sensing, these devices accurately determine the stroke of the cylinder. Rotary and linear devices are available, but rotary styles are most common. These devices measure position and resolution in pulses per revolution.

Image courtesy of Liebherr

Some rotary encoder designs use a wire-actuated sensor mounted directly inside the cylinder. Others are mounted externally. Others are based on Hall-effect, magnetostrictive, or inductive technologies.

Linear encoders are available with resistive, capacitive, optical or magnetic sensing with incremental or absolute position sensing to accurately determine the stroke of the cylinder.

UNDERSTANDING SENSOR OUTPUT

Sensors most often output a variable voltage or variable amperage signal. The most common outputs are 0-5 VDC, 0-10VDC and 4-20mA. Sensors taking advantage of Industry 4.0 operate using CAN networks, itself a variable voltage output. Going a step further, some sensors now employ wireless technology such as Bluetooth.

The output signal from a sensor connects to your PLC, if you have one, to help the machine recognize the state of the parameter its measuring. It may use a pressure signal to trigger the next sequence of operation or turn on a cooling fan when it measures excessive fluid temperature.

The sensor outputs may tell a smart relay to activate an auxiliary function. A differential pressure sensor relays to a warning lamp telling the operator that the hydraulic filtering is nearing its bypass pressure and provides maintenance with the opportunity to change the element.

Sensors may offer a visual display, either with integrated LCD/LED screens, or by sending its output to a secondary display screen. Maintenance parameters, such as contamination level, temperature, fluid level or backpressure, offer data on machine health critical to reliability.