Today’s Pressure Sensors: More Options to Meet Your Application Needs

Pressure sensing devices are prevalent in industrial machinery.  

 

There are three types of pressure measurements, each with their advantages and disadvantages. 

Absolute pressure (psia) is referenced to a perfect vacuum. This pressure measurement is always positive and is used in measuring barometric pressure or in altimeters.  

 

Gauge pressure (psig) measurement is measured relative to ambient pressure. Examples of gauge pressure include blood pressure and intake manifold vacuum in an engine. Typically, these types of sensors have an atmospheric vent hole located somewhere in the housing. An advantage to this type of sensor is it can measure both positive and negative, thus it can be used in vacuum applications such as robots picking up glass or other products with suction cups.  

 

Differential pressure (psid) measurement is the difference between two pressure sources. The gauge pressure measurement is really a differential measurement as one side is open to the ambient pressure and the other side is connected to the process pressure. However, for most differential pressure sensors the second pressure source is not the ambient air. 

In the past, pressure sensing was accomplished by mechanical switches that typically used Bourdon tubes, diaphragms or bellows. These devices caused a mechanical movement in the switch as the pressure increased or decreased. A course adjustment screw was used to set the desired set point to actuate the various controls. In addition to the switch, some sort of indication was needed so an analog gauge was also typically required. While these devices solved the control requirement, accuracy was not as reliable as the controls required or what was preferred. 

To convert pressure into an electrical output, several different technologies are used in electronic pressure sensors. The first type of strain gauge technology — Piezoresistive technology — is based on measuring the resistance of a deforming silicon semiconductor. stainless steel housing protects the silicon chip as pressure is indirectly transferred to the membrane with a liquid that is usually silicon oil. This type of measurement is most often used in high dynamic pressures.

Thin film technology utilizes a stainless steel carrier. The resistors and other circuitry are placed on the membrane, and measurement is based on the strain gauge technology. The advantage of the thin stainless steel film is its ability to withstand high peak pressures and burst pressures.  

Thick film technology, which also utilizes strain gauge concepts, uses a ceramic carrier. The resistors and other associated circuitry are placed on a membrane using a thick film process. Ceramic cells offer long-term stability and good corrosion resistance. 

In capacitive measuring cells, one electrode is fitted to an elastic membrane and the other electrode is on the support or housing surface on the opposite side. This forms a capacitor in which one electrode follows the movement of the membrane. As the pressure increases or decreases the distance between the electrodes change causing a change in capacitance. 

 

Today’s pressure sensors incorporate both the switching functions and the display of the current pressure. Since these devices are electronic, there are a multitude of output functions available as opposed to the simple on-off functionality of the mechanical pressure switch. These include multiple discrete PNP or NPN outputs from one sensor with multiple functionalities. In many cases a sensor will provide a single discrete output plus a continuous analog output proportional to the pressure value. The discrete output can provide an alarm function while the analog output provides a dynamic value of the process.

The discrete outputs can be programmed for various operations. First and most important are the set points, sometimes referred to as hysteresis, of when the output should activate (SP) and when the output should reset or turn off (RP). Hysteresis keeps the switching outputs stable even if the system pressure fluctuates around the set point.

In some applications it is desirable to know if the pressure is within operating range for machine functions to continue. The output or outputcan be programmed with a window function. The output will be active as long as the measured values fall between the defined low pressure and the defined upper pressure.

Pressure spikes can cause problems not only with the mechanics of the system but with the logic of electrical controls including outputs changing states quickly or chattering. The electronic sensors offered today include the ability to delay the switching outputs of the sensor. Typically, the delays are programmable up to 50 seconds. 

Pressure is usually measured in PSI or bar with one bar of pressure being equal to 14.5 PSI. When applying pressure sensors various pressures should be taken into consideration. First is the nominal operating pressure of the system. The pressure sensor applied to the system should in the 50 – 60% maximum rating of the sensor as this will provide a safety margin.  

 

Overload pressure can be caused by pressure spikes in the system from valves opening or closing or pump cavitation. These spikes can exceed the specified sensor limit, however, no permanent damage or change will occur. Burst pressure is the pressure that can cause permanent damage to the sensing device or mechanical damage to the sensor.   

 

What if the application involves a paste or thick substance that could potentially clog the orifice or dead space of the sensor? Some pressure sensor manufacturers offer flush mounted pressure sensors. These devices are perfect for detecting pastes, greases or thick substances as the bottom of the sensor has a protective membrane, typically stainless steel.  

 

Today’s display is multifunctional as not only does it display dynamic values but it is also used for programming or configuring the sensor. Included on the display is the pressure, parameters, parameter values, scaling of the device, and output(s) status. Also included are programming keys and, in some cases, keypad lock out functionality. 

The true epitome of a pressure sensor is one that can have all of capabilities I’ve mentioned as well as the ability to provide additional functionality and parameterization. Pressure sensors that connect to networks such as IO-Link can optimize processes allowing process monitoring, configuration and error analysis to take place through the system controller. Digital transmission of analog values ensures high signal quality over longer distances and signal delays and distortions are eliminated.  

 

Networkable sensors, such as IO-Link, reduce downtimes and possible configuration errors with plug-and-play functionality. Maximum system flexibility can be achieved during operation as parameters can be modified quickly and remotely. In addition, process diagnostics, data, and errors are reported directly to the controller and displayed on man-machine interfaces. 

 

Pressure sensors have come a long way from the multiple mechanical based components used in the past in both functionality and capabilities. 

 

Pressure-Rated Inductive Sensors Add Security in Mobile Equipment

Manufacturers of mobile equipment have long understood the benefits of replacing mechanical switches with the non-contacting technology of inductive sensors.  Inductive sensors provide wear-free position feedback in a sealed housing suitable for demanding environments.  But some applications may require a different approach if potential mounting issues or sensing ranges are a concern.  For instance, as the mobile machine ages and bushings wear due to typical daily operations, the sensing air gap between the linkage to be sensed and the sensor face may increase beyond the sensor’s optimum working range.   If this scenario is possible, periodic maintenance will be required to adjust the sensor mounting to compensate for the increasing wear.  Another consideration is the mounting bracket itself, and the likelihood of misalignment due to physical contact.

Many off road applications requiring sensor feedback involve hydraulic cylinders.  If these cases, a pressure-rated inductive sensor installed inside a cylinder or valve may be the better design choice.  Pressure-rated inductive sensors are offered with a variety of discrete outputs with numerous housing styles and connections.  Utilizing non-contact switching, stainless steel housings, and sealed to pressures up to 500 Bar, the sensors are designed to provide reliable feedback under the harsh conditions of off highway applications.

Mounting a pressure-rated inductive sensor into a cylinder or valve is straightforward, and very similar to the preparation of a hydraulic port:

      1. the sensor is threaded into the cylinder wall
      2. the sensing air gap is set
      3. the provided nut locks down the sensor
      4. a cable or connector is attached.

Day-to-day wear of the machine no longer affects the sensing gap and the sensor benefits from the additional protection of being installed into the cylinder, avoiding mounting mishaps and is better protected from external damage.

An outrigger application is a good example of the added benefits of using a pressure-rated inductive sensor.  Outriggers are used in cranes, firetrucks, aerial devices, and other mobile machines to provide lateral stability.  Mechanical switches and standard inductive sensors are used to denote when the outrigger is fully raised, lowered, etc.  A standard external sensor will do a good job as long as the mounting is intact and the sensing gap is within the proper range.  But a pressure-rated inductive sensor mounted internally into the hydraulic cylinder takes the worry out of those potential failure scenarios.

Applications with locking cylinders should also be considered.  Many locking cylinder applications are associated with a safety feature, where feedback that the cylinder is locked is critical.  An example would be the rear hatch of a refuse truck.  Occasionally, a worker may need to get inside the rear of a refuse truck.  With the rear hatch raised hydraulically, there’s a possibility that the rear hatch closes with gravity.  Positive feedback that the cylinder is locked is reassuring.

Therefore to reduce downtime caused by wear, to eliminate the misalignment of a mounting bracket, or to ensure your locking cylinder is absolutely locked, consider going “internal” to increase the quality and security of your application.

Error Proof Stamping Applications with Pressure Sensors

When improving product quality or production efficiency, manufacturing engineers typically turn to automation solutions to error proof and improve their application. In stamping applications, that often leads to adding sensors to help detect the presence of a material or a feature in a part being formed, for example, a hole in a part. In the stamping world, this can be referred to as “In-Die Sensing” or “Die Protection.” The term “Die Protection” is used because if the sensors do not see the material in the correct location when forming, then it could cause a die crash. The cost of a die crash can add up quickly. Not only is there lost production time, but also damage to the die that can be extremely costly to repair. Typically, several sensors are used throughout the die to look for material or features in the material at different locations, to make sure the material is present to protect the die. Manufacturing engineers tend to use photoelectric and/or inductive proximity sensors in these applications; however, pressure sensors are a cost-effective and straightforward alternative.

In today’s stamping applications, manufacturing engineers want to stamp parts faster while reducing downtime and scrap. One growing trend in press shops is the addition of nitrogen on the dies. By adding nitrogen-filled gas springs and/or nitrogen gas-filled lifters, the press can run faster and cycle parts through quicker.

Typically, the die is charged with nitrogen before the press starts running parts. Today, many stamping plants rely on an analog dial gauge (image 1) to determine if there is sufficient nitrogen pressure to operate safely. When a new die is set in the press, someone must look at the gauge and make sure it is correct before running the press. There is no type of signal or feedback from this gauge to the PLC or the press; therefore, no real error proofing method is in place to notify the operator if the pressure rating is correct or even present before starting the press. If the operator starts running the press without any nitrogen for the springs, then it will not cycle the material and can cause a crash.

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Another, likely more significant problem engineers face is a hole forming in one of the hoses while they are running. A very small hole in a hose may not be noticeable to the operator and may not even show up on the analog dial gauge. Without this feedback from the gauge, the press will continue to run and increase the likelihood that the parts will be stamped and be out of specification, causing unnecessary scrap. Scrap costs can be quite large and grow larger until the leak is discovered. Additionally, if the material cannot move through the press properly because of a lack of nitrogen pressure to the springs or lifters, it could cause material to back up and cause a crash.

By using a pressure sensor, you can set high and low pressure settings that will give an output when either of those is reached. The outputs can be discrete, analog, or IO-Link, and they can be tied to your PLC to trigger an alarm for the operator, send an alert to the HMI, or even stop the press. You can also have the PLC make sure pressure is present before starting the press to verify it was adequately charged with nitrogen during set up.

Adding an electronic pressure sensor to monitor the nitrogen pressure is a simple and cost-effective way to error proof this application and avoid costly problems.

Temperature sensing of process media — a hot topic in today’s manufacturing

Continuous control of process media significantly contributes to the reliability of industrial production. More and more process technology is involved in industrial manufacturing.  Besides pressure and level sensors, temperature sensors are also needed to monitor and control these media. Although new machine designs are often optimized in terms of energy efficiency, heat is added to the production equipment.

Thermal reading of media by temperature sensors

Process stability

To achieve a defined and stable temperature level (in many cases only slightly above the environmental temperature) the added heat dissipation of the production process constantly must be managed. Typically a coolant liquid or hydraulic fluid is cycling through the areas of the production equipment, which tend to heat up. It then runs to a heat exchanger system which cools down the liquid to a defined value. Some applications even require a defined viscosity of the liquids in use. Often the media viscosity depends on its temperature. Historically classic cylindrical housing temperature probes have been applied for temperature measurement. The values are transferred by cables to a PLC. For factory automation applications, housings with integrated display and an adjustable switching point (via pushbutton parametrization) have become more and more popular.

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Many housing styles now also include a digital display so in addition to the sensor transmitting temperature values via cable to the control system, they provide a visual monitoring functionality for the machine/plant operator.

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Hydraulic power pack

Monitoring of industrial processes

Monitoring of industrial processes has become more and more relevant. With increasing digitalization in manufacturing, the demand of transparent visualization of the production constantly grows.

 

Clamp Control of Tools and Workpieces

In Metalworking, the clamping status of tools and workpieces are monitored in many Image1applications. Typically, inductive sensors are used to control this.

Three positions are usually detected: Unclamped, clamped with object, and clamped without object. The sensor position is mechanically adjusted to the application so the correct clamping process and clamping status is detected with a proper switch point. Additionally, with the usage of several sensors in many cases the diagnostic coverage is increased.

For approximately 15 years, inductive distance sensors with analog output signals have been utilized in these applications with the advantage of providing more flexibility.

 Image2By using a tapered (conical) shape, an axial movement of the clamping rod can be sensed (as a change of distance to the inductive sensor with analog output). Several sensors with binary (switching) output can be replaced with a sensor using such a continuous output signal (0..10V, 4-20 mA or e.g. IO-Link). Let’s figure a tool in a spindle is replaced by another tool with a different defined clamping position. Now, rather than mechanically changing the mechanical position of the inductive sensor with binary output, the parameter values for the correct analog signal window are adjusted in the control system. This allows easy parameter setting to the application, relevant if the dimensions of the clamped object may vary with different production lots.

The latest state-of-the-art sensor solution is the concept of a compact linear position system which is built of several inductive sensor elements mounted in one single housing. Image3

Instead of a tapered (conical) shape, a disk shaped target moves lateral to the sensor. From small strokes (e.g. 14 mm) up to more than 100 mm, different product variants offer the best combination of compact design and needed lateral movement. Having data about the clamping force (e.g. by using pressure sensors to monitor the hydraulic pressure) will lead to additional information about the clamping status.

For more information on linear position sensors visit www.balluff.com.

For more information on pressure sensors, visit www.balluff.com.

 

Acids Can Put Your Sensors in a Pickle

In many types of metals production, pickling is a process that is essential to removing impurities and contaminants from the surface of the material prior to further processing, such as the application of anti-corrosion coatings.

In steel production, two common pickling solutions or pickle liquors are hydrochloric acid (HCl) and sulfuric acid (H2SO4). Both of these acids are very effective at removing rust and iron oxide scale from the steel prior to additional processing, for example galvanizing or rolling. The choice of acid depends on the processing temperature, the type of steel being processed, and environmental containment and recovery considerations. Hydrochloric acid creates corrosive fumes when heated, so it typically must be used at lower temperatures where processing times are longer. It is also more expensive to recover when spent. Sulfuric acid can be used at higher temperatures for faster processing, but it can attack the base metal more aggressively and create embrittlement due to hydrogen diffusion into the metal.

Acids can be just as tough on all of the equipment involved in the pickling lines, including sensors. When selecting sensors for use in areas involving liquid acid solutions and gaseous fumes and vapors, care must be given to the types of acids involved and to the materials used in the construction of the sensor, particularly the materials that may be in direct contact with the media.

PressureSensor
A pressure sensor specifically designed for use with acidic media, at temperatures up to 125° C.

A manufacturer of silicon steel was having issues with frequent failure of mechanical pressure sensors on the pickling line, due to the effects of severe corrosion from hydrochloric acid at 25% concentration. After determination of the root cause of these failures and evaluation of alternatives, the maintenance team selected an electronic pressure sensor with a process connection custom-made from PVDF (polyvinylidene fluoride), a VitonTM O-ring, and a ceramic (rather than standard stainless steel) pressure diaphragm. This changeover eliminated the corroded mechanical pressure sensors as an ongoing maintenance problem, increasing equipment availability and freeing up maintenance personnel to address other issues on the line.