linear position sensors Archives - AUTOMATION INSIGHTS

Choosing the Best Position Sensor: Does Yours Measure Up?

Reliable electronic measurement is something that is always needed in industrial automation. Production interruptions and unexpected downtime will cripple even the best manufacturers if they do not have the appropriate measurement technology in place.

Whether it’s monitoring the position of a hydraulic jack or determining the proper position of a flood gate on a dam, be sure to choose the best option for precision, accuracy, and most importantly, reliability.

Strings Holding Down Production

String potentiometers, also known as string pots, yo-yo sensors, cable-extension transducers, and a few other names, have been used for electronic measurement for the last 40 years.

These devices use braided steel wires (“strings”) wrapped around spools and require the release of the coiled string.

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In an industrial assembly application, a typical scenario might involve wire being integrated into a manufacturing platform that moves from one assembly station to the next. As the string pot’s spool extends or retracts, position is measured by a rotational sensor/potentiometer that rests outside the spool and will trigger based on the position of the metal wire.

While string pots are often used in many sectors (heavy industry, crude oil processing, waste water treatment, etc.), they come with potential issues that make then unsuitable for others:

The wire will eventually jam from rust, mechanical glitches, or other environmental factors
The springs in the reel often fail over time
The high contact nature of the devices causes friction among the components, which leads to excessive wear and failure after a few thousand rotations

Combined, these things lead to expensive downtime/loss of production and costly repairs. A measurement system should not be a consumable item or an item with an expected but unpredictable maintenance interval. A measurement system should be designed with longevity and reliability in mind.

Right Solution for Your Industry

The assembly industry is not the only one that benefits from highly accurate position measurement. Whether it’s metallurgy, plastics and rubber, energy, or woodwork — the advanced, versatile, and resilient technology is required to thrive in high speed and demanding applications.

Fortunately, magnetostrictive linear transducers were developed to provide the kind of reliable position measurement that industry demands.

Instead of a trouble-prone mechanism, magnetostrictive non-contact linear transducers work with a movable free-floating or captive magnet that rides the length of a sensing rod as it follows the target object.

During operation, a very short-duration pulse is generated along the sensing element. This is known as the waveguide. The resulting magnetic field interacts with the magnetic field of the position magnet and generates a mechanical force on the sensing element. This force ripples along the waveguide at a faster-than-sound velocity that is detected by the sensor electronics, and is converted into an electrical pulse.

Using a very accurate high-speed clock, the time interval between the initial current pulse and return pulse is measured and converted into an absolute position value.

The end result is constant, precise, accurate, and smooth position measurement.

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As an example: A high speed punch press requires position monitoring down to the millisecond. The punch press is designed to move very quickly back and forth in rows, punching holes in precise locations. When one row is finished, the unit moves forward and does the next. As the punch continues back and forth and up the rows, the sensor follows the position of the press, transmitting position feedback to the control system. This ensures the press stays on the appropriate track and punches where it should.

A contact device would not be suitable for this kind of operation, as the amount of friction caused from the speed and repetition would wear the sensor down too quickly and cause failure.

Fortunately, magnetostrictive linear sensors are widely available, come in a variety of form factors, and are truly non-contact, with some “floating” versions riding as much as 15mm off the surface of the transducer body. No contact means no wire binding issues and the lack of contact also means a lack of impact damage, which will help the sensor survive longer than a string potentiometer.

A measuring distance from 1 to 300 inches, offers short and long range capabilities.

Moreover, compared to string pots, magnetostrictive linear sensors, require fewer components. This means fewer parts to replace and maintain, which results in a reduction in overall equipment and maintenance costs.

Adaptable to nearly any industrial control system, these sensors are available in common analog (0….10V or 4….20 mA), as well as a variety of digital interfaces. This includes digital start/stop, synchronous serial interface, as well as network interfaces (IO-Link, Ethernet/IP, Profinet).

Tying it All Together

Though both string pots and magnetostrictive linear transducers are employed for electronic measurement, selecting the one that is best for your application will maximize manufacturing efficiency, increase machine uptime, and cut costs. All while ensuring your process keeps running smoothly and your customers get the parts and products they need on time.

Clamp Control of Tools and Workpieces

In Metalworking, the clamping status of tools and workpieces are monitored in many applications. 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.

By 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.

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.

IO-Link Measurement Sensors Solve Application Challenges

In industrial distance and position measurement applications, one size definitely does not fit all. Depending on the application, the position or distance to be measured can range from just a few millimeters up to dozens of meters. No single industrial sensor technology is capable of meeting these diverse requirements.

Fortunately, machine builders, OEM’s and end-users can now choose from a wide variety of IO-Link distance and position measurement sensors to suit nearly any requirement. In this article, we’ll do a quick rundown of some of the more popular IO-Link measurement sensor types.

(For more information about the advantages of IO-Link versus traditional analog measurement sensors, see the following blog posts, Solving Analog Integration Conundrum, Simplify Your Existing Analog Sensor Connection, and How Do I Make My Analog Sensor Less Complex?)

Short Range Inductive Distance Sensors

These sensors, available in tubular and block Scott Image1.JPG style form factors are used to measure very short distances, typically in the 1…5 mm range. The operating principle is similar to a standard on/off inductive proximity sensor. However, instead of discrete on/off operation, the distance from the face of the sensor to a steel target is expressed as a continuously variable value. Their extremely small size makes them ideal for applications in confined spaces.

Inductive Linear Position Sensors

Inductive linear position sensors are available in several block style form factors, and are used for position measurement over stroke lengths up to about 135 mm. These types of sensors use an array of inductive coils to accurately measure the position of a metal target. Compact form factors and low stroke-to-overall length factor make them well suited for application with limited space.

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Magnetostrictive Linear Position Sensors

IO-Link Magnetostrictive linear position sensors are available in rod style form factors for hydraulic cylinder position feedback, and in external mount profile form factors for general factory automation position monitoring applications. These sensors use time-proven, non-contact magnetostrictive technology to provide accurate, absolute position feedback over stroke lengths up to 4.8 meters.

Laser Optical Distance Sensors

Scott Image 4.JPG Laser distance sensors use either a time-of-flight measuring principle (for long range) or triangulation measuring principle (for shorter range) to precisely measure sensor to target distance from up to 6 meters away. Laser distance sensors are especially useful in applications where the sensor must be located away from the target to be measured.

Magnetic Linear Encoders

IO-Link magnetic linear encoders use an absolute-coded Scott Image 5 flexible magnet tape and a compact sensing head to provide extremely accurate position, absolute position feedback over stroke lengths up to 8 meters. Flexible installation, compact overall size, and extremely fast response time make magnetic linear encoders an excellent choice for demanding, fast moving applications.

IO-Link Measurement Sensor Trends

The proliferation of available IO-Link measurement sensors is made possible, in large part, due to the implementation of IO-Link specification 1.1, which allows faster data transmission and parameter server functionality. The higher data transfer speed is especially important for measurement sensors because continuous distance or position values require much more data compared to discrete on/off data. The server parameter function allows device settings to be stored in the sensor and backed up in the IO-Link master. That means that a sensor can be replaced, and all relevant settings can be downloaded from master to sensor automatically.

To learn about IO-Link in general and IO-Link measurement sensors in particular, visit www.balluff.com.

Hydraulic Valves – Customize your Feedback

Hydraulic actuators can be used to open and close a valve’s position. In automation architectures, a linear position sensor is used within the hydraulic actuator to provide continuous position feedback.

The linear position sensor is installed into the back end of the cylinder. The sensing element resides in a cavity that has been gun-drilled through the piston and cylinder rod, extending the full length of the mechanical stroke. A magnet ring is used as a position marker and mounted on the face of the piston. As the piston (and the position marker) move, the linear position sensor provides a continuous absolute position by way of an analog or digital signal.

In some applications, a cylinder’s position may only be moving across a small portion of the overall stroke or a specific portion of the stroke. The end user could benefit from altering the transducer’s signal based on the application’s specific stroke requirements instead of the entire cylinder’s stroke, thereby maximizing available position resolution. When this situation arises, most transducer manufacturers offer the ability to customize or “teach” a modified output of the stroke via push buttons or from wiring inputs. When this is done, the process does require the cylinder (and position marker) to move to these defined locations for a “teach”.

A more user-friendly and repeatable approach for customized stroke lengths with linear position sensors is to use a graphical software package. The software can be connected
from a PC via USB to a compatible linear position sensor. Starting and ending stroke values can be precisely entered into the software and a graphical representation of the output curve is created. For a more straightforward approach, you can also drag and drop these stroke points by a click of a cursor. The file can be saved on a PC and downloaded to the transducer. In either case, the cylinder’s piston doesn’t need to be actuated.

In projects where multiple, identical actuators and linear position sensors need to be customized, the setup would only need to be done once, the file saved, and simply uploaded to all the sensors for the project. A great time-saver over manually teaching each and every sensor.

Another benefit to using software with linear position sensors is to be able to upload programs for replacement units in a safe user environment (e.g. lab station or office) and shipping them to various job sites. These different locations (or locales) can be in harsh environmental conditions (extreme cold or heat) or areas that contain ignitable or explosive gases or dusts which may be difficult to work in.

Other software features include inverting the output curves, offering position or velocity outputs, and more.

For more information on Balluff’s Magnetostrictive Linear Position Sensors, visit www.balluff.com.

External Position Feedback for Hydraulic Cylinders

The classic linear position feedback solution for hydraulic cylinders is the rod-style magnetostrictive sensor installed from the back end of the cylinder. The cylinder rod is gun-drilled to accept the length of the sensor probe, and a target magnet is installed on the face of the piston. A hydraulic port on the end cap provides installation access to thread in the pressure-rated sensor tube. This type of installation carries several advantages but also some potential disadvantages depending on the application.

EDITED Image_HF — *Position Sensor Mounted Internally in a Hydraulic Cylinder*
*(Image credit:* Cowan Dynamics)

Advantages of in-cylinder sensor mounting include:

Simplicity. The cylinder manufacturer “preps” the cylinder for the sensor and may install it as an extra-cost option.
Ruggedness. The sensor element is protected inside the cylinder. Only the electronics head is exposed to the rigors of the industrial environment.
Compactness. The sensor is contained inside the cylinder, so it does not add to the cross-sectional area occupied by the cylinder.
Direct Position Measurement. Because the target magnet is mounted on the piston, the sensor is directly monitoring the motion of the cylinder without any interposing linkages that might introduce some position error, especially in highly dynamic, high-acceleration / deceleration applications.

Potential disadvantages of in-cylinder sensor mounting may include:

Sensor Cost. Cylinder-mounted position sensors require a rugged, fully-sealed stainless-steel sensor probe to withstand the dynamic pressures inside a cylinder. This adds some manufacturing cost.
Cylinder Cost. The procedure of gun-drilling a cylinder rod consumes machine time and depletes tooling, adding manufacturing cost over a standard cylinder. Refer to additional comments under Small Cylinder Bores / Rods below.
Cylinder Delivery Time. Prepping a new cylinder for a sensor adds manufacturing time due to additional processing steps, some of which may be outsourced by the cylinder manufacturer, increasing overall shipping and handling time.
Overall Installed Length. Because the sensor electronics and cabling protrude from the back end of the cylinder, this adds to the overall length of the installed cylinder. Refer to additional comments under Small Cylinder Bores / Rods below.
Service Access. In case sensor repair is required, there must be sufficient clearance or access behind the cylinder to pull out the full length of the sensor probe.
Small Cylinder Bores / Rods. Some cylinder bores and rod diameters are too small to allow for gun-drilling a hole large enough to install the ~10.2 mm diameter sensor tube and allow for proper fluid flow around it. In tie rod cylinders, the distance between the rod nuts may be too small to allow the flange of the position sensor to fully seat against the O-ring. In these cases, a mounting boss must be provided to move the mounting position back past the tie rods. This adds cost as well as increases overall installed length.

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In cases where the advantages of in-cylinder mounting are outweighed or rendered impractical by some of the disadvantages, an externally-mounted position sensor can be considered. The list of advantages and disadvantages looks similar, but reversed.

Position Sensor Mounted Externally on Hydraulically-Actuated Equipment

Advantages of external sensor mounting include:

Sensor Cost. Externally-mounted magnetostrictive position sensors are typically made from an aluminum extrusion and die-cast end caps with gaskets, saving cost compared to all-stainless-steel welded and pressure-rated construction.
Cylinder Cost. The cylinder can be a standard type with no special machining work needed to accommodate installation of the sensor.
Cylinder Delivery Time. Since no additional machine work is needed, the cylinder manufacturer can deliver within their standard lead time for standard cylinders.
Overall Installed Length. Typically, the external sensor is mounted in parallel to the cylinder, so overall length is not increased.
Service Access. The externally-mounted sensor is easily accessible for service by simply unbolting its mounting brackets and pulling it off the equipment.

Disadvantages of external sensor mounting may include:

Complexity. The machine designer or end user must provide the means to mount the sensor brackets and the means to position a floating magnet target over the sensor housing. Alternatively, a captive sliding magnet target may be used with a length of operating rod and swivel attachment hardware.
Exposure to Damage. Unless guarded or installed in a protected area, an externally mounted position sensor is subject to being mechanically damaged.
Space Requirements. There must be enough empty space around the cylinder or on the machine to accommodate the sensor housing and operating envelope of the moving magnetic target.
Indirect Position Measurement. Any time a floating target magnet is mounted to a bracket, there is the potential for position error due to the bracket getting bent, flexing under acceleration / deceleration, mounting bolts loosening, etc. In the case of operating rods for captive sliding magnets, there will be some mechanical take-up in the swivel joints upon change of direction, adding to position hysteresis. There is also the potential for rod flexing under heavy acceleration / deceleration – particularly when the rod is acting under compression vs. tension. Take note of the amount of sliding friction of the captive magnet on the sensor rails; some sensor magnet designs offer high friction and stiff resistance to movement that can increase operating rod deflection and resultant position error.

In conclusion, be sure to consider all aspects of an application requiring cylinder position feedback and choose the approach that maximizes the most important advantages and eliminates or minimizes any potential disadvantages. It may be that an externally-mounted position sensor will solve some of the challenges being faced with implementing a traditional in-cylinder application.

For more information about internally- and externally-mounted cylinder position sensors, visit www.balluff.com.

External Linear Position Sensors: Floating or Captive Magnet?

External Linear Position Sensors: Floating or Captive Magnet?
Linear position sensors that are designed to be mounted externally on a machine (as opposed to those designed to be installed into a hydraulic or pneumatic cylinder) are available in a variety of form factors that suit a variety of different applications and application requirements. One of the most common form factors, particularly for magnetostrictive linear position sensors, is a rectilinear aluminum extrusion that houses the sensing element, or waveguide, and the processing electronics. Commonly, you’ll hear these referred to as profile-style linear position sensors.

Captive magnet (left) and floating magnet (right)

With these types of sensors, the moving part of the machine to be measured or monitored is attached to a position magnet. The position magnet can be either captive or floating (see image to the right). Each of these magnet configurations offer some inherent advantages. We’re going to take a closer look at each.

Captive Magnet

A captive magnet glides along in a track that is an integral part of the extruded aluminum sensor housing. The magnet is attached to the moving part of the machine via a mechanical linkage. Advantages of a captive magnet arrangement include:

Mechanical flexibility: The magnet usually incorporates an articulating swivel or ball joint that is attached via a linking rod to the moving machine part. That means the sensor doesn’t need to be perfectly in line with the axis of movement.
Protection from damage – In some cases, it is necessary to move the sensor out of harm’s way (e.g., extreme heat, caustic chemicals, strong electromagnetic fields, etc.). The linkage can be as long as necessary in order to connect to the sensor, which will be located in a more hospitable environment.

Some things to consider when choosing to use a captive magnet configuration:

Binding of the magnet: A high-quality magnetostrictive sensor is going have a near-zero drag coefficient between magnet and extrusion. The magnet should not bind or drag. But in some applications, dirt, grease and particulates can accumulate and cause issues. For these applications, a floating magnet may be a better choice.
Mechanical overtravel: In a captive magnet arrangement, if the machine travel exceeds the physical length of the sensor, the magnet will (of course) fall off the track. If this is a concern, consider a floating magnet instead.

Floating Magnet

In a floating magnet arrangement, the sensor is located adjacent to the moving machine part. The magnet is attached to that machine part, usually on a rigid arm or bracket. Advantages of a floating magnet include:

No mechanical contact: The magnet never makes contact with the housing. This could be important in applications where dirt, grease or particulates tend to collect on the sensor (see photo below)
Machine overtravel: Since the magnet is completely uncoupled from the sensor, machine overtravel isn’t a problem. Obviously, if the magnet leaves the sensor, position feedback is lost, but the sensor will resume normal operation once the magnet re-enters the sensor’s range.

Some things to consider when choosing a floating magnet configuration:

Magnet-to-sensor gap: In some cases the movement of the machine does not allow a consistent magnet-to-sensor gap to be maintained. In some sensors, this can lead to inconsistent or erratic sensor operation. Fortunately, there are sensors available with innovative technology that automatically compensates for such gap fluctuations and maintain full performance and specifications even as the gap varies. Click below to see such technology in action.

Ultimately, the choice between a floating magnet and a captive magnet arrangement is going to be driven by the requirements of your particular application.

Click the link for more information on external-mount linear position sensors.

Quick field replacement for linear sensor electronics

Micropulse Transducers BTL 7 Rod-style with Rapid Replacement Module — Micropulse Transducers BTL 7
Rod-style with Rapid Replacement Module

When maintenance technicians replace linear position sensors (also known as probes or wands) from hydraulic cylinders, it can leave a terrible mess, waste hydraulic oils, and expose the individual to harmful hot fluids. Also, the change out process can expose the hydraulic system to unwanted contaminants. After the sensor replacement has been completed, there can also be more work yet to do during the outage such as replacing fluids and air-bleeding cylinders.

Hydraulic linear position sensors with field-replaceable electronics/sensing elements eliminate these concerns. Such sensors, so-called Rapid Replacement Module (RRM) sensors, allow the “guts” of the sensor to be replaced, while the stainless steel pressure tube remains in the cylinder. The hydraulic seal is never compromised. That means that during the replacement process there is no danger of oil spillage and no need for environmental containment procedures. There is also no need to bleed air from the hydraulic system and no danger of dirt or wood debris entering the open hydraulic port. Finally, there is no danger of repair personnel getting burned by hot oil.

The RRM is an option for Balluff’s BTL7 Z/B Rod Series used in applications for the lumber industry, plastic injection and blow molding, tire and rubber manufacturing, stamping presses, die casting, and all types of automated machinery where a continuous, absolute position signal is required. Applications in industries such as Oil & Gas and Process Control are especially critical when it comes to downtime. For these applications, this Rapid Replacement Module capability is especially advantageous.

You can learn more about linear position sensors with hazardous area approvals, by visting http://www.balluff.com/local/us/products/sensors/magnetostrictive-linear-position-sensors/

The video below shows a demonstration of the Rapid Replacement Module in action.

A superior non-contact sensing principle

Magnetostriction is a property of ferromagnetic (iron-based, magnetizeable) materials that causes them to change their shape or dimensions in the presence of a magnetic field. In addition to numerous other practical uses, this magnetostrictive effect is ideally suited for use in industrial linear position measurement sensors. Magnetostrictive linear position sensors use an iron-alloy sensing element, typically called a waveguide. Referring to the diagram at right, the waveguide (1) is housed inside a pressure-rated stainless steel tube or in an aluminum extrusion. The position magnet (2) is attached to the moving part of the machine, or the piston of a hydraulic or pneumatic cylinder. Measurements are initiated by applying a short-duration electrical pulse to a conductor (3) attached to the waveguide. The current creates a magnetic field (4) along the waveguide.

The magnetic field from the position magnet interacts with the generated magnetic field, inducing a torsional mechanical strain on the waveguide. When the current pulse stops, the strain is released, causing a mechanical pulse to propagate along the waveguide. This mechanical pulse travels at a constant speed, and is detected at the signal converter (5).

The time between the initial electrical pulse and the received mechanical pulse accurately represents the absolute position of the position magnet and, ultimately position of the machine or hydraulic cylinder. The position of the magnet along the waveguide is calculated by very accurately timing the interval between the initial current pulse, also known as the Interrogation Pulse, and the detection of the mechanical return pulse.