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.

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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 blockScott 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.


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.JPGLaser 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-codedScott 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.

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Measurement Fundamentals: Position Measurement vs. Distance Measurement

Continuous measurements on industrial machines or the materials that these machines are making, moving, or processing can be categorized into two main types of sensors:  position measurement sensors, and distance measurement sensors.  It’s a somewhat subtle distinction, but one that is important when evaluating the best measurement sensor for a particular application.

Position Measurement: When we speak in terms of position measurement, we’re typically talking about applications where a the sensor is installed onto a machine, and mechanically coupled to the moving part of the machine – or is installed into a hydraulic cylinder that is moving the machine – and is reporting the continuous position of the machine.  In a positioning application, the questions that need to be answered are: “Where is it?  Where is it now?  And now?”.

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Examples of position measurement sensors include magnetostrictive linear position sensors and magnetically encoded linear sensors.  With each of these sensor types, either the sensor itself, or the position marker, is typically attached to the moving part of the machine.

Distance Measurement: Distance measurement sensors, on the other hand, are used in applications that require accurate measurement of a target that is typically no part of the machine.  A good example would be an application where parts or components are moving along a conveyer belt, and the position of those parts needs to be accurately measured.  In this example, it wouldn’t be practical, or even possible, to attach a sensor to the moving part.  So its position needs to be measured from a DISTANCE.  In a distance measuring application, the question being answered is: “How far away is it?”.

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Examples of distance measuring sensors include photoelectric (laser) sensors and inductive distance sensors.  These types of sensors are usually mounted on the machine, or in the immediate vicinity of the machine, and are aimed at a point or a path where the object to be measured is, or will be, located.

In summary, while both position and distance sensors do much the same thing – provide continuous indication of position – the applications for each are generally quite different.  Gaining an understanding of the application and its requirements will help to determine which type of sensor is the best choice for the task.

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