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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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.

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.