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Understanding Inductive-Proximity Sensors for Die Protection

By: George Keremedjiev

Friday, February 9, 2018
 

Let's take a close look at inductive-proximity sensors, the most commonly used die-protection sensors. Available from numerous vendors, these come in a variety of sizes and geometries. Whether cylindrical, rectangular, threaded or nonthreaded, the majority of these long- and short-distance sensors have sensing fields that must, I repeat, must be thoroughly understood in order for the sensors to work properly, consistently and for the duration of a stamping die's life.

What's a Sensing Field?


Fig. 1—When determining the sensing field on a test bench, an inductive-proximity sensor can mount vertically on a fixture, typically a height gauge where a customized insert holds the sensor in position. The target, ideally a small metallic coupon mimicking the material and shape of the item to be sensed in production, is moved slowly and accurately right to left into the invisible radio-field signal coming out of the top of the sensor.
Many incorrectly refer to the sensing field as a magnetic field. If that were true, then how to account for their use with copper, brass and aluminum? No, it is not a magnetic field but rather an inductive field or radio field (RF). Think of your car radio, where your favorite station has a particular frequency, say 100 MHz. That is the number to which you tune the radio. That particular radio station sends out an electromagnetic signal with cycling waveforms traveling in a repeatable pattern of 100 million cycles/sec., or Hz. Likewise, an inductive-proximity sensor sends an RF signal, typically at or under 1 MHz. As with the radio station's signal, the RF signal from the sensor is invisible. With the sensor on, nothing seems to emanate from its sensing surface, but the invisible RF field is both water- and oil-proof.

Variables Determine the Field

A toolmaker or machinist must understand the exact size of that sensor's field to precisely detect the target within the die and make sure that the sensor has been located correctly for that detection. Why? Because the size and shape of the inductive-proximity sensor's RF field typically differs for each type of tool steel and strip material used. In other words, if an inductive-proximity sensor in a particular die works well detecting a block made of D2 material, the very same sensor will have a completely different RF shape and range when paired with 4140 or A2 tool steels. Ditto for strip materials. If a given inductive-proximity sensor works well detecting a target on a strip made of cold-rolled steel, the very same sensor will react differently when tasked with detecting a target made from Type 303 stainless steel-even if the parts are exactly the same and the sensor is located at the very same detection distance.



Fig. 2—Green marks indicate the various positions in space where the sensor has detected the coupon.

 

Fig. 3—The off point, where left-to-right movement of the target coupon causes the sensor to turn off, is indicated in red. Note the definite gap between the target coupon turning on the sensor, marked in green, and turning it off.

Some inductive-proximity sensors are marketed as having the same sensing field for all materials. Be careful here, as such claims require exacting proof. For example, even slight variations in the shape and range of the RF field in a sensor labeled as being insensitive to material types can lead to undetected strip misfeeds.

Use Test Bench to Determine Fields

To determine the exact sensing field, use a sensor test bench. This allows for experimentation without tying up production or damaging production equipment and tooling.

Figs. 1 through 5 outline the process by which an inductive-proximity sensor generates a specific RF field, and how it can be determined. In Fig. 1, the sensor mounts vertically on a fixture, typically a height gauge where a customized insert holds the sensor in position. The target is moved slowly and accurately right to left into the invisible RF signal coming out of the top of the sensor. Do not test large tooling components or parts on the sensor test bench. We have used sample coupons approximately 2 in. long by 1 in. wide and about 1⁄8 in. thick in the case of tool steels or aluminum. These coupons must be of the same alloy as the tooling component or part material to be detected. Also, the geometry of the coupon's surface to be tested must match that of the actual target. For example, to detect a cam return, the coupon surface representing that cam must have the same geometry-if the cam surface has a radius, then the coupon surface must have the same radius.

Move the coupon with precision on two axes, with the X axis as horizontal motion and the Y axis as vertical. Accomplish this via a three-axis micro-positioning table using digital micrometers connected to a computer running an Excel spreadsheet program. The toolmaker or machinist performing the test must design and build, or purchase, a small vise mechanism to hold the test coupon in place on this three-axis device. Power up the inductive-proximity sensor and move the coupon very slowly right-to-left until the sensor reacts. By pressing a switch, the digital micrometers will report their positions to the spreadsheet. The coupon then is indexed 0.002 in. and the process repeats. Fig. 2 shows the various positions in space where the sensor detected the coupon.

Hysteresis Defined and Determined

Fig. 4—With the data obtained as shown in Fig. 3, a macro function in an Excel spreadsheet connects the on and off points into lines. This sensing-area fish pattern is three-dimensional. Fig. 5—An Excel spreadsheet macro automatically generates a mirror image of the original on and off lines as a double-fish pattern. This indicates how the sensor should be positioned during production.
It is not enough to know where the sensor will detect the target but also, where the target will have to be to turn off that detection. That small travel distance is referred to as hysteresis. Unfamiliarity with hysteresis is the bane of many who attempt to use sensors in dies.

Imagine that you have completed the testing as described above and know exactly, to 0.001 in., where the sensor must be to detect the target within the die. But, the target exhibits a slight natural vibration. Perhaps it is vibrating a bit, due to the vibration of the stamping press. Or, maybe the target is located at the end of the strip where a short feed will be detected, but the unstable strip vibrates a few thousands of an inch. In any case, the natural movement of the target can cause the sensor to turn on and off. This, in turn, can cause nuisance stops of a press and create upheaval in the pressroom, perhaps to the extent that the "stupid sensor" is turned off completely. What to do? How much target movement is acceptable? Enter hysteresis.

In Fig. 3, the off point, the location where left-to-right movement of the target causes the sensor to turn off and indicated in red, is entered into the Excel spreadsheet. The same switch that sent data for the on position now is pressed to enter the off position. Fig. 3 shows a definite gap between the target turning on the sensor and turning it off. With this data, Excel, using a macro function, automatically generates a mirror image of the original on and off points and connects the points into lines. This sensing-area fish pattern is three-dimensional, as shown first in its one-sided stage in Fig. 4, and in Fig. 5 with the completed double-fish pattern, again using an Excel macro, with both the on (green) and off (red) lines shown.

Correctly Position the Sensor

Remembering that the double-fish pattern was generated using a sample coupon shaped like and made from the same alloy as the target, it is now time to position the sensor within a block of steel or aluminum and align the edge of the sensor so that the green line touches the target when it travels and stops in its natural in-die condition. Once the target stops and touches the green line, the target can jiggle all it wants, but as long as that motion does not touch the red line, the sensor will remain on and the die-protection control will allow the press to function.

The sides of the fish pattern typically are used for targets moving sideways to the inductive-proximity sensor, such as strips and cams. The tail end of the fish pattern usually finds use for head-on travel, such as a bottoming-out stripper plate or a cam that may be traveling toward the sensor and not sideways into the fish pattern.

Some metalforming companies have developed excellent libraries of hundreds of fish patterns and are so good with their testing and analysis processes that they can accurately and effectively specify where an inductive-proximity sensor must be located within a die-long before a die is actually built. Their experiments are so productive that they can digitize the fish patterns into their CAD programs and place the sensor and its respective fish pattern within the die design as if it were any other normal die component. In fact, electronic sensors are normal die components for these stamping companies. MF

MetalForming magazine's online-exclusive Metalforming Electronics column is aimed at understanding electronic sensors as they are applied to tool and die protection, in-tool part-quality monitoring and plant-wide value-added operations.

For more than 30 years, George Keremedjiev had authored the Metalforming Electronics column in MetalForming magazine and continues his efforts here. He regularly consults with metalforming companies worldwide and provides metalformers with training on the application and implementation of sensors for die protection. For more information on his seminars and consultancies, contact: Tecknow Education Services, Inc. P.O. Box 6448 Bozeman, MT 59771 phone: 406/587-4751 fax: 406/587-9620 www.mfg-advice.com E-mail: gk@mfg-advice.com.

 

See also: Tecknow Education Services, Inc.

Related Enterprise Zones: Sensing/Electronics


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