Understanding Sensors & Error-Proofing, Part 1: Inductive Proximity Sensors

December 1, 2015

Hundreds, if not thousands, of metalforming companies, toolmakers and pressroom personnel have tried and failed to properly implement electronic sensors within their dies. The sensors are consequently deemed by many to be ineffective, too delicate, impossible to properly align with a target, unable to survive the harsh environments within a die, etc. But in great measure, these failures are due to simple misunderstandings regarding sensor location, mounting and protection. We must correct these misunderstandings to ensure ideal sensor performance.

Fig. 1
Fig. 1—When using a test bench to determine the sensing field, 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.

Toolmakers worth their salt quickly acknowledge the precision with which tooling components are engineered, designed and located within dies, from manually fed, single-stroke types on through to automatic progressive and transfer dies. Every internal component has been carefully designed and placed with precision by toolmakers, and maintained with equal care by maintenance toolmakers during production cycles. Should not the same care be taken regarding selection and installation of electronic sensors to protect these valuable electronic investments?

Where is it written that it is okay to crash and repair a die? What genius came up with the phrase, “Oh, that die can take a bad hit?” Who designs dies to take such a bad hit? I have yet, in 30-plus years of consulting with companies on die protection, to see an addendum or note or scribbled phrase on a given die-setup sheet, design drawing or a purchase order, specifying that a die crash is acceptable.

Sensing Fields Explained

Fig. 4
Fig. 2—The green marks indicate the various positions in space where the sensor detected the coupon. Fig. 3—The OFF point—the location 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. 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.

Inductive proximity sensors, the most commonly used electronic die-protection sensors, are available from numerous vendors and come in a variety of sizes and geometries. The vast amount of these sensors used for die protection have a sensing field that must be thoroughly understood in order for them to work properly, consistently and in many cases, for the life of the die.

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, radio field or, in shorthand, 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 inductive proximity sensor is invisible. With the sensor on, nothing seems to emanate from its sensing surface, but the invisible RF field coming from the inductive proximity sensor is both water- and oilproof.

Knowing the Sensor Field a Must

Fig. 5
Fig. 5—A macro in an Excel spreadsheet 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.

A toolmaker or machinist must understand the exact size of that sensor’s field so that a precise detection of the target within the die can be made, and that the sensor is located properly 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 in the shop. In other words, if an inductive proximity sensor works well in a particular die 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 have a completely different reaction 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. Some inductive proximity sensors are marketed as having the same sensing field for all materials. Be very careful when dealing with these types of sensors, as you need exacting proof that these claims ring true. 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, for example, undetected strip misfeeds.

Use Test Bench to Determine Sensing Range

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 a sensor generates a specific RF field, and how it can be determined. In Fig. 1, the inductive proximity 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.

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 distance between the sensor turning on and turning off 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 wait—the target has a slight natural vibration to it. It could be the end of the strip where a short feed will be detected but the strip is unstable and vibrating a few thousands of an inch. Perhaps it is simply vibrating a little in sympathy with the vibration of the press. In any case, it is conceivable that the natural movement of the target can cause the sensor to first turn on and then off. This, in turn, can cause nuisance stops 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 to the sensor being tested? 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 the 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.

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 that move 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 in these stamping companies. MF

The next installment of this series will offer guidance on selection and testing of photoelectric sensors.

Industry-Related Terms: Edge, Fixture, Gauge, Insert, Lines, Plate, Stripper, Strips, Surface, Transfer, Case, Die, Drawing, Brass, CAD, Cam
View Glossary of Metalforming Terms


See also: Tecknow Education Services, Inc.

Technologies: Pressroom Automation, Sensing/Electronics/IOT


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