Retrofitting a tactile sensing head

Improve processing results by retrofitting a tactile sensing system with a capacitive height sensing system

Cliff Ankersen

Do you have a flatbed laser sheet-cutting machine equipped with a tactile sensor? In the early days of laser cutting, you had only the choice of "tactile" (contact) height sensors or the purely manual method of trying to locate the work beneath the cutting head. The manual method worked, albeit with manual effort and attention. The more automated "tactile" method used a height-sensing mechanism to maintain focus height relative to the surface of the work. This was an improvement from the manual approach, and, therefore, people migrated away from the pure manual method to the tactile sensor mechanisms.

Figure 1. A conventional tactile height-sensing head.

People gained knowledge of operating with tactile sensors. Programmers wrote routines that maximized the usefulness of these devices while working to minimize their shortcomings. Some of the typical problem areas with tactile sensors are their tendency to hang up when a cut part would flip up or out of position as the head passed over the work. This would cause the sensor to jamb the work into the cut sheet or bend the tactile contact device that rides on the work surface, sometimes called the "spoon." In addition, the design requirement of the spoon is such that it sometimes interferes with the nesting methods desired for optimizing sheet utilization, resulting in some loss of available cutting area particularly near the sheet clamps.

Tactile sensing requires a device that touches the work surface and provides feedback to a device that maintains the nozzle height (see Figure 1). For some types of materials such as plastics and wood, this remains the mode of operation today, but for metals and materials that conduct electricity, today's preferred method is the use of capacitive (non-contact) height sensing. This method allows the cutting head to "float" over the work surface maintaining a predetermined height during the cutting operation.

The electronics and mechanical components involved in producing a capacitive height-sensing system require designing and placing electronics between the machine controller and the laser beam cutting nozzle, through which the cutting beam is focused and assist gas is delivered to the work. Integrating these components to provide quick response while allowing user input and flexibility to adjust to specific wants and needs is what today's capacitive height-sensing systems offer. These components require tight machining tolerances and clever assembly techniques to provide industrial-strength products capable of operating unattended with fail-safe shut-down capability in automatic installations.

The evolution from the first prototypes to today's product offerings incorporates many trials and developments that evolved with the laser industry, as together they went through growing spurts evolving into more robust and efficient industrial laser systems. A typical system, using a Precitec cutting head, will offer cartridge-style lens holders, which allow both lens centering as well as lens height adjustments in one unit (see Figure 2). Another feature is the incorporation of all electronic connections to be made away from the cutting location, with connections up at the top of the cutting head for improved robust operation. Internal routing of assist gas and cooling is provided internal to the head without external lines or connections near the lower area of the head. Attention to the external finishing of the head provides for reduced spatter attraction for maximum up-time in industrial conditions.

Midbrook Products (Jackson, MI) has been running a Mitsubishi 3015 NC laser cutter since 1993. Mitsubishi built this system specifically for Midbrook, because the company needed a cutter with a large, 12 in., Z motion. This unit was equipped with a conventional tactile (contact)-sensing head.

According to Ray Cole, a supervisor at Midbrook, the contact head has always raised issues; cut parts handing-up and the cutting head hitting previously cut parts (a characteristic of any contact heads, he says). This problem necessitates the time and attention of a machine operator. Other issues, including use with materials that cannot be scratched due to the tactile sensor contact and the fact that thinner materials cut better, he claims, were factors in a decision to look for a change.

Even with these processing drawbacks Midbrook only recently decided to look into corrective action, brought about by a suggestion made six months ago by a representative of MC Machinery, the Mitsubishi distributor.

The recommended action was to retrofit the cutter with a new non-contact head from Precitec. Cole looked into the cost and decided, "It wouldn't take that long to recoup the investment by the things they could do with it [the non-contact cutting head]." In other words, it made sense and was doable.

Figure 2. A retrofit, capacitive height sensor.

The changeover took about two days, possibly because it was the first retrofit Precitec had performed. Programming made no difference, Cole says, as long as the head generated a signal, and they don't notice any difference.

Midbrook now experiences more efficiency from the retrofitted system. Cole mentions that nesting would work a lot better with the non-contact type head. With a contact head, if the operator didn't stop after each part was cut it might be caught with subsequent motion. In addition, the possibility of the operator trying to get the part out might get his hand caught.

After six months of use, Cole says they have no problems and they are happy with the change. Asked if Midbrook has ever considered replacing the old system when it had the contact head with a new more modern system, Cole says no. This unit was a special machine when they purchased it, and they had learned to live with the issues he identified. However the opportunity to retrofit, with the potential to make this unit more efficient, makes sense.

Midbrook operates four laser cutters, three with non-contact sensing heads and one with a tactile head, which is only three years old. Retrofitting this unit is not in the cards because the company has other plans for it.

If you review the equipment on your floor, there is a reasonable chance that you might have one of the earlier-model laser cutting systems—one that may have a tactile sensing system. If this is the case and on occasion you run short on capacity to produce customers' product where a non-contact, capacitive height sensing cutting head is required, there is an economic solution—retrofitting your old machine with a current state-of-the-art capacitive height sensing system.

Precitec currently offers turnkey retrofits with full warranty and only minimal machine downtime, generally performing a retrofit within two days.

Cliff Ankersen, North American Sales, laser products for Precitec Inc. (New Hudson, MI) can be contacted by telephone at (248) 446-8100 or e-mail - [email protected]