Height sensing makes sense

Cutting metal with a laser requires a lens that has to be focused correctly to obtain optimal results. Figure 1 shows the influence of lens defocusing by some 1/10 mm on the roughness of the cut kerf. Even if sub-optimal results concerning roughness and dross can be accepted, the metal should at least be completely cut through. Figure 2 shows the influence of defocusing on the maximum penetrable thickness.

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Cutting metal with a laser requires a lens that has to be focused correctly to obtain optimal results. Figure 1 shows the influence of lens defocusing by some 1/10 mm on the roughness of the cut kerf. Even if sub-optimal results concerning roughness and dross can be accepted, the metal should at least be completely cut through. Figure 2 shows the influence of defocusing on the maximum penetrable thickness.

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Obviously, laser cutting machines need height control features for the cutting lens. The height sensing has to be targeted at the small area of the workpiece that is hit by the laser radiation, an area of adverse conditions such as strong heat load, spatter, dust, and electrically charged metal plumes. Nevertheless sensing accuracy must be in the range of 1/10 mm.

In the early days of laser cutting no sensors were available until tactile height control systems emerged, which are still in use today. Exotic techniques, such as measuring the static pressure of a gas flow ejected by a nozzle close above the workpiece, were proposed. Then the idea of using capacitive height-sensing techniques for the distance between the cutting nozzle and the surface of the workpiece appeared. How should that work under laser cutting conditions? Simple theory and experience showed that the useful signal level created by a height variation of 1/10 mm would be overwhelmed by interfering short-term and long-term effects by a factor of 20 to 100. How then can such obstacles be safely overcome?

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Figure 1. Roughness of cut kerf versus deviation of focal point from optimum position.
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The electrical capacitance of two conducting parts facing each other at a certain distance, D, such as a cutting nozzle and a workpiece, results from the attraction force between the electrons located in one part and a positively charged or even grounded second part. That situation happens if a voltage is applied between the workpiece (solidly grounded) and the nozzle of the cutting head when more electrons will be present on the nozzle surface than there would be without voltage applied. Reducing the distance between both parts increases the attraction force of the workpiece to the electrons on the nozzle, causing additional electrons to flow to the nozzle, which can be measured.

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Figure 2. Maximum penetrable thickness of workpiece versus deviation of focal point from optimum position.
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To arrive at a formula, the electrical capacitance, C, of the nozzle can be increased by reducing D (C ~ 1/D). The capacitance also would be increased by enlarging the facing area, A, of the parts. Certain electrical properties, p, of the material between both parts are also influential. In total the capacitance of flat conductive parts facing each other at constant distance, D, can be calculated by C = p (A/D), if stray effects at the perimeter of the parts are neglected.

This formula offers an encouraging message: The capacitance, C, which is determining the level of the sensor signal output voltage, is most responsive for small values of D, which corresponds perfectly to normal cutting conditions. For such conditions a height control range of about D = 0 to 10 mm is usually desired while optimum accuracy has to be preferably obtained in the range of the relatively short working distance of about D = 0.5 to 1.5 mm. Figure 3 illustrates the response of the output voltage versus distance, D. Usually such characteristics will be linearized to be used for closed-loop control systems. Due to the strong inclination of the characteristic in the working distance region the error margins after linearization are smaller in that region than for larger distances.

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Figure 3. Typical stand-off distance sensor characteristic prior to linearization.
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The formula also shows some technological challenges involved in developing reliable capacitive height sensing systems. For example, the occurrence of p and A means that unintended variations of these parameters would also change the capacitance. Unfortunately such variations are common during laser cutting and they became more effective because of increases in laser power, cutting speed, and complexity of workpiece shapes and material composition.

However, the main problems of pioneering capacitive height sensing systems are not deduced from the formula. Issues at that time were the avoidance of stray capacitance effects of the cables, connectors, and so on and the instability of the sensor output voltage versus changes of temperature and humidity. To reduce the effects of stray capacitance, long cables had to be avoided and therefore parts of the sensor signal assessment electronics had to be installed in the cutting heads. These heated up occasionally and also provided no sufficient shielding against changes of atmospheric humidity. So the layout and integration of the electronic circuits had to be optimized steadily until common variations of temperature and humidity did not require recalibration procedures at every production shift.

Spatter was an early issue, because spatter hitting the nozzle changes its electrical potential, which in turn influences the measurement of its capacitance. Fortunately, spatter creates a specific signal pattern, which can be identified and compensated to some extent. The effect of excessive spatter impact cannot be compensated though, and therefore intelligent sensor electronics create an alarm signal. However, excessive spatter is an indication the process is running near the borderline of the stability region. In such cases a revision of all relevant parameters by the machine user is advisable rather than maintaining the situation and frequently resetting the height sensing electronics.

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Figure 4. Reduction of stand-off distance mismatch near obstacles by the active shielding technique.
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When 3D cutting started to become important, A in the above formula began to have effect. A represents the part of the surface of the nozzle that is in the immediate vicinity to the workpiece. 3D cutting heads are supposed to be as slim as possible, which calls for small dimensions of all parts, including the nozzle (and its area A) and components involved in electrical isolation and connection. Those components create increased internal stray capacitances, while the useful capacitance of the nozzle is reduced considerably (by a factor of about 10) compared to the situation of the bulky 2D cutting heads and nozzles. Because of the conical shape of the nozzles, the lower part of the cone contributes mostly to the useful capacitance against flat parts of the workpieces. However, if strongly curved 3D workpieces are concerned, there will be considerable side influence from walls coming near to the cone flanges, due to the increased relevant area A. That causes the control system to leave the programmed height position.

To reduce these problems, a new type of capacitive sensing, based on the active shielding technique, has been introduced by Precitec. This technique allows the use of union nuts, which shield the upper part of the nozzle thereby reducing the increase of the area A in the vicinity of walls of the workpiece. As a second advantage, that technique does not need ceramic parts for separating the nozzle electrically against the cutting head. Figure 4 is an example of the improvement of side influence reduction. As can be seen, there is still a residual deviation of the nozzle stand-off distance from the set point. Deviations in opposite direction occur if the workpiece is bent away from the nozzle. If such deviations are still too large to maintain acceptable cutting quality, the height control system may be switched automatically by the machine's CNC to a smaller or higher set point value for the wall-situation or the downward-bending situation, respectively.

Cutting zinc-plated mild steel became important in the early 1990s. That was the time, when the letter p in the formula drew more attention. Similar situations were created later during high-power, high-speed cutting of aluminum and stainless steel, where the space between the nozzle orifice and the workpiece is occupied by the cutting gas and with vapor created by the melt of the workpiece. The vapor of zinc-plated steel is highly conductive due to the ionized zinc, as opposed to the isolating cutting gas or the vapor of pure mild steel. Furthermore the ionized zinc concentration fluctuates rapidly, which adds considerable noise to the sensor signal. Depending on laser power and cutting speed the capacitive height sensing may be spoiled by that effect. One approach to overcome that problem simply freezes the electronic height sensing circuit for a short time (some hundredths of milliseconds) immediately after the noise overload starts. Of course, this method must fail if strong noise overload is present for a longer time period. Precitec has hardened the patented active shielding technique against strong variations of the p-parameter. Right now the hardened active shielding technique is almost as resistant against plasma as the traditional unshielded technique.

High-level capacitive sensing systems today are capable of offering a wide span of helpful features in addition to the height sensing function such as: auto calibration of the sensor output versus height characteristics; auto linearization of these characteristics; programmable set-point variation; body touch alarm signal; reverse spatter alarm signal; plasma occurrence alarm signal; and workpiece coordinates acquisition. For some of these functions the cooperation of the machine's CNC is needed.

The potential of capacitive sensing techniques is still not completely explored. At Precitec additional features are under development now that are part of an overall strategy to further increase the performance and smartness level of future cutting modules, which support the customers' needs for flexibility, reliability, and accuracy.

Dr. Wolf Wiesemann is CEO of Precitec KG, Gaggenau, Germany. 1The company's products are shown on its website, www.precitec.com

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