Waterjet cutting in the era of intelligent manufacturing

– Part IV: sensor technology

Dr. Jay Zeng, Shanghai Lionstek Co., Ltd

If the motion controller and the software (discussed in Part I and Part II) are compared to the human brain, the five-axis motion system (discussed in Part III) is compared to arms and legs, then what will be discussed in this article can be compared to human senses such as eyes and ears. Without eyes and ears, the human being will not be able to do any work and will not be aware if harm is coming in his/her way. If we want our machine to be smart enough to work independently without or with minimum human intervention, the machine will need to be equipped with sensor technologies. Therefore, this article will talk about the sensor technologies used on a smart waterjet cutting machine.

When cutting the aircraft engine part as shown in Figure 1, we learned a great lesson. The workpiece material is very expensive and workpiece scrapping is unaffordable. The total processing time for each part is about 33 hours of cutting time plus 2-3 days of setup time. The setup time can be significantly reduced if a customized production machine is built. The following problems occurred during the cutting process of the first couple of parts.

First of all, we did not have a proper way to find the position of the workpiece accurately. The workpiece came as a round disc with locating pin holes. To find the center of the workpiece, we had to make some trial cuts which were very time-consuming and inaccurate.

The jet made a big splash such that water and abrasive were everywhere on the machine. In additional to the jet splash, the machine was also surrounded by mist and fog. The abrasive feeding system utilized vacuum pressure created by the nozzle to suck in abrasive. It happened that the mist and fog was unfortunately sucked into the abrasive feeding system, causing stoppage of abrasive flow in the middle of the cutting process. When the operator found out and stopped the machine, it was already too late and the workpiece was scrapped.

The operator did not notice the abrasive shortage in the tank and the part would have been scrapped if  he had not acted fast enough to stop the machine.

The operator did not notice that a pin was in the path of the cutting head until a collision occurred. The workpiece was scrapped.

From this costly lesson we realized that our machine operation heavily relied on the experience and constant attention of the operator. For such a long duration cutting job, with so many potential problem areas, workpiece scrapping is a high probability event. Therefore we have to find ways to improve on the sensor technologies and rely more on automatic sensor technologies and CNC control system instead of human beings for critical machining jobs.

Figure 1 Cutting of aircraft engine part

There are several sensor technologies that are used in today’s smart waterjet cutting machines. I will discuss each of these technologies in turn.

Machine positioning sensor technology

Typically machine position is sensed by the encoder of servomotors. There are two types of encoders: rotary encoder and linear encoder. Rotary encoder (e.g. Figure 1(a)) converts angular displacement into electrical signal and is also called code disk. Most waterjet cutting machines use servomotors equipped with rotary encoders. The rotary motion is converted to linear motion through the use of ball screw or rack/pinion or other drive systems. Because the rotary encoder cannot sense the error of the drive system, the linear positional accuracy mainly depends on the accuracy of the drive system. A precision ground ball screw will provide a higher linear positioning accuracy than a rolled ball screw. Linear encoder converts linear displacement into electrical signal and is also called grating ruler. The linear positioning accuracy using linear encoder is higher than using rotary encoder because it does not depend on the accuracy of the drive system. There are two types of grating rulers: magnetic grating rulers (e.g. Figure 1(b)) and optical grating rulers (e.g. Figure 1(c)). Compared to optical version, magnetic grating rulers have lower precision, much lower cost, and are suitable for longer travel. In the past decade, waterjet machines that use magnetic grating rulers with rotary servo motors have been seen in the market. In the recent years optical grating rulers and linear motors have been found on some high precision models of waterjet cutting machines.

According to the working principle, the encoder can be divided into incremental type and absolute type. The incremental encoder converts the displacement into periodic electrical signal, and then converts the electrical signal into counting pulse. The machine position is determined by the pulse count in reference to the absolute origin of the machine. If the machine is powered off during motion, the machine position will be lost and the machine needs to be homed to the absolute origin to re-establish its position. For the absolute encoder, each position corresponds to a certain digital code, so the machine position is always known. If the machine is powered off during motion, the machine position will not be lost and homing the machine is unnecessary.

Figure 1 (a) Rotary encoder; (b) Magnetic linear encoder; (c) Optical linear encoder

Workpiece XY positioning sensor technology

When a workpiece is placed on the machine, it is necessary for the machine controller to recognize the position of the workpiece and associate its position with the CNC program. If you just want to cut a small part anywhere on a large plate of material, you can move the machine to where you want to start the cutting program. The CNC program will take the current machine position (no matter whether it is accurate or not) as the path origin, and start the cutting program there. This is quite common for waterjet cutting. However, if you are going to cut a full pattern on a large plate of material, it is necessary to find the edges of the plate so that the whole pattern can be cut within the edges of the plate. Sometime the plate is so large and so heavy that, once it is placed on the machine, moving around is almost impossible. If the plate orientation is not parallel to the machine XY axes, it is better to rotate the cutting program instead of rotating the material.

In some applications waterjet cutting is one of several machining processes. The positioning issue in multiple machining processes can be solved by using certain locating features. For example, the part shown in Figure 2 uses two locating pin holes (indicated with yellow arrows) for the purpose of positioning. A certain type of positioning device should be used to find these locating features.

Figure 2 A waterjet part with locating pin holes

Several positioning devices are available for the purpose of finding the position of workpiece. The simplest one is a precision laser pen like the one shown in Figure 3. It is held by the waterjet nozzle in place of the mixing tube. It generates a size-adjustable laser spot on the workpiece. You can move the machine so that the laser spot is on the edge of workpiece and then record the current machine position. Recording two spots on a straight edge will be able to define its orientation. Two straight edges will be able to define a corner. Recording three spots on an arc will be able to locate its center. The accuracy of the laser edge finder is usually within 0.05-0.10mm. If the laser edge finder has a different height from that of the nozzle, it will be necessary to move the Z axis up and down when using the laser edge finder and the perpendicularity error of Z axis to the XY plane will add to the positioning error.

Figure 3 Laser edge finder

A more accurate solution is an optical positioning device. It uses an industrial camera with a high resolution (e.g. 5-10 megapixel) and appropriate lighting device. Software tools such as crosshair can be used to capture spots on the edge of workpiece on the software screen. Its accuracy is typically within 0.02-0.05mm.

For waterjet cutting of electronic components, which has a higher standard in accuracy and speed, a CCD (Charge-Coupled Device) camera (as shown in Figure 4) with an even higher resolution (at the expanse of a smaller field of view) is used with the corresponding control software. The position of the workpiece can be quickly and automatically determined. Its accuracy can reach 0.005-0.02mm. Once the position of the workpiece is determined, the cutting program can be automatically moved and rotated accordingly.

Figure 4 CCD automatic positioning system

Workpiece height tracking sensor technology

Even though most waterjet cutting application are 2D cutting on a flat plate or sheet of material, the upper surface of the workpiece is not always on the XY plane because of the following reasons: the material may not be flat; the relief of internal stresses of the material during cutting causes material warpage; or the material support surface may not be parallel to the XY plane. Any of the above situations can cause the working distance between the nozzle and the workpiece to vary. The consequence could be one of these: (1) the nozzle is plugged because it is too close to the workpiece during piercing; (2) the mixing tube breaks because it touches and drags on the workpiece; (3) the cutting accuracy decreases because a large working distance causes the jet to spread and lose its cutting power and accuracy; (4) the cutting accuracy decreases because the working distance is different from the theoretical value required by the tilting head if used. Therefore, it is necessary to keep track of the working distance and make it relatively constant.  A common practice is to use a nozzle height tracking device as shown in Figure 5. The foot of the height tracking device is spring-loaded to keep in touch with the workpiece surface. The height variation of the workpiece surface will be sensed by a displacement transducer inside the device. The displacement analog signal will be sent to the CNC controller, which will command the Z axis to move up and down to maintain a constant working distance between the nozzle and the workpiece. The principle is quite simple. However, the biggest challenge is its reliability. The height tracking device works in an environment with water and abrasive as well as the deflected high speed abrasive waterjet. The device needs to be sealed against water and abrasive and be protected from the wear by the deflected high speed abrasive waterjet. Scraps and small parts may tipping and get in the way of the foot of the tracking device. Holes of the workpiece material may become traps for the tracking device. If the water tank is full of spent abrasive, the spent abrasive may form a hardened layer on the surface of the workpiece and “fool” the tracking device. Therefore measures are taken to overcome these obstacles. The foot of the tracking device is a ring surrounding the nozzle. This ring pushes away deposited abrasive if any. To prevent the tracking device falls over a “cliff” or climbs a “hill”, the tracking device is only allowed to respond to a slope of 10 degrees or less. To avoid the tracking device to collide with fixture or tipping parts or scraps, the tracking device will take a head-up mode (move up – traverse – move down) when it traverses.

Figure 5 Nozzle height tracking device

Machine collision sensor technology

Most waterjet cutting applications are for a small quantity of parts. It is not unusual to have several cutting jobs a day. Cutting programs are changed frequently. Workpiece fixture is often impromptu setup. Deformed workpiece and tipping scrap or small part can catch the nozzle. All these add to the risk of collision during waterjet cutting. The consequence of collision could be one or more of these: mixing tube breakage, workpiece scrapping or even permanent machine damage. A collision sensor can be used to prevent these damages.

For a machine equipped with a tilting head (e.g. as shown in Figure 5), the torque outputs of the A and B axis servomotors can be monitored to detect collision. Once the torque reaches a certain limit, the CNC controller will stop the machine immediately. For a two or three-axes machine, a collision detection device as shown in Figure 6 can be used. The collision detection device sends a switching signal to the CNC controller to stop the machine once a collision occurs.

Figure 6 Collision detection device

Machine fault sensor technology

A machine monitoring system is used to monitor the status of the machine. The statuses of all servomotors are monitored by the motion controller. The water pressure of the high pressure pump is monitored with a high pressure transducer (e.g. Figure 7). When the pressure is above or below a certain range, an alarm will be triggered. The abrasive level inside the pressurized abrasive tank is monitored with a proximity switch, which will be activated when the abrasive level falls below it. The air pressure is monitored with an air pressure switch, which will be triggered when the air pressure falls below a certain level. When the abrasive low-level switch is triggered, warning will be displayed on the controller screen and yellow warning light will be lit. But the machine is allowed to continue the cutting operation. Operator can select to add more abrasive or to stop the machine when it comes to an appropriate spot.  When any of the other sensor alarms is triggered, the machine will stop immediately. Operator or service personnel can then inspect the machine, solve the problems if any, and then resume cutting at the interrupted spot.

Figure 7 High pressure transducer

Cutting process sensor technology

According to the references [1] and [2], it is feasible to use a vacuum sensor to monitor the conditions of an abrasive waterjet, as shown in Figure 8.

Figure 8 Nozzle monitoring device with a vacuum sensor [1]

Figure 9 shows the vacuum signals for different jet conditions. When abrasive waterjet cutting starts in a normal condition, its vacuum signal can be used as a baseline for monitoring. If the red zone in Figure 9 is used as the boundary for vacuum signal, the vacuum signal can be used to detect the following problems:

Mixing tube plugged

Abrasive running out

Abrasive feed tube plugged

No cutting (no jet)

If the yellow zone in Figure 9 is used as the boundary for vacuum signal, the vacuum signal can be also used to detect the conditions of bad orifice and mixing tube breakage. However, because the stability of the vacuum signal is not good enough, the yellow zone may be too small and false triggering may occur frequently. Therefore it is not advised to use the yellow zone as the boundary for vacuum signal.

Figure 9 Detection of abrasive waterjet conditions with a vacuum sensor

It is my goal that one day the waterjet cutting machine will become a traditional CNC machine. The user can operate it to produce parts accurately, efficiently and reliably without having to worry about so many parameters and trivialities. With the advancement of CNC control technology and modern sensor technologies, we are one step closer to my goal. However we are not there yet. We need the whole waterjet community to share this goal, to   develop more sensor technologies, and more importantly, to make the waterjet machine more reliable, accurate, and efficient.

Again the author sincerely welcomes feedback, corrections, and discussions. Feedback can be received at my email address:  zengjiyue@lionstek.com.

References:

[1] J. Zeng and J. P. Munoz, “Feasibility of Monitoring Abrasive Waterjet Conditions by Means of A Vacuum Sensor”, Proceedings of the 12^th International Conference on Jet Cutting Technology, Rouen, France, Oct 25-27, 1994.

[2] J. Zeng and J. P. Munoz, “Adaptive Process Control system”, US Patent No. 5854744, 1998.