WO2003055644A1

WO2003055644A1 - System for regulating the abrasive mass flow rate in a waterjet cutting system

Info

Publication number: WO2003055644A1
Application number: PCT/EP2002/014884
Authority: WO
Inventors: Massimiliano Annoni; Angelo Galbiati; Michele Monno; Antonio Vergari
Original assignee: Politecnico Di Milano
Priority date: 2001-12-27
Filing date: 2002-12-24
Publication date: 2003-07-10
Also published as: AU2002364402A1; ITMI20012804A1

Abstract

A system for regulating the abrasive mass flow in a waterjet cutting system comprising; means for supplying pressurized water, means (20, 21) for supplying abrasive powder; means for transforming energy (12) into jet kinetic energy for supplying water at high speed; means for mixing (13) said water at high speed and said abrasive power and means (14) for directing said mixture against an object. The means (20, 21) comprise a system for measuring (22) the weight of said abrasive powder and supplying an electrical signal proportional to said weight and means (23) for processing an electrical signal proportional to the difference between the electrical signal proportional to the variation of weight, in a unit of time, and a preset signal of flow required. Batching of the abravise powder controlled by the difference between the signal proportional to the variation of weight and the preset signal of flow required.

Description

"System for regulating the abrasive mass flow rate in a waterjet cutting system".

* * * *

DESCRIPTION The present invention refers to a wateηet cutting system (Water Jet -

WJ). More particularly it refers to a system for regulating the abrasive mass flow rate in a wateηet cutting system.

Wateηet cutting was introduced at the beginning of the seventies and developed commercially only in the following decade, finding application in a good number of fields.

Initially a wide range of materials could be cut (cardboard, fabric, nylon, plywood, polyethylene and food products) by using a jet of pure water expelled at ultra-high speeds (up to 900 m/s) from a nozzle with a small diameter (0.1 - 0.5 mm) but to compete with the traditional cutting technologies, the new technology had also to cut metals.

At the beginning of the eighties grains of abrasive (typically garnet or silica sands) were added to the jet of water, obtaining an enormous increase in the cutting capacity (Abrasive Water Jet - AWJ).

This fact, together with the integration of robotized plants and computerized systems, enabled complex, three-dimensional cutting operations to be carried out, even on the hardest and the most difficult materials to machine with the traditional cutting technologies (for example titanium, stainless steel, marble, glass and composite materials) obtaining performances which were often unobtainable by the other cutting systems. The variations in the abrasive mass flow rate can have quite significant effects on the quality of the cutting. In fact it influences all the parameters that are normally used for assessing the quality of AWJ cutting, in particular the maximum depth of the cutting kerf, the average roughness of the surfaces obtained and the taper of the kerf. Systems for controlling the abrasive mass flow rate of the type based on laser sensors and photodetectors are known and have turned out to be difficult to execute, set up and control. This technology does not seem to be compatible with the aggressive and dusty environment of hydroabrasive jet plants. In addition the introduction of a component (measurer) downstream from the mixing chamber is provided for, and this configuration considerably complicates the overall system.

Systems for controlling the abrasive mass flow rate of the type based on the introduction of tracer particles are known. The characteristics of the tracer particles inserted have to be as similar as possible to the abrasive particles (mass, dimension, friction coefficient, etc.). In addition the system only traces a particle without knowing its position inside the hydroabrasive jet and without knowing the distribution of the particles inside the jet itself. For these reasons it is not certain that the tracer particle represents the entire population of particles inside the hydroabrasive jet. In addition the dimensions of the measuring system cannot be overlooked, as they are not very compatible with the geometric dimensions characteristic of the AWJ technology.

Systems for controlling the abrasive mass flow rate that use "mechanical" type sensors are known; they are indirect measure systems, that is systems that are based on the observation and measuring of a physical phenomenon introduced to be placed in relation (directly or indirectly) with the size whose measurement is required. For example vacuum sensors or strength sensors are used. As well as the problems connected with the measurement of the physical phenomenon, using this type of system, a relation between the quantity measured and the variable whose value has to be known is introduced.

In addition to inserting a physical phenomenon, used for tracing back to the measurement of the flow, it introduces many variables in the model; these variables cannot always be taken into consideration in the model and it is not always possible to carry out a measurement or a control. The Applicant has seen that some of the problems of the known art lie in the fact that attempt was made to assess the flow of the hydroabrasive jet at the output of the focusing tube, meeting quite a few difficulties such as, the speed of the jet of pure water is about three times that of sound in air; in the hydroabrasive jet, even though the speed is a lot less (about 1/3), it is till very high and therefore can be measured only by means of sophisticated, expensive equipment. In addition, studies conducted in the past have shown that the distribution of the speeds of the particles in the jet is comparable to a normal distribution around the axis of the jet itself, but the actual measurements of this distribution are not easy.

In general the system should carry out the measurement in a very short time and have very reduced reaction and adaptation times, to reply rapidly to the request for variation of the flow, the solutions proposed up to date do not always meet these fundamental characteristics. Another obstacle in realizing systems for controlling the abrasive mass flow rate has been the technical-economic aspect. Up to now the solutions found have been difficult to execute in practice because of their complexity and/or their not insignificant cost (above all if put in relation with that of the entire AWJ system). The Applicant has observed that considerable advantages could be derived if the abrasive mass flow rate was estimed upstream that is at the exit of the container of abrasive powder. And other advantages could be derived if the assessment of the abrasive mass flow rate could be inserted in a closed loop control system. The Applicant has also observed that considerable advantages could be derived by controlling the air flow in the path between the container of abrasive powder and the output nozzle.

In view of the state of the technique described, an object of the present invention is to provide a system for controlling the abrasive mass flow rate that does not contain the inconveniences of the known art. In accordance with the present invention, this and other objects are reached by means of a system for regulating the abrasive mass flow rate in a wateηet cutting system comprising: means suitable for supplying pressurized water; means suitable for supplying abrasive powder; means for transforming energy suitable for supplying water at high speed; means for mixing said water at high speed and said abrasive powder; means suitable for directing said mixture against an object; characterized in that said means suitable for supplying abrasive powder comprise a system for measuring the weight of said abrasive powder, suitable for supplying an electrical signal proportional to said weight; means for processing said electrical signal proportional to said weight suitable for generating an electrical signal proportional to the variation of weight of said abrasive powder in a unit of time, and suitable for supplying an electrical signal proportional to the difference between the electrical signal proportional to the variation of weight and a preset signal of flow required; and said means suitable for supplying abrasive powder comprise a system of batching said abrasive powder commanded by said signal proportional to the difference between the electrical signal proportional to the variation of weight and the preset signal of flow required. According to the present invention, such objects are reached also by a wateηet cutting system comprising a system for regulating the abrasive mass flow rate in accordance with claim 1.

Thanks to the present invention a system having a continuous feedback of the flow control signal can be executed. In fact by continuously measuring the variation of the mass of the abrasive powder the abrasive mass flow rate that has been delivered can be known at every moment and consequently, if it shifts from the required value, a continuous correction can be made to keep the abrasive mass flow rate within a suitable interval of values. The regulation made in this manner can also consider the influence of other technological parameters connected to the machining, such as the advancement speed of the cutting head and also any disturbance variables (for example humidity, vibrations etc).

With this last opportunity, provided by the regulating system proposed, the same quality of the cutting kerf is obtained for different advancement speeds, thus enabling profiles with any geometry to be made having uniform quality of the cutting kerf.

In fact, keeping the water pressure level and the distance between nozzle and piece fixed, the specific energy transferred by the hydroabrasive jet to the material being machined results directly proportional to the abrasive mass flow rate and inversely to the advancement speed of the head.

The characteristics and advantages of the present invention will be evident from the following detailed description of an embodiment thereof, illustrated as non-limiting example in the annexed drawings, wherein: Figure 1, represents schematicallya wateηet cutting system in accordance with the present invention;

Figure 2 shows an example of the valve group 21 for regulating the section of passage of the abrasive of Figure l.The water placed in container 10, possibly with the addition of a long-chain polymer for improving the coherence of the jet, is sucked into a pressure intensifier and brought to the required pressure; it is then pushed into an accumulator that acts as attenuator of the pressure pulsation, so that it arrives at the nozzle with quite a uniform pressure. The intensifier and the accumulator are represented schematically in Figure 1 with block 11. The primary nozzle 12 is the point at which the pressure energy is transformed into jet kinetic energy. This component has an orifice whose dimensions vary from 0.1 to 0.5 mm, and the speed of the water at this point can reach 900 m/s, that is about three times the speed of sound in air.

The thickness of the material that can be cut increases as the diameter of the primary nozzle 12 and the pressure set by the intensifier increase, while it diminishes as the relative speed between head and piece increases.

Generally the higher the quality of the cut, the greater the amount of energy is used per unit of length cut.

Following the flow of the jet, after the primary nozzle 12 there is the mixing chamber 13, where the jet of water meets the abrasive and pulls it through a focusing nozzle 14 that keeps the jet coUimated directing it against the piece being machined.

The addition of abrasive to the jet of pure water makes it possible to cut metals; in addition, at the same thickness cut, increasing the quantity of abrasive mixed the cut can be executed with greater advancement speeds of the head which thus lead to significant improvement of the system productivity.

A container 20 (for example a hopper) holds the quantity of abrasive to deliver. Container 20 communicates, at the lower end with a device 21 for regulating the section of passage of the product to be delivered (valve group). It is preferable that the section of device 21 is activated by means of an electrical signal coming from a control system 23. In particular, in a preferred embodiment, the section can be regulated continuously from a minimum section to a maximum section through a signal proportional to the diameter of the section required.

Container 20 and device 21 are weighed by means of, for example, a load cell 22, which preferably, weighs container 20, device 21 and the abrasive powder, and provides the control system 23 with a signal relating to the weight measured. The control system 23 (or processing means) has the task of processing the signal received from load cell 22 and consequently providing device 21 with the command signal.

In a preferred embodiment control system 23 also controls all the other technological parameters connected (for example the advancement speed of the cutting head) and provides device 21 with the activating signal for regulating the section of passage.

Preferably, control system 23 also comprises means of controlling the whole wateηet cutting system and consists of a suitably programmed computer. Preferably a sensor 24 for the clogging of the abrasive powder is installed in the near vicinity of the mixing chamber 13 on the pipe transferring the abrasive delivered and will have the task of detecting in very short times the clogging of abrasive near the cutting head, so that control system 23 can first stop the delivery of abrasive and then start up an automatic cycle which is preset for making the abrasive free the clogged zones and enable the machining to be resumed.

Control system 23 receives a continuous signal from load cell 22 that represents the weight of container 20, of device 21 and of the abrasive powder, it calculates the variation in weight in a preset unit of time, that is the abrasive mass flow rate. This signal is compared with a preset signal of flow required, set by the user on control system 23. If the two signals compared are different, device 21 for regulating the section of passage will be activated to vary the section of passage of the abrasive and modify the delivery flow. In particular, a signal is generated, preferably electrical, correlated to the difference between the electrical signal proportional to the variation of weight and the preset signal of flow required.

An example of the valve group 21 for regulating the section of passage of the abrasive is shown in figure 2.

It comprises a control valve 51 formed by a sleeve made of a cylindrical elastic membrane 52, in which the abrasive pass through. The membrane 52 is made for example with silicone rubber, but other elastic material resistant to the abrasive passage can be used. Around the cylindrical membrane 52 is placed a room 70 which is tillable with an oil supplied by a pipe 53 from the oil pump 54. The oil pump is for example controlled by the control system 23. Depending on the quantity of abrasive is required to pass through the valve 51, the control system 23 supply a signal to the oil pump 54, and consequently the oil pump 54 put in pressure the oil (in proportion to the signal value received) within the room 70. The cylindrical elastic membrane 52 deforms itself, in proportion of the oil pressure, regulating the abrasive passage. It is chosen to use preferably oil for its incompressibility characteristic, but other fluids can be used.

The valve group 21 comprises preferably also an on/off valve 55 formed by a sleeve made of a cylindrical elastic membrane 56, in which the abrasive pass through. The membrane 56 is made for example with silicone rubber, but other elastic material resistant to the abrasive passage can be used. Around the cylindrical membrane 56 is placed a room 71 which is fillable with air supplied by a pipe 57 from the air source 58. The air source 58 is controlled by the control system 23. On order of the control system 23 the air is supplied or not to the on/off valve 55, and accordingly the membrane 56 will be all close (membrane completely collapsed on itself) and the abrasive cannot flows through the membrane 56, or all open (cylindrical membrane not deformed) and the abrasive will have not obstruction in its flowing. Preferably, the on/off valve 55 is placed over the control valve 51. The. path between the container 20 and mixing chamber 13 is preferably airtight, and also the container 20 will have, preferably, a closure in order to have the entire path airtight.

In this case it is advantageously insert air at the air insertion points 59 and/or 62, placed in the path. The air insertion point 59 is placed over the control valve 51. The air insertion point 62 is placed above the control valve

51. Depending on the case, it is possible to use only one of the two air insertion point 59 and 62.

The air is supplied to the air insertion points 59 and 62 by the pipes 60 and 63 and it comes from respectively air valves 61 and 64. The air valves 61 and 64 receive the air from any source of air present in the system for example from the pipes 73 and 72. The air flow is controlled by the air valves 61 and 64 on order of the control system 23.

The abrasive disbursed by the container 20 reaches the mixing chamber 13 for the effect of the pneumatic transport that occurs in the pipe; the air is inhaled by the outside because of the Venturi effect produced by the high speed water in the mixing chamber 13. From these considerations is deduced that the presence of air in the pipe is essential; contemporarily an excess of air can be harmful because decreases the cut ability of the wateηet. The importance of the air in the AWJ working is remarkable because it occupies 95% in volume of the throw, and it brings to a diminution of the throw density and, consequently, of the drag strengths of viscous origin that act on the particles to accelerate them. From these considerations it is understood that the speed of the abrasive particles decrease at the increasing of air ratio in the throw. Besides, an increase of the air causes: a greater divergence of the throw due to the turbulences; an increasing, at constant pressure, of the drops separation due to the turbulent aerodynamic strengths; a diminution, at constant pressure, of the depression in the mixing chamber; and an instability of the throw in the time and in the space (possible asymmetrical diffusion of the throw in comparison to its axle).

The Applicant found that the erosion performances of the throw increase of 10% for a 45% air reduction (with parity of other conditions). Through the controlling of the air insertion points, the system is able to be adjusted in an optimal way, limiting the air passage at the narrow quantity necessary to realise the pneumatic transport. Further, preferably, the control system 23 receives a signal proportional to the advancement speed of the focusing nozzle 14 and is used to correct the value of the flow to deliver so as to consider the effects of the advancement speed of the focusing nozzle 14 on the required quality of the cut. With a system in accordance with the present invention the following advantages are obtained.

Higher quality level of the machining, in terms of uniformity of the surfaces, surface roughness, taper of the cutting kerf, machining depth.

Simplicity in execution and setting up. Possibility of varying the required value of abrasive mass flow rate also during machining time.

Possibility of also being applied in existing AWJ plants.

Flexibility: intended as capacity of the system to adapt itself to different environmental conditions (temperature, humidity, etc.), and intended as capacity to control the flow for different granulometri.es of abrasive.

Calibration in operative conditions.

In comparison with other systems it has the advantage of being able to carry out continuous dynamic adaptation of the delivery conditions, diminishing the downtimes of the machine and thus increasing the productivity.

The system is also capable of considering disturbances and variables that are not connected with the process, for example the variation of the quantity of abrasive present in the hopper that can influence the delivery flow rate. Maximum level of integration with the processing means, the system can be controlled directly by the CNC (Computerized Numerical Control) that commands the machine, so as to guarantee the control of the delivery of abrasive in every machining operation.

The control of the abrasive mass flow rate in closed loop can be applied to an external hopper (system beside the machine) or to a hopper positioned on board of the cutting head; the solution that offers the best performance is that which provides for installation of the system in the proximity of the cutting head; in this manner the path of the abrasive feeding tube is minimum, which determines clear advantages in the regularity of the abrasive mass flow rate and in the response times of the system to the requests for variation of the flow and in the event of clogging in the cutting head.

The reintegration of the abrasive into the hopper can also be made continuously, without stopping the machining and in an easily automated manner, it is however preferable to make the reintegration in the dead time of the system.

Possibility of controlling the regulation of the abrasive mass flow rate in combination with other technological parameters, such as the advancement speed of the cutting head, obtaining quality, of the machining and of the cutting kerf, also uniform in correspondence with slowing down of the cutting head. This allows cutting profiles and machining of any geometry to be carried out with uniform quality characteristics.

Limited system costs at a level of industrial production as the additional expenses in comparison to the present situation are connected with the presence of the load cell (which is a very common and economical component) and with the need to update the software and the hardware for controlling the machine (interventions that are only carried out occasionally).

Possibility to isolate the batching system from the aggressive and humid environment of the AWJ machine.

Claims

CLAIMS 1. System for regulating the abrasive mass flow rate in a wateηet cutting system comprising: means (11) suitable for supplying pressurized water; means (20, 21) suitable for supplying abrasive powder; means for transforming energy (12) suitable for supplying water at high speed; means for mixing (13, 14) said water at high speed and said abrasive powder; means (14) suitable for directing said mixture against an object; characterized in that said means (20, 21) suitable for supplying abrasive powder comprise a system for measuring (22) the weight of said abrasive powder, suitable for supplying an electrical signal proportional to said weight; processing means (23) for said electrical signal proportional to said weight suitable for generating an electrical signal proportional to the variation of weight of said abrasive powder in a unit of time, and suitable for supplying an electrical signal proportional to the difference between the electrical signal proportional to the variation of weight and a preset signal of flow required; and that said means (20, 21) suitable for supplying abrasive powder comprise a system for batching (21) said abrasive powder commanded by said signal proportional to the difference between the electrical signal proportional to the variation of weight and the preset signal of flow required.

2. System for regulating the abrasive mass flow rate in accordance with claim 1 characterized in that said processing means (23) comprise means for controlling said wateηet cutting system and consist of a computer.

3. System for regulating the abrasive mass flow rate in accordance with claim 1 characterized in that it comprises conduits for transporting said abrasive powder and a clogging sensor (24) of said conduits positioned near said mixing means (13) and suitable for supplying a clogging signal of said abrasive powder to said processing means (23).

4. System for regulating the abrasive mass flow rate in accordance with claim 1 characterized in that said processing means (23) receive in input a preset signal of flow required for said abrasive powder which is compared with said electrical signal proportional to the variation of weight of said abrasive powder in a unit of time and an electrical signal is generated correlated to the difference between said electrical signal proportional to the variation of weight and said preset signal of flow required.

5. System for regulating the abrasive mass flow rate in accordance with claim 1 characterized in that said means (20, 21) suitable for supplying abrasive powder comprise a container (20) of abrasive powder placed in the near vicinity of said mixing means (13).

6. System for regulating the abrasive mass flow rate in accordance with claim 1 characterized in that said processing means (23) receive in input a signal proportional to the advancement speed of said means suitable for directing said mixture against an object and correct the value of the abrasive mass flow rate to deliver so as to consider the effects of the advancement speed of the focusing nozzle (14) on the quality of the cut required.

7. System for regulating the abrasive mass flow rate in accordance with claim 1 characterized in that said system for batching (21) comprises a control valve (51), in which the abrasive powder pass through, formed by a sleeve (52) deformable on control of said processing means (23).

8. System for regulating the abrasive mass flow rate in accordance with claim 7 characterized in that said deformable sleeve (52) is surrounded of a room (70) fillable with a fluid having a pressure controlled by said processing means (23).

9. System for regulating the abrasive mass flow rate in accordance with claim 1 characterized in that said system for batching (21) comprises an on/off valve (55) in which the abrasive powder pass through, formed by a sleeve (56) deformable on control of said processing means (23).

10. System for regulating the abrasive mass flow rate in accordance with claim 9 characterized in that said deformable sleeve (56) is surrounded of a room (71) fillable with a fluid having a pressure controlled by said processing means (23).

11. System for regulating the abrasive mass flow rate in accordance with claim 1 characterized in that the path, comprised between said means (20, 21) suitable for supplying abrasive powder and said means for mixing

(13, 14) said water at high speed and said abrasive powder, is airtight.

12. System for regulating the abrasive mass flow rate in accordance with claim 11 characterized in that said path comprises at least one air insertion point (59, 62) for supply air to said path on control of said processing means (23).

13. Waterjet cutting system comprising a system for regulating abrasive mass flow rate in accordance with claim 1.