Centerless Grinding Machine Control System

This application is being filed with an appendix of computer program listings. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the document or the patent disclosure, as it appears in the Patent and Trademark Office files or records, but otherwise reserves all copyright rights whatsoever.

The present invention relates to an improved apparatus and method for controlling a centerless grinding machine to more accurately control the final configuration of objects having a cylindrical cross-section. The invention is particularly useful in situations where the final configuration of the ground object is characterized by fixed lengths of varying diameters and/or tapered sections. The invention has particular application to grinding guide wires for guiding catheters during angioplastic procedures or other procedures requiring catheterization, but may be applied to any product requiring varying fixed diameters or tapers, such as golf club shafts, arrows, whip antennae and fishing rods, which may be up to ten or more feet in length.

Generally, centerless grinders are used to grind the outer surface of a rod or wire. The object of the grinding operation is to produce a wire that is round and that has a diameter and surface finish in accordance with given specifications at any given cross-section along its length.

Typically, a wire is fed into a centerless grinder at one end and guided between two grinding wheels that rotate in the same direction at different speeds, known as the work wheel and the regulating wheel. The wire rotates as a result of its contact with the regulating wheel and is ground to a specified diameter dictated by the distance between the faces of the two grinding wheels. One of the grinding wheels, typically the regulating wheel, can be moved so that the distance between the faces of the grinding wheels may be varied during the grinding process.

The wire advances through the grinding machine as a result of its contact with the grinding wheels. Specifically, one of the grinding wheels, typically the regulating wheel, rotates along an axis that is almost parallel to the axis of rotation of the wire being ground, but slightly skewed in a vertical plane, so that its contact with the wire causes the wire to move forward through the machine. The angle at which the axis is skewed is commonly referred to as the tilt angle and generally varies between one and three degrees.

A number of factors can affect the rate at which the wire moves through the grinding machine. For example, temperature, regulating wheel RPM, regulating wheel tilt angle, slippage, type of coolant used, wire diameter, wire material, wire material uniformity, and grinding wheel material may affect feed rate. Thus, the feed rate cannot accurately be controlled and often varies substantially during the grinding process.

The prior art methods of producing wire of multiple fixed diameters and tapers are deficient because they do not account for varying feed rates. For illustrative purposes it is assumed that it is desired to produce a wire as depicted in FIG. 1 having a given fixed diameter section 10 for a first unit of length 17, a tapered section 11 for a second unit of length 16, and a second fixed diameter section 12 for a third unit of length 15.

In a prior art method of grinding such a wire the work wheel is dressed, i.e., formed, so that it will produce one tapered section and one fixed diameter section on a wire in any one given pass. For example, referring to FIG. 1, it can produce fixed diameter section 10 and tapered section 11 in one pass. More particularly, the grinding face of the work wheel is configured so that as the wire moves through the space between the work wheel and the regulating wheel the gap becomes more narrow until a certain point. At that point, the face of the work wheel becomes parallel to its axis of rotation so that the size of the gap between the wheels remains the same.

In operation, the wire is fed into the machine and allowed to progress until the point of transition between the parallel and tapered sections on the work wheel matches the point along the length of the wire where the taper is to begin. The wire is then withdrawn and the regulating wheel position is adjusted to grind the second fixed diameter and tapered section of the rod. If the taper angle of the second tapered section does not match the first taper angle, an entirely new work wheel with a different dress must be used. Thus, this prior art method is cumbersome and tedious. In addition, if the operator does not stop the grinding process at precisely the correct point, which can be difficult, the length of the fixed diameter sections of the wire will not meet specifications.

In another prior art method the wire is placed on an elongated feed bed and moves longitudinally along the feed bed as it is being ground. Sensors which sense the passage of the trailing end of the wire are mounted along the feed bed so that the end of the wire passes the sensors in succession. The position of the sensors are adjustable, and the operator positions the sensors on the feed bed so that when the end of the wire passes a sensor, that event corresponds to a point in the grinding process at which a transition from a fixed diameter section to a tapered section, or vice versa, should occur. The sensors generate signals and the commencement and cessation of movement of the regulating wheel is keyed to those signals. To produce a tapered section in this prior art method, when a signal is generated by one of the sensors indicating that a taper is to begin, the regulating wheel is moved away from the work wheel at a fixed constant rate while the wire continues to be fed into the grinding machine. The rate at which the regulating wheel is moved away from the work wheel is based on an assumed constant wire feed rate. However, since the actual feed rate may vary substantially, the produced wire taper may be of irregular shape and fall outside of specified limits. This prior art method is deficient because movement of the regulating wheel is not related to the actual position of the wire or rod.

A principal feature of the present invention is the provision of means to continuously monitor the position of a wire that is being ground in a centerless grinding machine and to control the position of the regulating wheel based on the known position of the wire. The position and feed rate of the wire is monitored and that information is then used to accurately determine when the distance between grinding wheels should be changed and the rate at which that distance should be changed. In this way precise, consistent tapers can be achieved and fixed diameter portions of the wire can be accurately produced to specified lengths.

The rod or wire that is to be ground is placed upon a feed bed and is advanced so that its leading edge enters the grinding area. At this point grinding commences and the wire begins to move through the grinding area. A plurality of sensors are mounted along the feed bed which sense the position of the wire. Signals from the sensors are processed by a central processor, which continuously calculates wire position and feed rate. The continuously updated wire position and feed rate information is used to calculate desired regulating wheel position and movement rate. Alternately, a sensor or sensors which measure feed rate directly are mounted along the feed bed and signals from such sensors are processed by the central processor to continuously calculate wire position.

The central processor also receives wire profile data through an input device from the machine operator indicating the dimensions of the wire as it is to appear after it has been ground. The operator provides the length and diameter of fixed diameter sections of the wire, and the length of the tapered sections of the wire between fixed diameter sections.

Using the position and feed rate information and the wire profile data from the operator, the central processor calculates the desired position of the regulating wheel throughout the grinding process, including the rate at which the regulating wheel must be moved to produce a given taper. The regulating wheel position calculation is constantly updated based on updated wire position and feed rate information. Thus, if a tapered section of the wire is being ground and the feed rate changes, a corresponding change is made in the rate at which the regulating wheel is being moved away or toward the work wheel.

The central processor also generates control signals which control a regulating wheel positioning mechanism, such as a stepper motor.

FIG. 1 depicts the profile of a wire after it has been ground by the claimed apparatus and method.

FIG. 2 is a schematic and block diagram showing details of a preferred embodiment of the invention.

FIG. 3 is a diagram showing the orientation of the grinding wheels and the wire.

FIG. 4 is a flow chart illustrating the method of the invention.

FIG. 2 shows a schematic of the preferred embodiment. A rod or wire 29 that is to be ground is placed upon a feed bed 28 so that it lies between pinch rollers 23 and 24. The pinch rollers are then clamped onto the wire and it is slowly advanced via stepper motor 25 until the leading end of the wire is sensed by initiating sensor 27. The wire is then advanced a further known fixed distance so that its leading end enters the gap between work wheel 20 and regulating wheel 21.

Once the wire enters the gap between the two grinding wheels, it comes into contact with them and grinding commences. The work wheel 20 and regulating wheel 21 can be composed of a variety of materials known to those skilled in the art, such as silicon carbide or aluminum oxide in vitrified form or rubber bonded. Grinding wheel material is selected in a manner known to those skilled in the art so that there is greater friction between the regulating wheel and the wire than between the work wheel and the wire, which allows the wire to be controlled by the rotation of the regulating wheel while the work wheel does all the grinding. Thus, by skewing the axis of rotation 41 of the regulating wheel in a vertical plane, as depicted in FIG. 3, the regulating wheel can be used to advance the wire through the machine.

In the preferred embodiment, work wheel 20 is approximately nine inches in diameter and spins at approximately 2500 RPM and regulating wheel 21 is approximately four inches in diameter and spins at approximately 50 RPM. The rod 29 spins as it is being ground.

Mounted along the feed bed 28 are sensors 30. In the preferred embodiment, the sensors are mounted at half-inch intervals and are photoelectric cells, such as those made by Keyence Company, which project light onto a target and sense whether light is reflected back. When the wire is present, reflected light is sensed and the sensor generates an electrical signal ("on"). After the trailing end of the wire has passed the sensor, reflected light is no longer sensed and the electrical signal is no longer generated ("off").

The sensors are connected to a pulse generator 31 which generates an electrical pulse each time a transition from "on" to "off" occurs in any of the sensors 30 on the feed bed. Thus, a pulse is generated each time the trailing end of the wire 29 passes a sensor 30 on the feed bed 28. The pulse generator 31 is known in the art and is often available from the manufacturer of the sensors being used. For example Banner Company makes pulse generators known as "one shot logic modules", which can be used with the photoelectric cells named above and used in the preferred embodiment. In addition, cells with built-in pulse generators are available on the market. For example, Banner Company manufactures a photoelectric cell with a built-in pulse generator known as "Multi-Beam.RTM.".

The pulse generator 31, or the photoelectric cells themselves if equipped with pulse generators, are connected to an event counter 32 for counting the number of pulses generated by the pulse generator 31. A register in the event counter 32 is reset to zero at the beginning of the grinding cycle after the leading end of the wire 29 has been inserted into the grinding area and is updated each time a pulse is received from the pulse generator 31. Event counters adapted for such use are available on the market. For example DGH manufactures sensor-to-computer interface modules under the D1000 series designation.

The event counter 32 is connected to a multitasking computer 33 via a standard computer interface such as an RS-232 serial interface. Any computer can be used provided it is capable of multitasking, i.e., running more than one program at a time. Many such multitasking computers are available on the market. An IWS-3025 workstation made by Nematron is used in the preferred embodiment.

The multitasking computer 33 is programmed via a feed rate calculation program to calculate the position and feed rate of the wire based on the information in the event counter. Specifically, the value in the event counter is polled continuously, that is, thousands of times a second, to see whether it has been updated. When a transition from one value to the next, for example from a "0" to a "1" or from a "1" to a "2", is noted a timer is polled to see how long it has been since the last time the event counter was updated. Since the distance between sensors is known, and the time it took the end of the wire to travel from one sensor to the next has just been measured, feed rate is easily calculated by dividing the distance travelled by the time interval. Each time the event counter 32 is updated, a new feed rate is calculated and the timer is reset to zero. Thus, in the preferred embodiment, the calculation of feed rate is updated for each half-inch of wire travel. Wire position during each half-inch interval is continuously computed based on the feed rate for the preceding half-inch interval.

The multitasking computer 33 also accepts data defining the wire profile to be achieved from the grinding process. This can be accomplished via dedicated port 34 which connects to an input device such as a keyboard or touch screen.

Referring to FIG. 1 there is depicted in schematic form a wire having a fixed diameter section 10, followed by a tapered section 11, followed by another fixed diameter section 12. The multitasking computer 33 accepts data from the grinding machine operator specifying the length of the fixed diameter sections, the diameter of the fixed diameter sections, and the length of tapered sections. Any number of fixed diameter sections can be specified. In addition, it may be specified that the wire increase in diameter and subsequently decrease in diameter.

The multitasking computer 33 is also programmed via a regulating wheel positioning program described below to calculate the position and rate of movement of the regulating wheel throughout the grinding process based on the input profile, the wire position and the feed rate. For example, referring to FIG. 1, the regulating wheel 21 must remain in one position while the first fixed diameter portion 10 of the rod is being ground. It must then be moved away from the work wheel 20 to produce the tapered section 11. The point in time at which the wheel begins to move and the rate at which the regulating wheel is moved away from the work wheel can be calculated by those skilled in the art based on the wire profile data and a feed rate. However, as discussed above, the feed rate is typically not constant during the grinding process, so if a constant feed rate is assumed, an improperly ground wire will result. In the present invention the feed rate calculation program is constantly polled to see whether a new feed rate has been calculated. As soon as a new feed rate is noted, the desired regulating wheel position and its desired rate of movement are redetermined.

The updated desired regulating wheel position and rate of movement are calculated based on the known positions of the wire and the regulating wheel. Thus, at any given time during the grinding of a tapered section the desired movement of the regulating wheel 21 is calculated based on the feed rate for the previous half-inch interval. When the next half-inch interval is achieved it is known precisely where on the length of the wire the grinding is taking place. If the feed rate has changed the actual position of the wire will not be precisely the same position that was assumed in calculating the regulating wheel rate of movement since the regulating wheel rate of movement was based on the previous feed rate. That discrepancy in wire position is taken into consideration in computing a new desired regulating wheel movement rate during the next half-inch interval.

To provide the adjustability of the position of the regulating wheel 21, so that the linear distance between it and the work wheel 20 is variable, the regulating wheel is slidably mounted and its position is controlled by stepper motor 36 and ball screw 35. Thus, the position of the regulating wheel 21 with respect to the work wheel 20 is a function of the angular position of the stepper motor 36 and the pitch of the ball screw 35. Likewise, its linear speed is a function of the angular speed of the stepper motor and the pitch of the ball screw. Stepper motors are readily available, such as the SXF stepper motor manufactured by Parker Compumotor which is used in the preferred embodiment, and typically come equipped with controllers that accept codes in ASCII format which dictate how the stepper motor should move. In the preferred embodiment, the stepper motor 36 is controlled by sending it codes via serial line 37 in ASCII format indicating the number of rotations, i.e., a distance code, and the speed at which the rotations should occur, i.e., a velocity code. The regulating wheel positioning program generates these commands in a manner known to those skilled in the art.

In addition to monitoring the wire position, the position of the regulating wheel stepper motor 36 is monitored as well. Stepper motors available on the market such as the one used in the preferred embodiment are equipped to transmit signals indicating the distance they have travelled. This information is transmitted to the central processor 33 via serial line 37 and is used in recalculating the desired regulating wheel position and rate of movement each time a new feed rate is generated. Thus, each time a new feed rate and wire position signal is received from the feed rate calculating means, the position of the regulating wheel stepper motor 36 is monitored to see how far the stepper motor has moved. A new taper angle is calculated based on the length remaining in the taper, which is known as a result of the rod positioning sensors, and the amount the regulating wheel must be moved to complete the taper, which is known from the stepper motor monitor. A new regulating wheel rate of movement is calculated based on the new taper angle.

In another embodiment of the invention the plurality of sensors may be replaced with a sensor which can read feed rate directly. For example Keyence Company makes a sensor which projects a laser pattern onto a moving part and computes feed rate based on a shift pattern reflected back to the sensor head. Such a sensor produces signals which vary in accordance with the feed rate. In this embodiment the pulse generator and event counter are not needed. Instead the rod position can be continuously calculated based on the continuously read feed rate. Desired regulating wheel position and movement rates are recalculated at fixed time intervals or are recalculated each time a change in feed rate is detected.

Referring to FIGS. 3 and 4, a schematic of the method of the control system is presented. At the start of the cycle the operator provides wire profile data in step 60 indicating the lengths and diameters of fixed diameter sections and the lengths of tapered sections.

Referring to step 61 of FIG. 4, wire 29 is placed on feed bed 28 so that it is positioned between the pinch rollers 23 and 24. The pinch rollers are then clamped onto wire 29 and the wire is moved along the feed bed until its leading end reaches the initializing sensor 27. The wire is then advanced a known fixed distance to bring its leading end into the nip between the work wheel 20 and the regulating wheel 21 while the regulating wheel is moved into position to begin grinding. The event timer is then reset, the pinch rollers 23 and 24 are released and grinding commences.

As grinding takes place the position of the wire is monitored by polling the event counter, as depicted in boxes 62 and 64 of FIG. 4. As stated above, this polling occurs thousands of times per second, so it is instantly known when the trailing end of the wire has passed a sensor. Feed rate is computed as described above and set forth schematically in box 64 of FIG. 4.

After the first fixed diameter portion of the wire has been ground the grinding of a tapered section begins. To control the position of the regulating wheel 21 the stepper motor 36 must be commanded with a distance code, indicating the total distance to travel to complete the taper, and a velocity code, indicating the speed at which the wheel should be moved while travelling that distance. Referring to the first equation in box 63 of FIG. 4, the distance is derived by computing the difference between the diameter of the wire at the end of the taper and the diameter at the beginning of the taper, i.e., the difference in diameters between the fixed diameter sections. Referring to FIG. 1, the distance is Diameter 2 minus Diameter 1. Referring to the second equation in box 63 of FIG. 4, the desired stepper motor velocity is computed by multiplying the feed rate by the computed distance and then dividing by the taper length. The commands are then sent to stepper motor 36.

As discussed above, the event counter is continuously polled to monitor wire progress, and each time the trailing end of the wire passes a sensor a new feed rate is calculated. Whenever this occurs the desired stepper motor velocity is recomputed using the equation in box 65 of FIG. 4 and a new velocity command is generated. Referring to the equation in box 65 of FIG. 4 the new feedrate value is the new feedrate which has just been received. The remaining taper length is known since the wire position is known. The remaining distance is derived by computing the difference between the desired diameter of the wire at the end of the tapered section and the then current wire diameter. The current wire diameter can be derived two ways. One way to derive it is by multiplying the velocity of the regulating wheel over the last half-inch interval by the time elapsed since the feed rate was last updated. Another way to derive it is by directly monitoring the position of the stepper motor via serial line 37. As discussed, stepper motors are available that provide information on their current position. Once the new distance is computed, velocity is computed by multiplying the remaining distance by the new feed rate and then dividing by the remaining taper length. Referring to box 65 of FIG. 4, a velocity command is recomputed each time a new feed rate is computed, and the stepper motor 36 is commanded accordingly, until grinding of the tapered section is completed.

Once the taper has been completed, the method progresses from step 65 back to step 62 of FIG. 4. If the entire wire has been ground the wire is removed from the machine in step 66 of FIG. 4 by pulling the wire back out of the machine via pinch rollers 23 and 24 and stepper motor 25. A new feed cycle, step 61, can then begin. If instead more grinding must take place the program progresses beginning with step 62 to continue grinding.

The above described method is applicable to wires of many different desired configurations. For example, wires that begin with tapered sections, and wires that have consecutive tapered sections but with different taper angles, can also be produced.

The programming of a system as described above will be apparent to all those skilled in the art. To provide further guidance, a copy of the source code used by the inventors in programming their system has been annexed as Appendix A. This code is provided for exemplary purposes only, and in no way is intended to limit the scope of this invention. ##SPC1##