ACTIVELY DAMPENED CENTERLESS GRINDING PROCESS

Abstract

The present invention relates to an actively dampened centerless grinding process for a centerless grinding machine having wheels, between which there is arranged a part to be ground, and heads carrying the wheels, which process has moving the wheels closer to one another by applying pressure on the part for the grinding thereof, such that vibrations are generated in the grinding machine due to grinding, measuring the vibrations due to grinding, and introducing, depending on the measurement, an active damping force (F.sub.act) parallel to the force flow of the grinding machine and by means of using an inertial actuator acting directly on one of the heads, such that the vibrations due to grinding are attenuated.

Claims

1. An actively dampened centerless grinding process for a centerless grinding machine having wheels, between which there is arranged a part to be ground, and heads carrying the wheels, the grinding process comprising: moving the wheels closer to one another, applying pressure on the part for the grinding thereof, such that vibrations will be generated in the grinding machine due to grinding, measuring the vibrations due to grinding, and introducing, depending on said measurement, an active damping force (F.sub.act) parallel to the force flow of the grinding machine and by means of using an inertial actuator acting directly on one of the heads, such that the vibrations due to grinding are attenuated.

2. The actively dampened centerless grinding process according to claim 1, wherein the active damping force (F.sub.act) is introduced in an upper portion of the head.

3. The actively dampened centerless grinding process according to claim 1, wherein the vibrations are measured in the heads, with an active damping force (F.sub.act) being introduced in each head by means of a respective inertial actuator, such that the vibrations due to grinding in the heads are attenuated.

4. The actively dampened centerless grinding process according to claim 1, wherein the active damping force (F.sub.act) is introduced at the same point of the machine where the vibrations are measured.

5. The actively dampened centerless grinding process according to claim 4, wherein the vibrations are measured in the upper portion of the head and the active damping force (F.sub.act) is applied in that upper portion of the head.

6. The actively dampened centerless grinding process according to claim 1, wherein the inertial actuator used is a hydraulic actuator, an electromagnetic actuator, or a linear motor in charge of accelerating a moving mass.

7. The actively dampened centerless grinding process according to claim 1, wherein the vibrations due to grinding are measured using detection means measuring low frequencies less than 300 Hz.

8. The actively dampened centerless grinding process according to claim 7, wherein the vibrations are measured in a frequency range between 10-300 Hz.

9. The actively dampened centerless grinding process according to claim 1, wherein the active damping force (F.sub.act) is introduced in the direction in which the wheels apply pressure on the part.

10. The actively dampened centerless grinding process according to claim 1, wherein the introduced active damping force (F.sub.act) is proportional to the speed of the vibrations due to grinding that have been measured, and it is introduced at the same frequency, and with an opposite phase, with respect to the speed of the measured vibrations.

11. The actively dampened centerless grinding process according to claim 1, wherein the introduced active damping force (F.sub.act) is a function of the frequency (.sub.a) and of the desired relative damping (.sub.a) in a virtual passive mass, such that the behavior of a passive damper is virtually simulated.

12. The actively dampened centerless grinding process according to claim 11, wherein the active damping force (F.sub.act) is a function of the following equation: $F_{\mathrm{act}}=-.Math..Math.m.Math.\frac{2.Math.{}_a.Math.{}_a.Math.\stackrel{\mathrm{.Math.}}{x}+{}_a^2.Math.\stackrel{.}{x}}{\stackrel{\mathrm{.Math.}}{x}+2.Math.{}_a.Math.{}_a.Math.\stackrel{.}{x}+{}_a^2.Math.}$ where: m is the mass of the grinding machine, is the ratio between the virtual passive mass of the inertial actuator m.sub.a and the mass of the machine m. (=m.sub.a/m), x is the movement measured due to vibrations, .sub.a is the desired oscillation frequency in the virtual passive mass, .sub.a is the desired relative damping in the virtual passive mass.

Description

DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 shows an example of a centerless grinding machine in which the grinding process of the invention can be applied.

[0039] FIG. 2 shows a schematic view of the two main vibration modes of the machine of FIG. 1.

[0040] FIG. 3 shows a schematic view of an actuator arranged in series with the force flow of a grinding machine.

[0041] FIG. 4 shows a view like that of the preceding figure, but with the inertial actuator arranged parallel to the force flow of the grinding machine as proposed by the invention.

[0042] FIG. 5 shows a comparison of the damping force used in a cancellation control strategy according to the state of the art.

[0043] FIG. 6 shows a comparison of the damping force used in an active damping control strategy like the one proposed by the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0044] FIG. 1 shows an example of a centerless grinding machine in which the grinding process of the invention can be applied. The grinding machine comprises two wheels (1, 2), a grinding wheel (1) and a regulating wheel (2), between which there is arranged a part (3) to be ground supported on holding means (4). Each wheel (1, 2) is arranged in a head (5, 6) and each head (5, 6) is arranged on translation means (8, 9).

[0045] The machine configuration depicted in FIG. 1 is not limiting for the invention, where it is obvious for one skilled in the art that the grinding process may be applied in centerless grinding machines with a different machine configuration.

[0046] By means of the movement of the heads (5, 6), the wheels (1, 2) move closer to/farther away from one another in a direction perpendicular to the part (3). Therefore, in operation, the wheels (1, 2) apply pressure on the part (3) in the direction in which the wheels (1, 2) move closer to one another, such that the part (3) is retained between the wheels (1, 2), being supported such that it freely rotates about the holding means (4), so the part (3) is ground by means of rotation of the wheels (1, 2).

[0047] The grinding process itself creates forces which cause the excitation of the vibration modes of the different components of the machine, such as the heads (5, 6), which generates vibrations that are transmitted to the machining point located in the contact area between the part (3) and the wheels (1, 2). This causes geometric defects or excessive wear of the machining tool, or a poor surface finish of the part (3), among other factors. As can be seen in FIG. 2, the centerless grinding machine has two main vibration modes which cause the heads (5, 6) to bend, resulting in them moving farther away from or closer to one another.

[0048] The invention proposes a grinding process for centerless grinding machines whereby those vibrations due to the grinding process itself can be attenuated or even eliminated.

[0049] The grinding process proposed by the invention therefore comprises moving the wheels (1, 2) closer to one another by applying pressure on the part (3) for the grinding thereof, measuring the vibrations generated due to grinding, and introducing, depending on said measurement, an active damping force (F.sub.act), which is introduced parallel to the force flow of the grinding machine, and by means of using an inertial actuator (10) that acts on one of the heads (5, 6), such that the vibrations due to grinding are attenuated.

[0050] The measurement of the vibrations due to grinding is taken using detection means (9), such as accelerometers, for example. The detection means (9) are particularly configured for measuring low frequencies of less than 300 Hz, and preferably a frequency range between 10-300 Hz. To that end, the use of filters which discriminate noise due to frequencies that are outside the preferred range indicated above has been envisaged.

[0051] As shown in the example of FIG. 1, the arrangement of the inertial actuator in the upper portion of the head (5, 6), which is the part of the machine where the greatest movement due to vibrations of the grinding process occurs, has been envisaged.

[0052] To improve machine precision, an inertial actuator (10) is arranged in each head, such that an active damping force (F.sub.act) is introduced in each head (5, 6).

[0053] The detection means (9) are arranged in the head (5, 6), such that the detection means (9) and the inertial actuator (10) introducing the force are co-located at the same point.

[0054] In the example of FIG. 1, the vibrations are measured in the two heads (5, 6), with an active damping force (F.sub.act) being introduced in each head (5, 6) by means of a respective inertial actuator (10), such that the vibrations due to grinding in the two heads are attenuated (5, 6).

[0055] FIG. 3 shows a schematic view of an actuator (9) arranged in series with the force flow of the grinding machine, as proposed by the documents of the state of the art ES2278496A1 or JP2005199410A, whereas FIG. 4 shows another schematic view of an inertial actuator (9) arranged parallel to the force flow of the grinding machine, as proposed by the invention.

[0056] The force flow of the grinding machine, also referred to as path force, is depicted in the drawings by a line with arrows. This flow refers to the path the transmission of the force required for grinding the parts would follow inside the machine.

[0057] When the actuator is arranged in series, as proposed by documents ES2278496A1 or JP2005199410A, as the force flow passes through the actuator, the actuator can modify the original dynamic properties of the machine, and thereby dampen the created vibrations, while at the same time having to withstand the grinding forces. To that end, the actuators arranged in series must provide a very high rigidity and in the event that the actuator fails, the machine would not perform proper grinding.

[0058] In contrast, if the inertial actuator is arranged in parallel, as proposed by the invention, the damping force is introduced as if it was an external force and it has no effect whatsoever on the original rigidity of the machine. To that end, even in the event of the actuator failing, the machine would still have its original rigidity and perform proper grinding.

[0059] According to one embodiment of the invention, a control strategy which is based on measuring the vibration speed of the head (5, 6) and applying, by means of the inertial actuator (10), a force proportional to the amplitude of that speed, is used to perform active damping.

[0060] In that sense, the force required by the actuator would not be as large as in the case of cancellation and would not require knowledge about the dynamics of the machine or the process to be dampened.

[0061] The use of active damping by means of feedback, as proposed by the invention, the objective of which is to provide the machine with a higher dynamic rigidity depending on the measured vibration, is highly effective for eliminating process vibrations. This feedback strategy is not valid for forced constant vibrations, like in the case of the vibration due to imbalance in the wheel, but it is indeed highly effective against process vibrations, because the increase in the dynamic rigidity of the machine causes these vibrations to be completely attenuated using a force that is less than the vibration force.

[0062] According to another embodiment of the invention, to perform active damping, a control strategy which is based on a dynamic model of the machine is used to cause the inertial actuator to behave like a passive damper (tuned mass damper). This virtual passive damper can attenuate different frequencies and can be designed for simulating different mass magnitudes, thereby eliminating the drawbacks of passive dampers. Furthermore, the force required of the actuator would also be much less than the force required in the cancellation, as it is also based on feedback of the vibration parameters. Although the need of the dynamic model makes it less attractive compared to the simple feedback, the fact that the dynamic behavior does not vary according to the position of its components means that it is not necessary to adjust the model in each process, since the control strategy does not require knowledge about the conditions of the grinding process.

[0063] A table comparing a control strategy based on cancellation, like in the case of documents JP2005199410A or JP2002254303A, and a control strategy based on active damping, like in the case of the two strategies proposed by the present invention, is show below.

[0064] As can be seen, among other advantageous aspects, in the case of active damping, the force required by the inertial actuator is much less than the force required in the cancellation, and furthermore cancellation is a pre-control strategy in which a precise knowledge about the vibration to be cancelled is required, whereas active damping is a feedback strategy in which a precise knowledge about the vibration is not required in order to be able to attenuate it.

TABLE-US-00001 Active damping Speed Virtual passive Cancellation feedback damping F.sub.act F = (m{umlaut over (x)} + + kx) F.sub.act = G .Math. {dot over (x)} [00002]$F_{\mathrm{act}}.Math..Math.\text{?}=-m.Math.\frac{2.Math.{}_a.Math.{}_a.Math.\stackrel{\mathrm{.Math.}}{x}+{}_a^2.Math.\stackrel{.}{x}}{\stackrel{\mathrm{.Math.}}{x}+2.Math.{}_a.Math.{}_a.Math.\stackrel{.}{x}+{}_a^2.Math.x}$ Type Pre- Feedback Feedback control/Feedforward indicates data missing or illegible when filed

[0065] FIG. 5 shows a comparative graph for a cancellation control strategy according to the prior state of the art in which the damping force (F.sub.act), depicted by lines, has the same amplitude and an opposite phase with respect to the force (F) to be cancelled, whereas FIG. 6 shows a comparative graph for an active damping control strategy in which it is clearly seen that the required damping force (F.sub.act) is less than the damping force (F.sub.act) used for the cancellation.