Vibration Isolator Device for a Percussion Tool

Technical Field

This invention relates generally to a vibra- tion isolator device for a tool such as an .impact chipping hammer or the like, and more particularly to an active vibration isolator device therefor.

Background Art

In the usual chipping hammer or rock breaker a piston is pneumatically reciprocated at a rapid rate, for example 50 Hertz or 50 cycles per second, to impact upon a distal work element such as a chisel. The dynamic forces resulting from the blows and the action of the chisel is unfortunately transmitted back to the human operator or alternate supporting member. Because the dynamic force interaction between the chipping hammer and supporting member has not been heretofore fully appreciated, the prior attempts to reduce the shocks and vibrations transmitted back to the supporting member have met with only very limited success.

For example, many pneumatic hammers incorporate rubberized handles or resilient mounts in the form of resilient annular rings therein inserted in series with certain portions of the tool on the central axis thereof. Still other hammers include one or more coiled metal compression springs between the elements of the tool to reduce the oscillatory forces and the noise levels being transmitted to the operator. These forms of shock absorbing devices are generally passive and/or simplistic in nature and are not as satisfactory as is desired.

Because of the vexatious nature of the vibra¬ tions of these percussion tools it is highly desirable to mount them on mechanical manipulators or industrial robots, for in this way the human operators can be relieved of the drudgery usually associated with the operation of such tools. Of course, the development of an improved vibration isolator device for reducing the transmission of oscillatory forces from the percussion tool to the supporting member of an industrial robot can be expected to involve principles applicable as well to hand held percussion tools.

Vibration isolation systems are denoted as being active or passive depending on whether or not external power is required for their operation. Conven- tionally vibratory disturbance is isolated from a mechanical system by interposing elements of compliance and energy dissipation, such as springs and dampers, ^' between the disturbance and the system. In most cases the masses of the mechanical systems are predetermined, such that only spring rates and damping coefficients can be selected. For the most part their isolation performance is limited because the various elements thereof are dependent on the relative displacements and velocities of their attachment points. Use of active elements in vibration isolation offers greater system construction flexibility and per¬ formance. An active suspension element is ideally a programmable force generator operating from a controlled energy source, with the element being controlled to produce functions of a sensed system variable. Active systems are generally more complex and more expensive than passive systems. Hence, such active systems have been heretofor limited to vehicle suspensions, vehicle seating, and the aerospace industry. Representative of the work being done in this area are an article by

D. W. Schubert and J. E. Ruzicka on "Theoretical and Experimental Investigation of Electrohydraulic Vibration Isolation Systems" in the Journal of Engineering for Industry, November, 1969; an article by M. J. Crosby and D. C. Karnopp on "The Active Damper - A New Concept for Shock and Vibration Control" in Bulletin No. 43, The Shock and Vibration Bulletin, Part 4, Shock and Vibration Center, Naval Research Library, June, 1973; and an article by D. C. Karnopp on "Active and Passive Isolation of Random Vibration" from Isolation of Mechani¬ cal Vibration, Impact and Noise, ASME Design Technical Conference Publication, 1973. While these and other publications present relationships on active vibration isolation systems that are of general background interest, they are primarily of the type that isolates a body on a vehicle from roadway disturbances rather than being the type that isolates a vibrating device from a human operator or other support.

The present invention is directed to over- coming one or more of the problems as set forth above.

Disclosure of the Invention

In one aspect of the present invention a vibration isolator device is provided for a tooi having a work element movable at a preselected oscillatory rate. The device includes a support member and isolator means for dynamically absorbing the transmission of vibratory forces and motion from the tool to the support member, the isolator means having a supplemental mass of preselected weight and being actively tuned to the forces applied to the tool in response to a sensed system variable.

The instant vibration isolator device features both active means and passive means to reduce the oscillatory motions transmitted back to the support member. Advantageously, the active means is responsive

to a sensor signal reflecting the vibration rate of the tool, and the supplemental mass provides an inertial body on which an active member thereof can push. The active member provides a force to the percussion tool, to oppose the external disturbance force, and an equal but opposite force to the supplemental mass. In effect, the supplemental mass acts as a dynamically tuned vibration absorber.

In another aspect of the invention, a vibra- tion isolator device is provided for a percussion tool such as a pneumatically operated chipping hammer. The device includes a support member, passive means for mounting the percussion tool on the support member, an accelerometer on the percussion tool, and force- transmitting actuator means for providing a force on the percussion tool in opposition to the external disturbance force in response to a signal from the accel'erometer. Preferably, the force-generating actuato means includes a cooperating piston member and housing, and means for supplying significant hydraulic energy thereto.

In a still further aspect of the invention, first resilient means is provided for axially guiding and resiliently biasing a casing connected to the percussion tool toward an axially centered position with respect to a supporting member, a supplemental mass is provided, and second resilient means is provided for axially guiding and resiliently biasing the supple¬ mental mass toward an axially centered position with respect to the casing.

In still another aspect of the invention, a vibration isolator device is located between a per¬ cussion tool and an industrial robot for minimizing the transmission of vibratory motion thereto. The device includes both passive and active system means, with the

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active system means including a supplemental mass and means for hydraulically applying a force upon the mass in opposition to the movement of the percussion tool. Preferably the active system means is responsive to a feedback signal reflective of the acceleration and velocity of the percussion tool.

Brief Description of the Drawings

Fig. 1 is a diagrammatic perspective eleva¬ tional view of one embodiment of the vibration isolator device of the present invention as mounted on an indus¬ trial robot and for supporting a conventional percus¬ sion tool.

Fig. 2 is a fragmentary and enlarged diagram¬ matic elevational view of the vibration isolator device illustrated in Fig. 1 with portions of the device and the percussion tool broken open to better illustrate details of construction thereof.

Best Mode for Carrying Out the Invention

In the illustrated embodiment of the invention shown in Fig. 1 a vibration isolator device 10 is connected between an articulated mechanical manipulator or industrial robot 12 and a conventional percussion tool 14 such as a pneumatically reciprocated chipping hammer. The industrial robot 12 is preferably computer controlled and has a multi-segmented arm 16 that can be automatically positioned in any one of a plurality of working attitudes. For example, a support member or end-effector 18 connected to a first portion 20 of the arm can be rotated about a first axis 22 to provide a variable degree of roll as is illustrated by the arrow identified by the letter R in Fig. 1. While the first portion 20 of the arm is rotatably connected to a second portion 24 of the arm to provide such roll, the second portion 24 is rotatably connected to a third

portion 26 to provide a variable degree of yaw about a generally upright second axis 28 as is indicated by the arrow identified by the letter Y. Similarly, the third portion 26 is pivotally connected to a fourth portion 30 to provide a preselected degree of pitch about a third axis 32 as indicated by the letter P, and the fourth portion 30 is pivotally connected to a fifth portion 34 to provide a preselected degree of elbow extension as indicated by the letter E about a fourth axis 36. Lastly, the fifth portion 34 is pivotally connected to a sixth portion 38 to provide a preselected amount of shoulder swivel as indicated by the letter S about a fifth axis 40, and the sixth portion 38 is mounted on a fixed base pedestal 41 secured to a floor 42 for arm sweeping rotation as indicated by the letter A about a vertical axis 44. Thus, it can be appreciated that the support member 18 can be conveniently positione where and when desired by the industrial robot 12. Such robot can be of conventional construction, and reference is made to the commercial line of industrial robots and associated automatic control systems produced by Cincinnati Milacron, Inc. of Cincinnati, Ohio under the trademark T Industrial Robot, the full construction and operation of which is incorporated herein by refer- ence.

Turning now to the construction of the chippin hammer 14 shown in Fig. 2, it may be noted to include a barrel-shaped or tubular housing 46 defining an elongate chamber 48 in which a piston hammer 50 is slidably received. Pressurized air is alternately supplied to the opposite ends of the chamber to reciprocate the piston hammer and to impact it against an end 51 of a distal work element 52. In the illustrated embodiment the work element is a chisel having a rearwardly facing shoulder 53 and a chisel shank 54. The chisel shank is

reciprocatably supported in a sleeve or bearing insert 55, and a tool retainer 56 limits the forward movement of the chisel via contact with a forwardly facing shoulder 57 thereof. The chisel 52 is specifically adapted to chip an unwanted cast metal fin 58 from a workpiece 61, and the time to apply the loads for impact fracturing of the fin is significantly less than the characteristic period of vibration so that the loading is dynamic and of an impact nature. The chipping hammer 14 can be of any well known construction so that, although not shown in detail, it is to be understood that it has valve and passage means for directing air or another fluid alter¬ nately to the opposite ends of the chamber 48 to recipro- cate the piston hammer 50. One suitable construction is illustrated and described in U. S. Patent No. 3,727,700 to L. A. A tsberg, on April.17, 1973.

As stated earlier with respect to Fig. 1, the vibration isolator device 10 is connected to the support member 18 of the robot 12, and supports the chipping hammer 14. As is shown best in Fig. 2, the device 10 includes isolator means 59 comprising both passive system means 60 and active system means 62 for dynami¬ cally isolating and absorbing the transmission -of vibratory forces and motion from the chipping hammer to the support member.

The passive system means 60 basically consists of a passive, parallel spring-damper isolator that can provide a substantial reduction in force and velocity transmission from the housing 46 of the chipping hammer 14 to the support member 18. More specifically, the passive system means 60 includes a cylindrical outer casing 64 constrained to motion along the central axis 22 of the chipping hammer by a plurality of parallel shafts 66 . The shafts are individually fixed to a

plurality of radially outwardly extending flanges 68 of a cylindrical inner casing 70 having a rear support plate 72 releasably connected to the support member 18. Cooperating pairs of radially inwardly extending flanges 74 on the outer casing 64 axially slidably support the shafts via bearings or ball bushings 76. First resilien mounting means 78 is connected between the outer casing- 64 and the inner casing 70 for resiliently biasing the outer casing and the hammer housing 46 connected thereto toward a preselected centered axial' position with respect to the support member. In the instant embodi¬ ment the first resilient mounting means includes a plurality of cooperating pairs of coiled metal compres¬ sion springs 79 peripherally spaced about and parallel to the central axis 22, for example four pairs or sets of linear compression springs. Preferably, the passive system means 60 also includes one or more energy absorb¬ ing hydraulic damping devices or shock-absorbing struts 80 connected between the inner and outer casings. In the instant embodiment two hydraulic damping devices 80 are utilized, and as representatively illustrated in Fig. 2, each is connected between the flange 68 of the inner casing and a front plate 81 of the outer casing. Preferably, each hydraulic damping device is self- contained and includes a tubular housing 82 and a piston member 83 reciprocatingly disposed therein and defining first and second work chambers 84,85 therewith. An adjustable flow control valve or orifice 86 is disposed in a passage 87 interconnecting the work chambers to allow simple variations of compliance and energy dissipation within the vibration isolator device 10.

Referring now to the active system means 62 shown in Fig. 2, it can be noted to advantageously include a resiliently centered supplemental mass 88 of

preselected weight and an active piston member 89 cooperatively associated therewith which together define a force-generating actuator 95. The supplemental mass includes a cylindrical rear guiding plate 90, and a hollow cylindrical housing 91 is releasably connected to the guiding plate and is of a construction sufficient to receive the active piston member 89 slidably therein. Second resilient mounting means 92 is connected between the supplemental mass 88 and essentially the hammer housing 46 for resiliently biasing the supplemental mass toward a preselected centered axial position with respect to the hammer housing. In the instant embodi¬ ment the second resilient mounting means 92 includes a plurality of cooperating pairs of coiled compression springs 93 spaced peripherally about and parallel to the central axis 22, for example two sets of linear compression springs. A plurality of parallel shafts 94 are fixedly connected at one end to a rear plate 96 connected in turn to a cylindrical inner wall 98 of the outer casing, and at the other end the shafts are individually connected to an annular flange 100 secured to the inner wall. Thus, the springs 93 surround two of the shafts 94 in much the same way as the springs 79 surround the shafts 66 in the passive system means 60, and a bearing or ball bushing 102 is provided in the guiding plate 90 for each of the shafts 94.

First and second work chambers 104, 106 are defined by the active piston member 89 in the hollow cylindrical housing 91. Such head end and rod end chambers ^' are respectively connected to first and second hydraulic fluid lines 108,110 leading to a controlling actuator or electrohydraulic control valve means 112. The control valve means 112 is hydraulically connected to a conventional energy source 114 of fluid under pressure via a plurality of conduits 116 and is electri-

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cally connected to control means or signal processor means 118 via a plurality of electrical command signal lines 120. A sensor such as an accelerometer 122 is connected to the chipper hammer housing 46, and a plurality of electrical lines 124 are utilized to electrically couple the sensor to the control means for signal -feedback purposes. Collectively, the fluid source 114, the electrohydraulic control valve means 112, the control means 118 and the sensor 122 comprise a closed loop, active vibration control system of the electrofluidic type for controllably directing pres¬ surized fluid to a preselected one of the first and second work chambers 104,106 as a sensed function of the vibration excitation of the chipping hammer 14 as the chisel 52 is rapidly urged against the fin 58 by periodic blows of the piston hammer 50.

Industrial Applicability

The subject invention addresses the removal of excess metal from foundry iron castings using the pneumatically operated chipping hammer 14 as controlled by the industrial robot 12. The vibration isolator device 10 minimizes the input disturbances to the robot so that structural wear and fatigue, inaccurate~robot positioning, and maintenance problems are greatly reduced. Essentially, the device 10 filters and smooths the vibratory load variations and places an essentially more constant load on the robot.

In the instant example, the percussion tool 14 that was considered was Chipper Model CP-4125 manu- factured by Chicago Pneumatic Tool Company of New York, New York. In that model the piston hammer 50 traverses the cylinder chamber 48 at a rate of 42 cycles per second with a nominal air supply gauge pressure of 620 Kpa (90 psig) . With the chisel 52 positioned against

the fin 58 at a preselected angle of inclination, as illustrated by the angle A in Fig. 2., the impact energy is transferred through the material of the chisel as it penetrates slightly. Some energy is absorbed by the workpiece 61, while the remainder, for example 30 percent, is reflected back against the tool housing 46.

Several possible sources of vibration exist in such chipping operations. First, the pressure variations in the forward regions of the chamber 48 generate a dynamic force on the housing 46. Second, the ^" reflected energy from material impact travels back through the chisel 52 directly to the housing via the shoulder 53. Third, friction and binding forces exist between the chisel shank 54 and the bearing insert 55 and allow some of the piston impact energy loss to be transferred to the housing. Fourth, operation of the chipping' hammer 14 without striking the workpiece 61 allows the chisel to move forwardly or outwardly against the tool retainer 56. Piston impact energy is delivered through the chisel shank 54 causing relatively high impact forces between the chisel shoulder 57 and the tool retainer. The other vibration sources are distur¬ bances due to transverse chisel motions, chise'l point penetration, and workpiece vibration. These, however, can usually be neglected in a systems analysis.

Chipping hammer 14 was initially mounted through a resilient support system substantially solely like the passive system means 60 onto a test machine, not shown, for linear removal of a fin 58. Instrumenta¬ tion techniques and equipment subsequently provided measurement data and analysis of tool chipping forces, acceleration, displacement, and the feed rate of the chipper housing 46. Two types of accelerometers 122 were used: an Entran Devices Model EGC-500-DS strain- gauge accelerometer, and a Bruel and Kjaer Model 4332

piezoelectric accelero eter. Also a Schaevitz D. C. linear variable displacement transformer was used to indicate displacement of the chipper housing relative to the support 18. Three different types of behavior of the tool were considered which are dependent upon the chisel placement and action. The three types result from the piston hammer 50 impacting upon the chisel 52 (a) while the chisel is pressed against the fin 58, Cb) following removal of small fragments, and (c) following removal of long chips. Spring rates of

62.5 Kg/cm (350 lb/in.) and 125 Kg/cm (700 lb/in.) were used in the experimental passive resilient support to provide suspension resonant frequencies of 12 and 17 Hertz, respectively. Based upon the understanding gained of the initial experiments discussed briefly above, a hybrid computer simulation was formulated for studying the robot-controlled fin chipping process along a single geometric axis. The simulation served as a design vehicle for evaluating the specifics of construction of the vibration isolator device 10. However, the vibra¬ tory loading on the industrial robot 12 imposed by the chipping process is not only dependent upon the chipping action and characteristics of the chipping hammer 14, but also on the dynamics of the robot.

The formulated models of the chipping action, the dynamics of the chipper hammer 14, and the dynamics of the robot 12 were combined to simulate robot con¬ trolled fin chipping for determining the desired vibra- tion isolation characteristics. In order to guarantee effective chipping, the extent that the chipper hammer housing 46 can be allowed to move relative to the support member 18 should be limited to a preselected maximum value, for example about 0.5 cm (0.20 in.) inward movement toward the robot.

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The instant vibration isolator device 10, incorporating both the passive systems means 60 and the active system means 62, can be extremely effective for use with a percussion tool without excessive complexity in construction. In operation, the active system means 62 provides both vibration isolation and vibration absorption and is basically a servomechanism including the response sensor or accelerometer 122, the sensor signal processor 118, the controlling actuator 112, and the force-generating actuator 95. The accelerometer supplies a signal proportional to the vibration excita¬ tion value of the chipper hammer housing 46. The signal processor 118 reacts to such sensed signal and is preprogrammed to create a command signal to the controlling actuator 112. Thereafter, the controlling actuator communicates pressure fluid to and relieves pressure fluid from the work chambers 104,106 to induce compensating forces upon which the reciprocating hammer housing 46 can react. More specifically, in operation, air pressuri- zation of the left end of the chamber 48 when viewing Fig. 2 will cause the piston hammer 50 to travel to the right and to impact upon the end 51 of the chisel 52. Whereupon the hammer housing 46 and the accelerometer 122 secured thereto will experience negative accelera¬ tion or a reactive movement to the left when viewing the drawing. Such negative acceleration is converted into an electrical signal proportional thereto and is delivered to the signal processor 118 via the electrical lines 124. Preferably, the signal processor internally processes this acceleration signal and electrically integrates it to provide a command signal at the electri¬ cal lines 120 that is proportional to both acceleration and velocity. We have determined that the performance

of the active system means 62 can be significantly improved when both acceleration and velocity of the hammer housing is used over effecting a command signal by acceleration alone. Adding velocity in the closed loop artifically increases energy dissipation.

The extent to which the force-generating actuator 95 can cancel the external disturbance force and reduce the peak suspension force level, is primaril dependent upon the capability of the controlling actua 112 to produce a force function identical with the external disturbance. Typically, the external• chippin force function has a frequency content up to about 1000 Hz. Preferably, the electrohydraulic control valve 112 has a construction sufficient for producing forces fro command signals with frequencies in the range of 150 to 300 Hz without attenuation. The limitation of response speed is primarily due to servovalve and hydraulic actuator dynamics and fluid compressibility effects.. With these factors in mind, a piston bore of about 6.30 cm (2.5 in.) and an effective piston area of about 26 cm 2 (4 in.2) for the active piston member 89 is needed for a hydraulic power supply pressure at the source 114 of about 8,300 Kpa (1,200 psi) at 23 litres/ min. (6 gpm) . Increasing the supply pressure -can allow a smaller diameter controlling actuator to be utilized.

Assuming that the hammer housing is still moving to the left with negative acceleration, the hydraulic spool or spools within the controlling actuat 112, not shown, respond to the command signal via electrical lines 120 and quickly move to a position of communicating pressure fluid from the source 114 to the first hydraulic fluid line 108 and to the primary work chamber 104, and to simultaneously communicate the opposite or second work chamber 106 to drain. The

flow rate to the work chamber 106 will provide a force on the supplemental mass in weighted proportion to the sum of the acceleration and velocity signals. This advantageously provides an inertial body on which the active piston member 89 can push. The supplemental mass system resonance can be selected to be low since the size 'and motion thereof are limited only by the weight constraints on the isolator from the loading capability of the industrial robot 12. If the force generated in the work chamber 104 approximately cancels the external disturbance force, then the chipping hammer 14 is disturbed primarily by the induced motion of the supplemental mass 88 from its passive attachment to the hammer housing 46. The result is the artificial addition of inertia to the chipping hammer and the attenuation of the external force through the low frequency supplemental mass system. In effect, the supplemental mass acts as a dynamically tuned vibration absorber. It is of note that the force-generating actuator 95 requires a substantial amount of external energy for its operation, for example several horsepower. When the hammer housing 46 experiences positive acceleration, or movement to the right when viewing the drawing, then the control means 118 can be operated to supply a command signal to the control actuator 112 sufficient to controllably direct pressure fluid via fluid line 110 to the second work chamber 106 of the force-generating actuator 95, and to controllably open the first work chamber 104 to drain. Thus, energy can be supplied to the force-generating actuator in con¬ trolled opposition to either direction of movement of the hammer housing 46.

The instant acceleration and velocity feedback control system, in essence, makes the mass of the hammer housing 46 and associated members 64, 81, 89,

96, 98, etc., act as if it were very large. Conse¬ quently, it will follow that the hammer housing will desirably not move as much. In the simulation, a

2 supplemental mass of 0.007 Kgf. - sec. /cm (0.0389 2 lbf. - sec. /in.), which is equivalent to about 6.8 Kgf

(15 lbf.) weight, was used with a chipper hammer housing weight of 11.3 Kgf (25 lbf.) . Increasing the supple¬ mental mass can further smooth the robot response oscillatory behavior by lowering the frequency of the supplemental mass dynamic system. We theorize that the supplemental mass 88 should be within a practical range of about 50 to 100% of the mass of the hammer housing 46 and the associated members moving therewith as a unit, and preferably about 70 to 75% thereof. Alter- natively, the supplemental mass should be about 20 to

50% of the total mass of the entire vibration isolation device 10 and the chipping hammer 14 for optimum results, or approximately one third the total weight • thereof. Simulation results showed that a direct mounting of the chipper hammer 14 on the support member 18 of the industrial robot would exhibit impulsive forces with magnitudes of up to about 900 Kgf. (2000 lbf.) and vibratory displacements of the support member of about 1.15 mm (0.045 in.) as the support member 18 is advanced or moved to the right when viewing the drawing at a feed rate of about 2.5 cm/sec. (1 in./sec) The resulting motion of the support member is very oscillatory, representing intense shaking of the robot. In contrast, the use of the vibration isolator device 10 of the present invention can result in impulsive forces with magnitudes of up to about 40 Kgf. (88 lbf.) and vibratory displacements of about .025 mm (0.010 in.) . Compared with a passive suspension system only, the vibration isolator device 10 is expected to reduce vibratory motions by an additional 50 percent.

With respect to the passive compression springs 79 it is to be noted that in the illustrated example four sets are used, with each spring having a deflecting spring rate of about 17.8 Kgf/cm (100 lbf/in.) On the other hand, the active compression springs 93 individually have a deflecting spring rate of about 8.9 Kgf/cm (50 lbf/in.), and the instant example has two sets. Therefore, the active system means 62 has a combined deflecting spring rate of about one fourth that of the passive system means 60 for respectively positioning the hammer housing 46 toward a centered axial location with respect to the support and for positioning the supplemental mass 88 toward a centered axial position with respect to the hammer housing. Fig. 2 illustrates what can be referred to as the preferred parallel arrangement of the passive system means 60 and the active system means 62 in that the passive system means is generally disposed encir- clingly about the active system means. Such construc- tion minimizes the length of the vibration absorber device 10 while contributing to its fairly significant overall diameter. Alternately, we contemplate that the active system means and passive system means can be disposed in end-to-end series in order to minimize the diameter at the expense of additional overall length.

In summarizing, the subject vibration isolator device 10 can smooth or minimize the vibratory motion of the chipping hammer housing 46 and of the support member- 18 on the robot 12 by utilizing a force- generating actuator 95 controlled by a closed-loop electrohydraulic control system 112, 114, 118, 122. Acceleration is used as the feedback signal to generate a force at the force-generating actuator 95 which can oppose the motion of the chipper housing 46. The result is a substantially constant load on the robot

with a minimum of vibratory motion, such that the robot can be preprogrammed for accurately and positively positioning the chisel 52 against a workpiece for definning purposes. This will relieve the usual human operator of the percussion tool from this onerous task, and increase the total amount of work done because the tools never get tired.

Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.