Abstract
The present invention relates to a drilling device. The device comprises at least one drill rod, the or each drill rod having a first cylindrical wall defining an elongate chamber for receiving a working fluid to form a fluid column, the length of the fluid column being equal to a total length of the elongate chambers of the or each drill rod. The device also comprises a displacement excitation device arranged at a proximal end of the fluid column and configured to excite the fluid column to cause the working fluid in the fluid column to oscillate, wherein the excitation device is configured to excite the fluid column at an excitation frequency at or within 10% of a natural frequency of the fluid column determined based on the fluid column having a fixed boundary condition at a proximal end thereof. The device further comprises a tool piston moveably mounted at a distal end of the fluid column and a drilling tool connected to the tool piston such that the oscillation of the working fluid in the fluid column imparts an oscillating force to the drilling tool.
Claims
1. A drilling device for forming a drilled hole, the drilling device comprising: at least one drill rod, the or each drill rod being disposed in the drilled hole during use and having a first cylindrical wall defining an elongate chamber for receiving a working fluid to form a fluid column, the length of the fluid column being equal to a total length of the elongate chambers of the or each drill rod; a displacement excitation device arranged at a proximal end of the fluid column and configured to excite the fluid column to cause the working fluid in the fluid column to oscillate, wherein the excitation device is configured to excite the fluid column at an excitation frequency at or within 10% of a natural frequency of the fluid column determined based on the fluid column having a fixed boundary condition at the proximal end thereof; and a tool piston moveably mounted at a distal end of the fluid column and a drilling tool connected to the tool piston such that the oscillation of the working fluid in the fluid column imparts an oscillating force to the drilling tool, wherein the displacement excitation device reciprocally moves the working fluid in the fluid column in a longitudinal direction of the fluid column.
2. The drilling device as claimed in claim 1, wherein the drilling tool is a rotary tricone bit and wherein the drilling device is rotatable about a longitudinal axis.
3. The drilling device as claimed in claim 1, wherein the working fluid is a liquid.
4. The drilling device as claimed in claim 3, wherein the liquid is hydraulic oil.
5. The drilling device as claimed in claim 3, wherein the liquid is water.
6. The drilling device as claimed in claim 5, further comprising: at least one outlet for water at a distal end of the fluid column; means for pumping water into the fluid column at an input flow rate; such that the water flows along a leakage fluid path between the first cylindrical wall and the tool piston and out of the at least one outlet at a leakage flow rate equal to the input flow rate.
7. The drilling device as claimed in claim 6, wherein: the or each drill rod comprises a second cylindrical wall arranged outside at least a portion of the first cylindrical wall such that an annular flushing channel is defined between the first and second cylindrical walls and the annular flushing channel is configured to receive a flushing fluid at a proximal end thereof and discharge the flushing fluid at a distal end thereof; and the outlet for water is provided adjacent the distal end of the flushing channel.
8. The drilling device as claimed in claim 6, wherein: the outlet is provided at a distal face of the drilling tool.
9. The drilling device as claimed in claim 1, wherein: the or each drill rod comprises a second cylindrical wall arranged inside the first cylindrical wall such that the elongate chamber is an annular elongate chamber defined between the first and second cylindrical walls and a flushing channel is defined within the second cylindrical wall and the flushing channel is configured to receive a flushing fluid at a proximal end thereof and discharge the flushing fluid at a distal end thereof.
10. The drilling device as claimed in claim 1, wherein each elongate chamber has a length l and the length of the fluid column is an integer multiple of l.
11. The drilling device as claimed in claim 1, wherein the displacement excitation device comprises an excitation piston disposed in a proximal end of the chamber such that a forward end of the excitation piston forms a proximal end wall of the fluid column and coupled to a crankshaft mechanism such that the piston is driveable reciprocally in a longitudinal direction of the fluid column to reciprocally displace the proximal end wall of the fluid column.
12. The drilling device as claimed in claim 1, wherein the displacement excitation device comprises a cam mechanism arranged at a proximal end of the chamber such that each of a plurality of pistons is driveable reciprocally in a radial direction by a rotatable cam, to change the volume of the chamber in which the fluid column is established in a reciprocal fashion.
13. The drilling device as claimed in claim 1, wherein the displacement excitation device comprises an epicycloid mechanism comprising a multi-lobed rotor having N lobes arranged to orbit within a multi-lobed stator having N+1 lobes, such that N+1 cavities of varying volume are created between the rotor and the stator, and wherein a first group of the N+1 cavities are in fluid communication with each other and with the chamber to change the volume of the chamber in which the fluid column is established in a reciprocal manner.
14. The drilling device as claimed in claim 13, wherein a second group of the N+1 cavities are in fluid communication with each other and connected to a source of fluid at a substantially constant pressure.
15. A method of controlling an excitation frequency of a drilling device for forming a drilled hole, the drilling device including at least one drill rod, the or each drill rod being disposed in the drilled hole during use and having a first cylindrical wall defining an elongate chamber for receiving a working fluid to form a fluid column, the length of the fluid column being equal to a total length of the elongate chambers of the or each drill rod, a displacement excitation device arranged at a proximal end of the fluid column and configured to excite the fluid column to cause the working fluid in the fluid column to oscillate, wherein the excitation device is configured to excite the fluid column at an excitation frequency at or within 10% of a natural frequency of the fluid column determined based on the fluid column having a fixed boundary condition at the proximal end thereof, and a tool piston moveably mounted at a distal end of the fluid column and a drilling tool connected to the tool piston such that the oscillation of the working fluid in the fluid column imparts an oscillating force to the drilling tool, the method comprising: setting an input torque for the displacement excitation device, whereby the excitation frequency is determined based at least on the input torque and a frequency response of the fluid column, wherein the displacement excitation device reciprocally moves the working fluid in the fluid column in a longitudinal direction of the fluid column.
16. The method as claimed in claim 15, wherein the excitation frequency is further based on a condition of a material to be drilled.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a part-schematic cross-sectional view of a drilling device according to a first embodiment of the invention;
(2) FIG. 2 is a graph of pressure in bar along the length of the fluid column of the drilling device shown in FIG. 1;
(3) FIG. 3 is a graph of displacement along the length of a fluid column having a fixed boundary condition at its proximal end for the first, second and third natural frequencies of the fluid column;
(4) FIG. 4 is a graph of pressure along the length of a fluid column having a fixed boundary condition at its proximal end for the first, second and third natural frequencies of the fluid column;
(5) FIG. 5A is a part-schematic cross-sectional view of a drilling device according to a second embodiment of the invention;
(6) FIG. 5B is a magnified view of a distal end of the device shown in FIG. 5A;
(7) FIG. 6A is a part-schematic cross-sectional view of a drilling device according to a third embodiment of the invention;
(8) FIG. 6B is a magnified view of a distal end of the device shown in FIG. 6A;
(9) FIG. 7 is a cross-sectional view of a proximal end of a drilling device according to an embodiment of the invention, in which the displacement excitation device comprises a crankshaft;
(10) FIG. 8A is a longitudinal cross-sectional view of a proximal end of a drilling device according to an embodiment of the invention, in which the displacement excitation device comprises a cam mechanism;
(11) FIG. 8B is a transverse cross-section of the device of FIG. 8A, taken along line A-A;
(12) FIG. 9A is a transverse cross-section of an epicycloid mechanism, suitable for use as a displacement excitation device in a drilling device according to the present invention;
(13) FIG. 9B is a side elevation view of the rotor of the epicycloid mechanism of FIG. 9A;
(14) FIG. 10A is a transverse cross-section of an alternative epicycloid mechanism, suitable for use as a displacement excitation device in a drilling device according to the present invention;
(15) FIG. 10B is a perspective view of the epicycloid mechanism of FIG. 10A;
(16) FIG. 11 is a schematic representation of a system comprising the epicycloid mechanism of FIGS. 10A and 10B connected to a drilling device according to the present invention;
(17) FIG. 12 is a graph of frequency response versus input torque for a drilling device of according to the present invention;
(18) FIG. 13 is a graph of frequency response versus input torque for different drilling conditions for a drilling device according to the present invention;
(19) FIGS. 14A and 14B are schematic representations of control arrangements for the system of FIG. 11;
(20) FIGS. 15A and 15B are schematic representations of alternative control arrangements for the system of FIG. 11; and
(21) FIG. 16 is a part-schematic cross-sectional view of a drilling device according to an embodiment of the invention
DETAILED DESCRIPTION OF THE DRAWINGS
(22) FIG. 1 shows a drilling device 1 according to an embodiment of the present invention. The device 1 comprises a drill rod 2 having a first cylindrical wall 3 defining an elongate chamber 4. The chamber 4 receives a working fluid, such as hydraulic oil or water, to form a fluid column. In the embodiment shown in FIG. 1, only a single drill rod is included and so the fluid column has a length L equal to the length of the elongate chamber 4. As will be described in more detail below, additional drill rods may be added to the device such that the fluid column has a length that is an integer multiple of the length of the elongate chamber 4. The drill rod or rods are disposed in the drilled hole during drilling. The drill rod 2 also comprises a second cylindrical wall 10 arranged outside the first cylindrical wall. An annular flushing channel 11 is defined between the first and second cylindrical walls.
(23) The device 1 further includes a displacement excitation device 5 arranged at a proximal end 6 of the fluid column. In the embodiment shown in FIG. 1, the displacement excitation system comprises a crankshaft arrangement. This will be described in more detail below in relation to FIG. 7. The displacement excitation device is configured to excite the fluid column at a frequency close to a natural frequency of the fluid column determined based on a fixed-fixed boundary condition, to cause the working fluid in the fluid column to oscillate. For a fixed-fixed boundary condition, the natural frequencies of the fluid column can be determined using the equation:
(24)
where f.sub.n is the natural frequency, k is the order of the natural frequency, L is the length of the fluid column, B.sub.fluid is a fluid bulk modulus and .sub.fluid is fluid density. Selection of the excitation frequency is described in more detail in relation to FIGS. 12 and 13.
(25) The drilling device 1 further comprises a tool piston 7 moveably mounted at a distal end 8 of the fluid column and a drilling tool 9 connected to the tool piston such that the oscillation of the working fluid in the fluid column imparts an oscillating force to the drilling tool. In the embodiment shown in FIG. 1, the drilling tool 9 is a rotary tricone bit and the drilling device is rotatable about a longitudinal axis as indicated by the arrow.
(26) FIG. 2 illustrates the pressure oscillation along an exemplary fluid column, such as that of FIG. 1, when the fluid is excited at an excitation frequency close to the second natural frequency of the fluid column. As shown in FIG. 2, in this embodiment, the fluid column is 20 metres in length and pressure nodes (where the pressure has a constant value of p.sub.static) are seen at 5 meters and 15 metres from the proximal end of the chamber, respectively. Pressure antinodes (where the pressure has the highest amplitude) are seen at the proximal end of the chamber, at the midpoint of the chamber and at the distal end of the chamber. At the pressure antinodes, the pressure varies between p.sub.static+p.sub.osc amplitude and p.sub.staticp.sub.osc amplitude. The static pressure, p.sub.static, can be generated by a feed force F.sub.feed (using Weight on Bit) or by pressurising the fluid column, or both.
(27) Due to the pressure oscillation in the fluid column, the force on the tool piston and thus, the drilling tool, will oscillate accordingly. Where the drilling device is a rotary drilling device, as shown in FIG. 1, the drilling will mainly be done as conventional rotary drilling (using Weight on Bit and rotation) and the oscillating force imparted to the tool is used to enhance the drilling speed. Alternatively, the high frequency, high amplitude oscillating force alone may be used to perform a drilling action. In this embodiment, it is preferable to have the static pressure p.sub.static very close to the pressure oscillation amplitude so that the force on the tool face is close to zero when the pressure is p.sub.staticp.sub.osc amplitude. This allows the drill bit or tool to be rotated for the purpose of bit indexing, without severe wear on inserts in the cutting face of the tool.
(28) FIGS. 5A and 5B show another embodiment of a drilling device according to the present invention. This embodiment is similar to that shown in FIG. 1 and uses water as the working fluid. As shown in FIG. 5A, in this embodiment, the annular flushing channel 11 is configured to receive a flushing fluid, such as air, at a proximal end 12 thereof via inlet 13 and discharge the flushing fluid through outlets (not shown) in a distal face 21 of the drilling tool.
(29) The drilling device 1 further comprises a plurality of injection holes 15 for water at a distal end 8 of the first cylindrical wall 3, adjacent the distal end 14 of the flushing channel. The device 1 also comprises a pump 16 for pumping water into a proximal end 6 of the fluid column at an input flow rate. A check valve 17 is provided to prevent back flow and a seal 23 is provided at the excitation device 5 to prevent leakage of water from the proximal end of the drilling device. As shown in FIG. 5B, water flows along a leakage fluid path 22 having a length L.sub.leak between the first cylindrical wall 3 and the tool piston 7 and out of the outlets 15 at a leakage flow rate equal to the input flow rate.
(30) In use, flushing air is supplied to the flushing channel and discharged into the drilled hole through the drilling tool to evacuate cuttings from the drilled hole. Water is supplied to the fluid column at an input flow rate and water pressure at the tool 9 induces leakage through the clearance between the piston 7 and the first cylindrical wall 3. This leakage water enters the drilled hole via the injection holes 15 in the wall 3, where it mixes with the flushing air and drill cuttings, providing dust suppression. The leakage flow is dependent on the length L.sub.leak of the leakage fluid path. The shorter the length of the path, the higher the leakage flow rate. If more water is pumped in by the pump 16 than is leaking out, the tool piston 7 will be pushed out in a distal direction, thereby maintaining a constant static pressure in the fluid column. This, in turn, decreases the length of the leakage path L.sub.leak, increasing the leakage flow rate so that the tool piston 7 is automatically driven to a position where the leakage flow rate is the same as the input flow rate.
(31) Another embodiment is shown in FIGS. 6A and 6B, in which the water from the fluid column is itself used to flush the drilled hole. In this embodiment, the drill rod comprises only a single cylindrical wall 3. The device 1 further comprises a pump 16 for pumping water into a proximal end 6 of the fluid column at an input flow rate. A check valve 17 is provided to prevent back flow and a seal 23 is provided at the excitation device 5 to prevent leakage of water from the proximal end of the drilling device. In this embodiment, the tool piston 7 and drilling tool 9 are integrally formed with one another. As shown in FIG. 6B, a fluid channel 24 is provided through the tool piston and drilling tool between inlets 19 in the tool piston and an outlet 20 in the distal or cutting face 21 of the drilling tool. As also shown in FIG. 6B, water flows along a leakage fluid path 22 having a length L.sub.leak between the cylindrical wall 3 and the tool piston 7 and into undercuts 18 provided in an inner surface of the wall 3 at a distal end thereof. From there, the water flows into the drilling tool 9 via the inlets 19, and is conducted through the drilling tool to the outlet 20 at the distal face 21 of the tool.
(32) As in the previous embodiment, in use, water is supplied to the fluid column at an input flow rate and water pressure at the tool 9 induces leakage through the clearance between the piston 7 and the cylindrical wall 3. This leakage water enters the drilled hole via the outlet 20 in the cutting face of the tool, where it is used to flush cuttings from the hole. As before, the leakage flow is dependent on the length L.sub.leak of the leakage fluid path. The shorter the length of the path, the higher the leakage flow rate. If more water is pumped in by the pump 16 than is leaking out, the tool piston 7 will be pushed out in a distal direction thereby maintaining a constant static pressure in the fluid column. This, in turn, decreases the length of the leakage path L.sub.leak, increasing the leakage flow rate so that the tool piston 7 is automatically driven to a position where the leakage flow rate is the same as the input flow rate.
(33) A further embodiment of the invention is shown in FIG. 16. The device 1 comprises a drill rod 2 having a first cylindrical wall 3 defining, with a second inner cylindrical wall 10 arranged inside the first cylindrical wall, an elongate chamber 4. That is, an elongate annular chamber 4 is defined between the first and second cylindrical walls. The chamber 4 receives a working fluid, such as hydraulic oil or water, to form a fluid column. In the embodiment shown in FIG. 16, only a single drill rod is included and so the fluid column has a length L equal to the length of the elongate chamber 4. Additional drill rods may be added to the device such that the fluid column has a length L that is an integer multiple of the length of the elongate chamber. The drill rod or rods are disposed in the drilled hole during drilling.
(34) The device 1 further includes a displacement excitation device 5 arranged at a proximal end 6 of the fluid column. In the embodiment shown in FIG. 16, the displacement excitation system comprises a crankshaft arrangement. The drilling device 1 further comprises a tool piston 7 moveably mounted at a distal end 8 of the fluid column and a drilling tool 9 connected to the tool piston such that the oscillation of the working fluid in the fluid column imparts an oscillating force to the drilling tool.
(35) In this embodiment, an inner flushing channel or pipe 11 defined by the inner cylindrical wall 10 is configured to receive a flushing fluid, such as air, at a proximal end 12 thereof via inlet 13 and discharge the flushing fluid through outlets in a distal face 21 of the drilling tool 9. In use, flushing air is supplied to the flushing channel and discharged into the drilled hole through the drilling tool to evacuate cuttings from the drilled hole. Where the working fluid is water, a leakage flow of water may be provided, similar to the arrangements described above. Where the working fluid is oil or another fluid, there is no leakage of working fluid from the chamber.
(36) In the embodiment shown in FIG. 16, the drilling tool 9 is a rotary tricone bit and the drilling device is rotatable about a longitudinal axis. The drill tool may be rotated with the inner flushing pipe 11. The outer cylindrical wall 3 may also rotate, or it may remain stationary.
(37) FIG. 7 illustrates a first embodiment of a displacement excitation device for use in the present invention. The displacement excitation device 5 is arranged to reciprocally move the fluid in the fluid column in a longitudinal direction. In this embodiment, the displacement excitation device 5 comprises a crankshaft 25 having an eccentricity e arranged to drive an excitation piston 26 disposed in a proximal end of the chamber 4 in a reciprocal manner. A forward end 29 of the excitation piston forms a proximal end wall of the fluid column. Driving the excitation piston has the effect of reciprocally displacing the proximal end wall of the fluid column in a longitudinal direction. The excitation piston has a stroke length of 2e and reciprocates at a frequency .sub.oscillation equal to the drive frequency .sub.drive of the crankshaft. In this embodiment, as the excitation piston diameter is relatively large and the pressure amplitude of the fluid oscillation is relatively high, the pressure force on the crankshaft mechanism is relatively high. This means that the mechanism must be quite strong and, therefore, heavy with the result that the dynamic forces generated when the mechanism is running at high frequency may be substantial.
(38) FIGS. 8A and 8B illustrate another embodiment of a displacement excitation device for use in the present invention. In this embodiment, the displacement excitation device comprises a cam mechanism, in which three pistons 27a, 27b and 27c are driven reciprocally and simultaneously in a radial direction by a cam 28. This has the effect of changing the volume of the chamber in which the fluid column is established in a reciprocal fashion, thereby reciprocally moving the fluid in the fluid column in a longitudinal direction. The cam 28 is rotated at a drive frequency .sub.drive, so that the pistons 27a, 27b, 27c are driven at an excitation frequency of .sub.oscillation=3 .sub.drive, thereby achieving a higher excitation frequency than the crankshaft mechanism for the same drive frequency. This mechanism is therefore more compact than the crankshaft described above and the dynamic forces generated are cancelled out due to the symmetrical nature of the mechanism.
(39) FIGS. 9A and 9B illustrate a further embodiment of a displacement excitation device for use in the present invention. In this embodiment, the displacement excitation device is based on an epicycloid mechanism similar to a gerotor or geroller type of hydraulic motor. The epicycloid mechanism comprises a rotor 30 having a plurality of lobes 32 which orbits at a frequency .sub.orbit with an eccentricity of e about the centre of a stator 31 which also has a plurality of lobes, one greater than the number of lobes on the rotor. Stator pins 33 provide a seal between the stator casing and the rotor and also receive pressure-induced forces from the rotor. While orbiting, the rotor is also spinning about its centre at a frequency .sub.spin. The orbit and spin frequencies are related to one another by the equation .sub.orbit=N .sub.spin, where N is the number of lobes 32 on the rotor 30. In the embodiment shown, the rotor has five lobes so that the orbit frequency is five times the drive frequency, in the opposite direction.
(40) The arrangement of the stator and the rotor is such that N+1, or in this case, six cavities are formed between them as the rotor rotates. The volume of each cavity changes in a harmonic fashion with a frequency .sub.orbit. When used as a motor, each of these cavities is connected to high and low pressure lines with a valve system such that the cavity receives high pressure liquid when the cavity volume is increasing and the cavity is connected to a low pressure line when the cavity volume is decreasing. When used as an excitation mechanism, as in the present application, the cavities are divided into two sets, labelled A and B, respectively in FIG. 9A. All of the cavities in the same set are connected to each other by way of a groove 36 provided in a bottom face plate 37 of the stator. This means that the pressure in each of the cavities in the same group is equalised. As the rotor orbits, the total volume of each of the sets of cavities changes in a harmonic fashion. The displacement excitation for the fluid column is achieved by connecting the fluid column to one of the sets of cavities. The frequency of the excitation is the same as the orbit frequency .sub.orbit of the rotor. The rotor may be driven in a number of ways. In the embodiment shown in FIG. 9B, the rotor is driven by a cardan shaft 38, connected to a drive shaft 39 at a first end and the rotor 30 at a second end. The rotor spins at a frequency .sub.spin which is equal to the drive frequency .sub.drive of the drive shaft. As set out above, the orbit frequency .sub.orbit, and thus the excitation frequency is N times the drive frequency, so that the system has a built-in step up gear.
(41) An alternative drive arrangement for the rotor 30 is shown in FIGS. 10A and 10B. In this arrangement, the rotor is connected directly to the drive shaft 39 and has an eccentricity e with respect to the centre 40 of the drive shaft. In this case, the rotor is forced to orbit about the centre of the stator at an orbit frequency .sub.orbit which is equal to the drive frequency .sub.drive. This is also the excitation frequency of the fluid column. The orbital motion induces the spinning motion. This arrangement requires a higher drive speed to achieve the same excitation frequency as the previous arrangement, but allows for a more compact layout. In this case, the pressure forces are carried by a bearing element (not shown) rather than the stator pins as in the previous arrangement.
(42) In the embodiments described above in relation to FIGS. 9 and 10, only one of the two sets of cavities is connected to the fluid column. In this case, the rotor is subjected to pressure forces as follows:
F.sub.max=p.sub.max*A.sub.pressure
F.sub.min=p.sub.min*A.sub.pressure,
where p.sub.min and p.sub.max are the maximum and minimum pressures in the fluid column and A.sub.pressure is the area upon which the pressure is acting. In an alternate embodiment, the second set of cavities may be connected to a constant pressure source with a pressure equal to the mean pressure of the fluid column, p.sub.mean. This reduces pressure forces on the rotor substantially:
F.sub.max=(p.sub.maxp.sub.mean)*A.sub.pressure
F.sub.min=(p.sub.meanp.sub.min)*A.sub.pressure,
or
F.sub.max/min=(p.sub.amp)*A.sub.pressure,
where p.sub.amp is pressure oscillation amplitude in the fluid column. Thus, the maximum force on the rotor is at least 50% lower than in the case where the second set of cavities is not connected to a pressure source. The constant pressure source may be provided by a gas accumulator 41 connected to the B cavities, as shown in FIG. 11. The A cavities are in fluid communication with the fluid column as before and the rotor is driven by driver motor 42. There will be a slight variation in the pressure supplied by the accumulator, but the variation is small once the gas accumulator is relatively large. As shown in FIG. 11, the pressure forces on the rotor are much reduced as compared with the pressure variation in the fluid column. As the rotor 30 has no seals, there will be leakages between the A and B cavities and from the cavities to the driver shaft casing. The arrangement shown in FIG. 11 also allows for compensation for the leakages 49 between the cavities and the driver shaft casing by connecting the B cavities to a pressure source 43 at the same pressure as the mean pressure in the fluid column.
(43) FIG. 12 illustrates the required input torque to excite a drilling device according to the present invention at various frequencies. The peaks in the response 1201 correspond to the natural frequencies of the system. As the excitation frequency gets close to one of the natural frequencies, the additional torque required to increase the excitation frequency increases. If the torque input to the displacement excitation device is C.sub.1 and the system starts from rest, it will seek to operate at an excitation frequency .sub.1, close to the first natural frequency .sub.n1 of the device. If the input torque is increased to C.sub.2, the excitation frequency is .sub.2, which is closer to the first natural frequency .sub.n1. A further increase of the input torque to C.sub.3 causes a jump in excitation frequency to .sub.3, which is close to the second natural frequency .sub.n2. A further increase in input torque to C.sub.4 increases the excitation frequency to .sub.4, close to the third natural frequency. Thus, by selecting an appropriate the input torque, the distance of the excitation frequency from the natural frequency, and thus, the drilling speed, can be selected.
(44) FIG. 13 illustrates how the frequency response varies based on differing rock conditions. A first response curve 1301 corresponds to a first rock condition and a second response curve 1302 corresponds to a second rock condition. For a constant input torque C, the excitation frequency will vary depending on rock conditions. In other embodiments, rather than torque, the control input may be input pressure or input power to the driver motor 42. FIGS. 14A and 14B illustrate possible control arrangements for the system shown in FIG. 11. In FIG. 14A, the control arrangement comprises a pressure compensated pump 44 which is controlled by a control unit 45 (or manually) to provide a constant drive pressure for the motor 42. Alternatively, the pump can be controlled to provide a constant output power to the drive motor. In FIG. 14B, the pump is a fixed displacement pump 46 and an adjustable pressure relief valve 47 is controlled by the control unit 45 to provide the required input pressure p.sub.control to the drive motor.
(45) Further examples of control arrangements for the system shown in FIG. 11 are illustrated in FIGS. 15A and 15B. In these embodiments, an adjustable flow restrictor, such as a needle valve 48, is provided in a supply line to the driver motor 42 (as shown in FIG. 15A) or in a tank line (as shown in FIG. 15B). The driver pressure of the motor 42 is p.sub.constant, less the drop across the needle valve 48. The pressure drop is a function of the opening of the valve and the flow rate through the valve, that is, the speed of the motor. The needle valve can be adjusted by the control unit 45 or manually. The control unit may comprise a solenoid as an actuator, a voltage or current regulator and a potentiometer to control the regulator output.
(46) The words comprises/comprising and the words having/including when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
(47) It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.
