Feed rate control for jet piercer

Abstract

An automatic drilling feed rate control for a jet piercing drilling machine. A weight cell generates a signal proportional to the resistance to penetration force experienced by the jet piercer burner assembly while piercing a drill hole in a rock formation. This signal is processed by a transistor amplifier, then is used for controlling the feed rate of the jet piercer into the drill hole. Means are provided for automatically adjusting the feed rate in response to drill tip melt build-up or encoutering a zone of fractured rock.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to the field of jet piercing drilling machines capable of forming blast holes in rock or ore formations by spalling or melting the rock or ore. More particularly, this invention pertains to apparatus for controlling the rate at which the jet piercer burner is lowered into a drill hole.

For a number of years jet piercing drilling machines have been used in the mining industry for making vertical blast holes in hardd rock or ore formations. After a pattern of vertical blast holes have been formed in the formation, explosives are placed in the holes and detonated to break up the ore for removal from the mine. Instead of chopping or abrading holes in the rock as is normally done with conventional drills, a jet piercer directs a high velocity, high temperature flame against the rock fromation causing disintegration by spalling or melting action. In order to achieve optimum performance from a jet piercing drilling machine, the rate at which the burner is lowered to form the drill hole must be carefully controlled. Maximum efficiency results when the tip of the burner is about 3 inches from the bottom of the hole, because it is this distance that the high velocity flame is the most effective. If the burner is lowered too quickly, the burner comes to close to the bottom of the hole resulting in reduced efficiency, or the rotating burner parts may contact the bottom or sides of the hole clogging it with debris. On the other hand, if the burner is lowered too slowly, drilling speed will be reduced resulting in a lowered productivity.

Because most rock formations vary considerably in chemical compositions and physical characteristics from one drill location to another, and also at different levels in the same drill hole, it is necessary that the drilling feed rate be continuously monitored and adjusted if optimum efficiency is to be attained. Often drilling speed is controlled manually by an operator who adjusts a rheostat in response to a resistance to penetration meter and other machine parameters. However, manual control in general is not satisfactory because of inherent lags in the operator's response to the changing conditions.

Accordingly, automatic drilling feed rate controls have been used in an attempt to increase efficiency. These prior art automatic controls are electro-mechanical devices in which the drilling feed rate is controlled by a motor driven potentiometer, which in turn is activated by switches operated by a resistance to penetration sensor. With this type of arrangement, a low resistance to penetration activates a switch which causes the motor driven potentiometer to turn in a first direction, causing the burner to accelerate in downward travel. When the burner catches up with the bottom of the hole because of too high a feed rate, the resistance to penetration increases causing activation of another switch. This switch causes the motor driven potentiometer to turn in the opposite direction which causes the burner to decelerate. Thus it is readily apparent that a major disadvantage of this prior art automatic control is that it continually hunts for the optimum drilling rate by a continuous acceleration-deceleration cycle, but that it is incapable of finding or maintaining the optimum feed rate. Further it is incapable of precisely tracking the optimum feed rate which continues to vary while the hole is being formed.

The present invention overcomes these problems by providing an electronic automatic drilling feed rate control which is capable of maintaining the optimum feed rate and precisely tracking the optimum feed rate as drilling progresses. The present invention uses a solid state amplifier to provide proportional feed rate control to the burner in response to instantaneous variations in the resistance to penetration.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an automatic drilling feed rate control for a jet piercing drilling machine having weight cells means associated with the drilling machine for producing control signals indicative of the resistance to penetration, a variable speed driving circuit for controlling the motor which lowers the burner into the drill hole, and a transistor control circuit for receiving signals from the weight cell means, processing them, and delivering them to the driving circuit so as to control the drilling feed rate in response to the instantaneous resistance to penetration experienced by the burner. Additional circuit elemlents associated with the transistor control circuit cooperate therewith to provide additional modes of operation in the event that a zone of fractured material is encountered, or that drill tip melt build-up is experienced.

DESCRIPTION OF THE DRAWINGS

In FIG. 1 there is shown a schematic drawing of the jet piercing drilling machine;

FIG. 2 shows a block diagram of a control system incorporating the present invention, for use on the machine of FIG. 1;

FIG. 3 is a schematic drawing of the automatic drilling feed rate control of the present invention; and

FIG. 4 is a graph of pertinent wave forms illustrating the operation of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, reference numeral 10 generally designates a jet piercing drilling machine. Drilling machine 10 comprises a support member 11, and a frame member 12 which is connected to support member 11 by a pivoting joint 13. Frame 12 supports a hoist assembly 14 which comprises a cable drum 15, a high speed electric motor 16, and a low speed electric motor 17. Motors 16 and 17 are drivably connected to drum 15 via clutches and suitable gearing (not shown). A mast 18 supports a pulley 19 which carries a cable 20 from hoist 14 to the top of a blowpipe 21. At the tip of blowpipe 21 is a burner assembly 22 which provides the flame for the piercing operation. An electric motor and suitable drive gearing (not shown) are included with blowpipe 21 to provide rotary motion to the burner assembly. For operation, suitable conduits are provided (not shown) for supplying fuel, oxygen, and cooling water to the blowpipe, and electrical power to the rotary drive motor.

Also mounted on frame 12 is a power cabinet 23 which receives three phase AC power from transmission lines 69, for distribution to various electrical equipment on the drill. A control panel 24 is also mounted on frame 12, and contains the automatic and manual controls for operation of the machine, including a rheostat 25, and a meter 26, the functions of which are described in a subsequent paragraph.

During normal drilling operation, blowpipe 21 is suspended above the bottom of the drill hole 27 so that substantially all of its weight is supported by cable 20. Under these conditions, the weight of blowpipe 21 acting upon hoist 14 exerts a clockwise moment about pivot 13. This moment is normally balanced by a counterclockwise moment about pivot 13 due to the weight of frame 12 and the equipment attached to it. A bias spring 28 is provided for making adjustments so that the two moments balance. Bias spring 28 exerts force on the frame 12 and on a retaining nut 29 which is threaded on a strut 30. Strut 30 is anchored at its lowered end to a member 31 which is attached to the lower end of support 11, and which may be the main frame of the machine.

A weight cell 32 is mounted on member 31, and has a movable shaft 33 attached to frame 12. Line 34 conveys a pneumatic pressure output generated by weight cell 32 to meter 26 which is the resistance to penetration meter. In operation, with the full weight of blowpipe 21 being supported by cable 20, retaining nut 29 is adjusted so that the moments about pivot 13 are in balance. At this point meter 26 is calibrated to indicate zero resistance to penetration. If during operation burner assembly 22 approaches the bottom of drill hole 27, increasing resistance to penetration forces support a portion of the weight of blowpipe 21, and the tension on cable 20 decreases. In this situation, the weight of frame 12 and attached equipment causes it to pivot downwardly pushing shaft 33 and registering an increasing resistance to penetration on meter 26.

FIG. 2 shows a block diagram of the controls for the jet piercing drilling machine of FIG. 1. In FIG. 2, weight cell 32 connects through pneumatic line 34 to meter 26, as in FIG. 1. In addition, line 34 branches to connect to the automatic feed rate control 50 and an automatic retract control 35. The output of automatic feed rate control 50 connects through lead 51 to one terminal of manual-automatic switch 36. Manual control rheostat 25 connects through a lead 37 to another terminal of switch 36. The output terminal of switch 36 connects through lead 38 to a low speed motor driver 39. Low speed motor driver 39, which may include a thyratron circuit, functions to vary the amount of power delivered to low speed motor 17 via leads 40 in response to speed command signals received from lead 38, either from automatic feed rate control 50 or from manual control 25, depending upon the position of switch 36. Low speed motor 17 operates the hoist during the drilling operation through suitable clutches and gearing which are indicated in FIG. 2 by reference numeral 41. Similarly, a high speed motor driver 42 is provided for supplying power to high speed motor 16 through leads 43. High speed motor 16 is also capable of operating the hoist when connected thereto through clutches and gearing 41. A master control 44 is provided for controlling low speed motor driver 39, clutch and gears 41, and high speed motor driver 42 through leads 45, 46 and 47, respectively. The automatic retract control 35 is connected to master control 44 by lead 49.

Also shown in FIG. 2 is a rotary ammeter 54 which connects to automatic feed rate control 50 via lead 55, and a water pressure switch 56 which connects to the automatic feed rate control through a lead 57, and to master control 44 through a lead 58. The functions of automatic retract control 35, rotary ammeter 54, and water pressure switch 56 are described following the description of automatic feed rate control 50.

FIG. 3 is a schematic diagram of automatic feed rate control 50 of FIG. 2. In FIG. 3, a lead 60 is connected to the positive terminal of a suitable DC power supply (not shown). The negative terminal of the power terminal of the power supply is connected to signal ground, as indicated by reference numeral 61. A resistor 62 is connected to lead 60. Connected between resistor 62 and signal ground is a potentiometer 63. Potentiometer 63 is a pressure responsive potentiometer, and it is connected through conduit 34 to weight cell 32 (FIG. 1), so that its wiper arm 64 (FIG. 2) moves in response to varying pressures on the weight cell.

A transistor 70 has its emitter connected to signal ground, and its base connected to wiper arm 64 by the series connection of resistor 71, and variable resistors 72 and 73. The collector of transistor 70 is connected by a resistor 74 to the positive power supply on lead 60. The output terminal of automatic feed rate control 50 is lead 51, which connects to the collector of transistor 70. As shown in FIG. 2, output lead 51 connects through switch 36 to low speed motor driver 39. A variable load resistor 75 is connected from collector to emitter of transistor 70. A second output load resistor 76 connects from the emitter of transistor 70 to normally open the contacts 82, which in turn are connected to the collector of the transistor. The normally closed relay contacts 83 are connected across variable resistor 72. The operation of relay contacts 82 and 83 is controlled by relay 81, which is energized by timer 80. Timer 80, which may be an electronic timer, is connected by leads 87 to a suitable source of electrical power. The input of timer 80 is controlled by a set of relay contacts 85. When triggered, timer 80 functions to energize relay 81 which is connected to its output, and to maintain relay 81 in an energized state for a predetermined time interval.

A time delay relay 84 is controlled by lead 55 from the rotary ammeter 54 of FIG. 2. Relay 84 controls one set of contacts 85 associated with timer 80, previously described, and another set of normally open contacts 86 which is connected from the junction of resistors 62 and 63 to the junction of resistors 71 and 72. The last mentioned circuit mode is also connected to signal ground by a normally open set of relay contacts 88. Relay contacts 88 are operated by a time delay relay 87, which in turn is connected by lead 57 to the water pressure switch 56 of FIG. 2.

The automatic feed rate control of FIG. 3 is capable of operating in several different modes and of automatically switching from one mode to another as different drilling conditions are encountered. During normal drilling operations, the automatic feed rate control circuit of FIG. 3 functions to provide speed command signals at lead 51 for operating the low speed motor driver, in response to control signals received by wiper arm 64 from the weight cell. The drilling feed rate is thus controlled, between predetermined maximum and minimum values, in inverse proportion to the instantaneously experienced resistance to penetration. The normal mode of operation is as follows.

Resistor 62 is a current limiting resistor to control the current through potentiometer 63. Together, resistor 62 and potentiometer 63 comprise a voltage divider so that the voltage picked off by wiper arm 64 varies between zero and some maximum value determined by the values of the resistances. This voltage is coupled to the base of transistor 70 through resistor 71, normally closed relay contacts 83, and variable resistor 73. The position of wiper arm 64 thus controls the base current to transistor 70, which in turn controls the collector current, the voltage drop across resistor 74, and hence the output voltage at terminal 51.

When wiper arm 64 is at the bottom of its travel, corresponding to low resistance to penetration, the base of transistor 70 is grounded and the transistor is cut off. At this point the speed command voltage appearing at lead 51 on the collector of transistor 70 is determined by the relative values of the collector load resistance 74 and variable resistor 75. In practice, variable resistor 75 is adjusted to provide an output voltage corresponding to the desired maximum drilling feed rate which the automatic control can command.

As the resistance to penetration increases, wiper arm 64 moves upward increasing the voltage and base current applied to the transistor. During this increase, the collector current through the transistor increases causing an increasing voltage drop across the collector load resistor 74. The speed command voltage on lead 51 therefore decreases. When the resistance to penetration has reached a predetermined maximum value, wiper 64 is at the top of potentiometer 63. Variable resistor 73 is adjusted so that the base of transistor 64 will receive enough current at this point to drive the transistor into saturation. At this point the voltage on output lead 51 will be equal to the saturation collector to emitter voltage drop of the transistor, corresponding to the minimum drilling feed rate.

The normal mode of operation of the automatic feed rate control described above would be adequate if only "normal" drilling conditions were encountered. However, the lack of uniformity in the rock or ore to be drilled creates nonuniform drilling conditions which require additional modes of operation of the drilling apparatus. For example, the hardness and resistance to piercing of a particular layer of rock may be so great that even the minimum drilling feed rate is too fast. In this case, the automatic retract control 35 of FIG. 2 comes into operation. When the resistance to penetration has increased to the point that wiper arm 64 has reached the top of travel, resulting in the automatic feed rate control 50 commanding its minimum feed rate, if the resistance to penetration should continue to increase beyond a predetermined threshold, automatic retract control 35 is energized by a pressure responsive switch contained therein which is in communication with weight cell 32 by way of pneumatic conduit 34. When energized, automatic retract control 35 causes master control 44 to engage high speed motor 16 momentarily to lift the drill a few inches above the obstruction and hold it there for a predetermined time interval, usually a few seconds, while the flame can burn away the obstruction. At the end of this time interval, control of the drilling feed rate is returned to automatic feed rate control 50, operating in conjunction with low speed motor driver 39.

Other drilling conditions requiring special modes of operation occur when the jet drill encounters an area of cracked or fractured material, or in the case of build-up of melted rock on the drill tip. These two conditions are normally handled by a crack-and-melt control subsystem within master control 44. The automatic drilling feed rate control of the present invention is particularly adapted to operate in conjunction with the crack-and-melt control subsystem as follows.

If the jet drill encounters an area of cracked or fractured material, rotary ammeter 54 of FIG. 2 will detect an increase in current flowing to the motor which rotates the blowpipe, reflecting the increased load on the motor caused by the cracked or fractured rock. The required response to this condition is a decrease in the drilling feed rate, which is accomplished by the automatic feed rate control as follows. The high current level detected by rotary ammeter 54, causes it to energize time delay relay 84 (FIG. 3), closing contacts 85 and 86. Closure of contacts 85 triggers timer 80, which energizes relay 81, closing contacts 82 and opening contacts 83. In this configuration, wiper arms 64 is bypassed and full voltage from the top of potentiometer 63, corresponding to maximum resistance to penetration, is applied to contacts 86, resistors 72 and 73 to the base of transistor 70. Resistor 72 is introduced by the opening of contacts 83, and is adjusted to take the place of resistor 71 which is switched out of the base circuit. Transistor 70 is driven to saturation, resulting in a minimum feed rate command at lead 51. This situation is maintained for the duration of the time delay of relay 84, which in the preferred embodiment is 30 seconds. During this interval, the drill blowpipe is lowered at the minimum feed rate to clear the fractured zone.

At the end of the 30 second time period, relay 84 returns to normal, opening contacts 86. However, relay 81 remains energized for an additional period of time controlled by timer 80. In the preferred embodiment, timer 80 is adjusted to provide an additional two minute time delay. During this 2 minute time delay, contacts 86 are open, allowing transistor 70 to follow the movements of wiper arm 64. However, relay contacts 82 remain closed, paralleling resistance 76 with resistance 75. The maximum feed rate is thus reduced to an intermediate value which is less than the normal maximum value. After timer 80 completes its cycle, the feed rate control returns to normal operation. Thus, in the case of a cracked or fractured zone of rock, the drilling feed rate is automatically reduced to minimum for 30 seconds, after which automatic feed rate control is resumed with the restriction that the maximum possible drilling feed rate is held to an intermediate value for a period of 2 minutes.

In the event that an area of drill tip melt (liquid rock) build-up is incurred, the required response is an increase in the rotary speed of the blowpipe, and an increased drilling feed rate to "punch through" the area in which melt exists. The indication that an area of melt build-up has been encountered is a drop in the rate of water flow to the blowpipe. This drop in water flow is detected by water pressure switch 56 (FIG. 2) which communicates via control line 58 to the crack and melt control subsystem of master control 44, to cause an increase in the rotary speed. At the same time water pressure switch 56 communicates via control line 57 to the automatic feed rate control 50.

In FIG. 3, actuation of the water pressure switch causes energization of time delay relay 87, resulting in the closing of relay contacts 88. Closure of contacts 88 effectively grounds the base of transistor 70, corresponding to a minimum resistance to penetration condition. With transistor 70 cut off, the speed command signals appearing at lead 51 will correspond to the maximum drilling feed rate or to the intermediate feed rate, depending upon conditions encountered just prior to the melt build-up. For example, if an area of cracked or fractured material has been encountered prior to the melt build-up, timer 80 and relay 81 may still be energized, placing resistor 76 in the output circuit. In this case, the closing of relay contacts 88 will result in the intermediate feed rate output. However, if normal drilling conditions were encountered prior to the melt build-up, relay contacts 82 would have been open, and the energization of time delay relay 87 upon encounter of melt build-up will result in commanding the maximum drilling feed rate. Time delay relay 87 remains energized so long as the rate of water flow is low. After the water flow returns to normal, upon clearing of the melt condition, time delay relay 87 remains energized for an additional thirty seconds. The purpose of this time delay is to prevent control cycling which could otherwise occur. For example, if the automatic drilling feed rate control were allowed to return to normal operation immediately after clearing the melt build-up, another melt build-up sequence could immediately occur. Repeated cycling from normal mode to melt build-up mode and back to normal mode could occur. The use of the time delay avoids excess control cycling by mantaining operation in the melt build-up mode for an additional 30 seconds to insure that the drilling punches through the melt zone.

Operation of the jet piercing drilling machine incorporating the present invention will now be explained. The machine operator using the master control 44 (FIG. 2) to control high speed motor 16 and clutch and gears 41 to quickly lower the blowpipe to the place where drilling is to begin. At this point, the operator uses master conrol 44 to energize low speed motor driver 39, and control the clutch and gears 41 so that low speed motor 17 is engaged for lowering the blowpipe. At this point, the burner is lit and drilling begins. If switch 36 (FIG. 2) is in manual position, the operator controls the drilling feed rate by varying rheostat 25 based upon his observation of resistance to penetration meter 26. If automatic feed rate control is desired, switch 36 is placed in the automatic position bringing automatic feed rate control 50 into the control loop.

FIG. 4 shows pertinent wave forms which illustrate the operation of the automatic feed rate control, throughout its various modes of operation. In all three graphs of FIG. 4, the horizontal axis represents the commmon time basis for all of the graphs. In the first graph, the vertical axis represents the resistance to penetration experienced by the burner assembly, as measured by weight cell 32 of FIG. 2. The second graph shows the output of the transistor amplifier at lead 51 (FIGS. 2 and 3). The third graph shows the drilling feed rate, which equals the rate at which the burner is lowered into the drilling hole in response to speed command signals generated by the automatic feed rate control. In the first graph, broken line 91 represents a lower threshold value of the resistance to penetration, and broken line 92 represents the maximum speed command voltage output of the transistor amplifier, which corresponds to this lower resistance to penetration threshold. Broken line 93 represents the maximum drilling feed rate, corrresponding to the maximum output voltage 92. Broken line 94 represents an upper threshold of the resistance to penetration. Broken line 95 represents the minimum speed command voltage output of the amplifier corresponding to threshold 94, and broken line 96 represents the minimum drilling feed rate, corresponding to minimum voltage 95.

For purposes of illustration, assume that drilling has begun under automatic control. From time t.sub.0 to time t.sub.1, the burner is encountering generally decreasing resistance to penetration, which might result for example from a layer of softer rock. During this time interval the output of the transistor amplifier is correspondingly increasing in inverse proportion to a decreasing resistance to penetration. The drilling feed rate is increasing in proportion to the transistor amplifier output. At time t.sub.1, the resistance to penetration has reached the minimum threshold 71. At this point, wiper arm 65 (FIG. 3) has reached the bottom of its travel, and the transistor has reached a cut off condition, resulting in a maximum voltage output. From time t.sub.1 to time t.sub.2 the resistance to penetration briefly dips below threshold 91, but since the transistor amplifier is in cut off, the output voltage remains constant at the maximum value, and the drilling feed rate remains maximum. From time t.sub.2 to time t.sub.3, the resistance to penetration is increasing, which might result from the burner encountering a harder layer of rock. During this time interval the amplifier output in the drilling feed rate decreases, in inverse proportion to the increasing resistance to penetration. At time t.sub.3 the resistance to penetration reaches the upper threshold 94, corresponding to the maximum upward travel of wiper amr 64 of FIG. 3. At this point the transistor reaches saturation, thereby commanding the minimum drilling feed rate. From time interval t.sub.3 to t.sub.4 the amplifier output and drilling feed rate remain at minimum value even though the resistance to penetration continues to increase.

At time t.sub.4, the resistance to penetration reaches the upper threshold 97 which activates the automatic retract control 35. Initiation of the automatic retract cycle results in lifting of the blowpipe, as indicated by the negative value of the drilling feed rate from interval t.sub.4 to t.sub.5. At t.sub.5 normal drilling resumes, with resistance to penetration at a minimum value since the burner has been lifted a distance from the bottom of the hole. At time t.sub.6 the lowering blowpipe again begins to encounter an increasing resistance to penetration which exceeds the minimum threshold 91 at time t.sub.7. From t.sub.7 to t.sub.8, the transistor amplifier output and drilling feed rate once again track the resistance to penetration.

At time t.sub.8 a zone of cracked or fractured material is encountered, resulting in energization of relays 84 and 81 of FIG. 3. Time delay relay 84 remains energized until time t.sub.9. During the interval t.sub.8 to t.sub.9, transistor 70 is maintained in saturation, and the transistor amplifier output and drilling feed rate are held at the minimum values. At t.sub.9 relay 84 de-energizes, but relay 81 remains energized for an additional time period, to t.sub.10. During this additional time period, the transistor amplifier output and the drilling feed rate are maintained at the intermediate value. At time t.sub.10, relay 81 is de-energized, and normal drilling operation is resumed with the transistor amplifier output and the drilling feed rate tracking the resistance to penetration.

At time t.sub.11, a melt build-up is encountered, resulting in the energization of time delay relay 87 of FIG. 3. This in turn, causes maximum output of the transistor amplifier and maximum drilling feed rate for the duration of the time delay of relay 87. At time t.sub.12, relay 87 de-energizes, and normal drilling mode resumes. From the time interval t.sub.12 to t.sub.13, the transistor amplifier output and the drilling feed rate once again track the resistance to penetration, in inverse proportion thereto.

Claims

We claim:

1. A jet piercing drilling machine having automatic drilling feed rate control, comprising, in combination:

a. a frame support member;

b. a frame pivotally attached to the frame support member;

c. a burner assembly;

d. hoist assembly means mounted on the frame for supporting the burner assembly so that the burner assembly normally counterbalances the weight of the frame and hoist assembly about the frame support member;

e. weight cell means associated with the frame and operable to produce control signals indicative of the resistance to penetration of the burner assembly, in response to the pivoting of the frame about the support member;

f. motor means drivably connected to the hoist assembly for raising and lowering the burner assembly into a drill hole;

g. a motor driving circuit having an input for receiving a speed command voltage, and having an output connected to the motor means, the motor driving circuit operable to control the speed and direction of the motor means in response to a received speed command voltage;

h. a transistor amplifier normally connected to receive the control signals from the weight cell means, and connected to the motor driving circuit for supplying a speed command voltage thereto, the transistor amplifier operable in a normal mode to vary the speed command voltage in response to the control signals so as to maintain the drilling feed rate substantially inversely proportional to the instantaneous resistance to penetration of the burner assembly, over a predetermined range of operation; and,

i. means for switching a bias signal into the input of said transistor amplifier when a predetermined drilling condition is encountered, whereby a predetermined fixed drilling speed command voltage is produced in response thereto.

2. Apparatus according to claim 1 wherein said means for switching includes means for sensing a drill tip melt build-up condition, and wherein said bias signal corresponds to a minimum resistance to penetration condition, so that a maximum drilling feed rate is commanded.

3. Apparatus according to claim 2 wherein said means for switching includes means for sensing the encounter of an area of cracked material, and wherein said bias signal corresponds to a maximum resistance to penetration condition, so that a minimum drilling feed rate is thereby commanded.

4. Apparatus according to claim 1 wherein the weight cell means comprises a pneumatic weight cell, a pressure responsive variable resistor in communication with the pneumatic weight cell, and an electrical power supply connected to the pressure responsive variable resistor, whereby the control signals produced by the weight cell means have a voltage proportional to the resistance to penetration of the burner assembly.

5. Apparatus according to claim 4 wherein said means for switching includes means for sensing drill tip melt build-up and a time-delay relay connected for operation by said sensing means, said time-delay relay having contacts for connecting the minimum voltage from said variable resistor to the input circuit of said transistor amplifier.

6. Apparatus according to claim 4 wherein said means for switching includes means for sensing the encountering of a zone of fractured material, and a time-delay relay connected for operation by said sensing means, said time-delay relay connected for operation by said sensing means, said time-delay relay having contacts for applying the maximum voltage for said variable resistor to the input circuit of said transistor amplifier.

7. Apparatus according to claim 1 wherein the transistor amplifier includes a transistor connected in common emitter configuration, and further including a variable resistor in the base circuit of the transistor for adjusting the minimum drilling feed rate.

8. Apparatus according to claim 7 further including a second variable resistor connected across the transistor from collector to emitter for adjusting the maximum drilling feed rate.