Hydraulic-electric Control System For Hydraulic Motor

Abstract

A hydraulic motor is driven by hydraulic fluid supplied by a hydraulic pump. The pump has a servo-stem control thereon which regulates fluid flow to the motor and thereby controls motor speed and direction of rotation. The servo-stem control is operated by a movable spring-centered piston of a hydraulic actuator. The actuator piston is movable in response to fluid flow conditions in a rotary servo-system which comprises a hydraulic sensing motor (driven by the hydraulic motor) and a control pump which supplies fluid to the sensing motor. The direction, speed and degree of rotation of the control pump regulates the direction, speed and degree of rotation of the hydraulic motor. The control pump is driven by an electric stepping motor and the latter is operated by electric control means. The electric control means comprise variable frequency pulse generating means to establish stepping motor speed and pulse counting means to establish the extent of stepping motor rotation. The electric control means also includes selector switches for presetting a predetermined direction, speed and degree of rotation for the stepping motor. The hydraulic motor is used to drive a movable bridge member of an overhead crane.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to hydraulic-electric control systems for hydraulic motors. More specifically, it relates to such control systems wherein a hydraulic rotary servo-system, including a control pump, controls direction, speed, and degree of rotation of the hydraulic motor and wherein direction, speed and degree of rotation of the control pump is regulated by an electric stepping motor under the remote control of electronic control means.

2. Description of the Prior Art

Some large overhead cranes, for example, comprise components such as a bridge horizontally movable on rails, a carriage or trolley movable on the bridge, and a rotatable hoist mounted on the trolley. Each component is driven by a separate hydraulic motor which is rotatable in either direction at some desired rate of speed for a desired length of time. Each hydraulic motor is operated by a hydraulic control system in response to control commands given by a human operator stationed in a crane cab or station mounted on the crane bridge. Hydraulic control systems of this character are relatively complex and require a great many hydraulic fluid control lines to be connected between the several crane motors and the operator's cab, thereby adding to the cost and complexity of the crane as a whole. Furthermore, in operating such cranes, it was necessary for the crane operator to actively direct and continuously monitor the direction, speed and extent of movement of each crane component when moving components to desired locations with respect to a load. This demanded the full attention of the crane operator and was unnecessarily fatiguing. For example, movement of any crane component required the crane operator to manually move and hold a control lever in a certain control position and simultaneously observe component movement until the desired movement was completed.

It is desirable, therefore, to provide improved hydraulic-electric control systems for hydraulic motors, especially such as are used on overhead cranes, which enable the crane operator to preselect the direction, speed and distance the crane components are to be moved by the hydraulic motor and thereby relieve him of the necessity of continuously directing and monitoring such movements.

SUMMARY OF THE INVENTION

A hydraulic-electric control system for a hydraulic motor in accordance with the invention is especially well adapted for use with hydraulic motors used on overhead cranes to drive crane components such as the bridge, trolley and hoist, but is also suitable for other applications.

The control system contemplates a main hydraulic motor which is driven by a variable volume main hydraulic pump having a servo-stem speed and direction control thereon. Direction, volume and duration of fluid flow from the main pump to the main motor determine direction, speed and duration of rotation of the main motor. These factors are controlled by the position of a spring-centered piston in a hydraulic actuator which is connected to the servo-stem control on the main pump.

A rotary servo-system comprising a small hydraulic sensing motor, a small hydraulic control pump and the actuator determine movement of the servo-stem control on the main pump. The sensing motor is responsive to operation of the main motor (i.e., is driven by it) and is supplied with fluid from the control pump. The sensing motor is a fixed volume variable speed unit as is the control pump, and the actuator is responsive to pressure differentials in the servo-system. Therefore, the direction, speed and degree of rotation of the control pump determine operation of the actuator and thereby control the direction, speed and degree of rotation of the main motor. The servo-system also maintains the speed of the main motor constant at whatever speed is chosen.

U. S. Pat. Application Ser. No. 195,093 filed Nov. 3, 1971 for "Speed Control System for Hydraulic Motor" by Richard O. Gordon and assigned to the same assignee as the present application discloses a hydraulic control system similar to that described above.

In accordance with the invention, the control pump is operated by an electric stepping motor which is remotely controlled by electric control means. In practice, the stepping motor (and therefore the control pump) is rotatable in a desired direction at a desired rate of speed for a desired number of degrees.

The electric control means comprises a presettable variable frequency pulse generating means which establishes stepping motor speed, a presettable pulse counter means for establishing the amount or extent of stepping motor rotation, and selector means for establishing the direction of rotation of the stepping motor. The electric control means further comprises selector switches, for use by the operator to preselect direction, speed, and duration of rotation of the main hydraulic motor.

In the embodiment disclosed, a hydraulic-electric control system in accordance with the invention is used to control the hydraulic motor for the bridge component of an overhead crane. But it is to be understood that the control system could be applied to the hydraulic motors which effect operation of other crane components, such as the trolley motor and the hoist motor.

DRAWINGS

FIG. 1 is a schematic diagram of a hydraulic-electric control system in accordance with the invention;

FIG. 2 is an elevational view of an overhead crane in which the control system of FIG. 1 is embodied to effect control of the bridge motor thereof;

FIG. 3 is a top plan view of portions of the crane shown in FIG. 2; and

FIG. 4 is a circuit diagram of the electric control means for the system shown in FIGS. 1, 2 and 3.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 is a schematic diagram of a hydraulic-electric control system in accordance with the invention.

The control system contemplates a high torque low speed piston type hydraulic motor 10 which is driven by a variable volume piston type main hydraulic pump 12 having a servo-stem control 18, including a control member 18a, thereon. Pump 12 is driven by a constant speed electric motor 13. Direction, volume and duration of fluid flow from main pump 12 is controlled by a spring-centered hydraulic actuator 14 which has a movable piston rod 16 connected to servo-stem control member 18a. A small hydraulic piston type fixed volume sensing motor 20, responsive to operation of (i.e., driven by) main motor 10, is supplied with control fluid from a small hydraulic fixed volume variable speed control pump 22, Actuator 14 is responsive to fluid flow conditions between sensing motor 20 and control pump 22. Sensing motor 20, control pump 22 and actuator 14 cooperate to serve as a rotary servo-system. The direction, speed and degree of rotation of control pump 22 determines operation (i.e., piston movement) of actuator 14 and thereby controls direction, speed and degree of rotation of main motor 10 through main pump 12. The servo-system also maintains the speed of main motor 10 constant at whatever speed is chosen.

The control pump 22 is operated by an electric stepping motor 24 which is remotely controlled by electric control means 25. In practice, stepping motor 24 (and therefore control pump 22) is rotatable in steps of predetermined size in a desired direction at a desired rate of speed for a desired number of steps.

The electric control means 25 comprises a power supply 26, a presettable variable frequency pulse generating means 28 which establishes stepping motor speed, a presettable pulse counter means 30 for establishing the amount or extent of stepping motor rotation, and a translator means for adapting the pulses for use by stepping motor 24. The electric control means 25 further comprise remote control selector or input devices, such as a speed selector switch 34, a distance selector switch 36, and a direction selector switch 38, for use by the operator to preselect or establish speed, extent and direction of rotation of stepping motor 24.

As FIGS. 2 and 3 show, overhead crane 40 on which the hydraulic-electric control system is used comprises a bridge component 42 having wheels 47 and 48 (driven by series connected hydraulic bridge motors 10 and 11, respectively) which adapt the bridge for movement in either direction on rails 50 and 51, respectively. Brake means 66 and 68 are provided for the bridge motors 10 and 11, respectively. Crane 40 further comprises a wheeled carriage or trolley 58 movable on bridge 42 by a hydraulic motor 44. A hoist 56 is mounted on trolley 58 and comprises a hydraulic motor 46, a hoist drum 60 rotatable thereby, and a hoist line 62 for a load 64. A crane operator's control station or cab 52 is suspended from bridge 42 by rigid supports 54 and various crane controls are located thereon.

The hydraulic-electric control system is disclosed herein as associated with main hydraulic bridge motor 10 (and series connected bridge motor 11) but it could also be applied to trolley motor 44 and hoist motor 46.

As FIGS. 2 and 3 further show, motors 10 and 11 are supplied with hydraulic fluid by main pump 12 which is connected to a hydraulic fluid reservoir 117. A port 100 of main pump 12 is connected by a fluid line 102 to a port 103 of bridge motor 10. Another port 104 of bridge motor 10 is connected by a fluid line 105 to a port 106 of series-connected bridge motor 11. Another port 107 of motor 11 is connected by a fluid line 108 to a port 109 of main pump 12. Fluid lines 102, 105 and 108 are part of the main hydraulic circuit. The direction, rate and duration of fluid flow from main pump 12 to the motors 10 and 11 determine their direction, speed and duration of rotation.

The brake means 66 and 68 for motors 10 and 11, respectively, each comprise parking brakes in the form of fail-safe caliper type brakes which are spring-applied to a brake disc 110 and released by pressurization of a plurality of brake cylinders 111 in response to operation of a manually controlled parking brake valve 112 on cab 52. Furthermore, the brake means 66 and 68 each comprise working brakes which are spring-released from brake disc 110 and pressure-applied in response to pressurization of a plurality of brake cylinders 113 in response to operation of a pedal controlled working brake valve 114 on cab 52. A hydraulic pump 115, which is driven by an electric motor 116 and connected to reservoir 117 through a fluid line 118, supplies brake valves 112 and 114 with fluid through a fluid line 119. Valve 112 is connected to cylinders 111 by fluid lines 120 and 121. Valve 114 is connected to cylinders 113 by fluid lines 122 and 123.

Direction and volume of fluid flow from main pump 12 (i.e., direction and speed of motors 10 and 11) is controlled by the direction and degree of movement of servo-stem control member 18a of servo-stem control 18 on pump 12. Control member 18a, in turn, is connected to and operated by movement of piston rod 16 of actuator 14. Actuator 14 comprises a cylinder 124 wherein piston rod 16 and a piston 17 are maintained centered by the action of a compression spring 125. When piston 17 is centered, no fluid flow exists at either port 100 or 109 of pump 12 and motors 10 and 11 are stationary. Cylinder 124 of actuator 14 comprises a chamber 126 (and a port 127) on one side of piston 17 and another chamber 128 (and a port 129) on the other side of the piston. Pressurization of either chamber through its respective port, as hereinafter described, effects appropriate movement of piston 17 and piston rod 16 against the centering bias of spring 125 and causes corresponding movement of servo-stem control member 18a. This, in turn, causes appropriate operation of motors 10 and 11. The ports 127 and 128 of cylinder 124 of actuator 14 are connected by fluid lines 130 and 131, respectively, to fluid lines 132 and 133, respectively. Fluid line 132 is connected between port 134 on sensing motor 20 and port 135 on control pump 22. Fluid line 133 is connected between port 136 on sensing motor 20 and port 137 on control pump 22.

Sensing motor 20 (which is driven by main motor 10), control pump 22, actuator 14 and their interconnecting fluid lines cooperate to provide a rotary servo-system whereby direction, speed and duration of rotation of motors 10 and 11 are controlled. This servo-system, furthermore, maintains the chosen speed of motors 10 and 11 constant, despite variations in load thereon.

A pair of series connected check valves 140 and 141, poled as shown in FIGS. 2 and 3, are provided between fluid line 132 and port 109 of main pump 12. A pair of series connected check valves 142 and 143, poled as shown in FIGS. 2 and 3, are provided between fluid line 133 and port 100 of main pump 12. A point in the line between each pair of check valves is connected by a fluid line 145 to a booster pump 146 driven by electric motor 13. The check valves serve as an anticavitation circuit and also as a means whereby booster pump 146 can maintain fluid pressure constant in both the main hydraulic circuit (i.e., between main pump 12 and motors 10 and 11) and the rotary servo-system comprising sensing motor 20, control pump 22, and actuator 14. By means of this arrangement, fluid leakage which normally occurs from the hydraulic motors and pumps when they are maintained stationary under load is compensated for. Consequently, no slippage or creepage of the motors or pumps can occur.

The rotary servo-system operates as follows. With main pump 12 in operation but with servo-stem control member 18a in neutral and piston 17 of actuator 14 in spring-centered position, motors 10 and 11 and sensing motor 20 are stationary. Upon rotation of control pump 22 in a desired direction at a desired rate of speed for a desired angular distance (effected by stepping motor 24 in response to control means 25, as hereinafter explained), fluid pressure begins to build up in one or the other of the fluid lines 132 or 133. The pressure differential between fluid lines 132 and 133 effects pressurization of one or the other of the ports 127 and 129 of actuator 14 and results in proportional movement in an appropriate direction of piston 17 and servo-stem control 18a on main pump 12. This, in turn, effects rotation of motors 10 and 11. Rotation of motor 10 causes corresponding rotation of sensing motor 20 and, as sensing motor speed increases, a reduction in the pressure differential between fluid lines 132 and 133. When the speed of sensing motor 20 matches that of control pump 22 (i.e., when both are handling the same volume of fluid) the pressure differential between lines 132 and 133 becomes such that the speed of motors 10 and 11 is maintained constant at whatever speed was chosen. When rotation of control pump 22 stops, motors 10 and 11 also stop. Direction of rotation of control pump 22 determines direction of movement of piston 17 of actuator 14 and direction of rotation of the motors 10 and 11.

Stepping motor 24 for driving control pump 22, shown in FIGS. 1, 2, 3 and 4, has the following characteristics. The stepping motor is a digital motor wherein each shaft revolution is divided into a plurality of discrete identical steps or increments of rotation. Each step of rotation is triggered by a single pulse from control means 25 and is on the order of 10.degree. per step, i.e., 36 steps for one revolution. A preferred stepping motor for use in accordance with the present invention has high torque, power and speed. Furthermore, it has a travel time per step on the order of 4 milliseconds and is operable in forward or reverse direction. A stepping motor suitable for use in accordance with the invention is a 3 phase variable reluctance 36-step stepping motor designated Model SM-036-0030- AB and manufactured by the Warner Electric Brake & Clutch Company, Beloit, Wis.

FIGS. 1 and 4 show that electric control means 25 for stepping motor 24 comprises power supply 26, presettable variable frequency pulse generating means 28, presettable pulse counter means 30, translator means 32, speed selector switch 34, distance selector switch 36 and direction selector switch 38.

Power supply 26 comprises a connector 300 for connection, for example, to a 110 volt a.c. power source, a fuse 301, and an on-off switch 302 (shown open). A step-down transformer 303 has its primary connectable across the 110 volt source by switch 302 and provides 6.3 volts a.c. to the input terminals of a voltage rectifier 304 which provides a 6.3 volt d.c. supply at its output terminals to a voltage regulator which comprises a transistor 305, a diode 306, a capacitor 307, a resistor 308 and a capacitor 309. The regulator provides a 5 volt regulated d.c. supply between its output terminal 310 and ground terminal 311 for driving solid state circuit 28.

Pulse generating means or square wave generator circuit 28 functions as a crystal controlled oscillator circuit driving a shaper circuit which feeds a series of digital dividers to provide a series of variable frequency square waves or pulses. Circuit 28 comprises a crystal 312 of desired frequency response connected in parallel with a variable capacitor 313 and a resistor 314 across terminals G and S of a field effect transistor 315. These components cooperate to serve as an oscillator circuit for the desired frequency, i.e., 1,000 kHz. The output of the oscillator circuit appears at a junction point 316 between a resistor 317 and an oscillator or choke coil 318 which are series connected between 5 volt d.c. terminal 310 of the power supply and terminal D of field effect transistor 315. The output at junction point 316 is provided to the base terminal 319b of a transistor 319 which has its emitter-collector circuit connected across d.c. power supply terminal 310 and ground in series with a resistor 320. Transistor 319 serves as a shaper which provides a square-edged signal for driving a diode-transistor logic (DTL) frequency divider chain comprising twelve series-connected Motorola type MC853P dual-JK flip-flop type integrated circuit components designated 321a through 321l. The integrated circuit components 321a through 321l are arranged and connected as shown in FIG. 4, so as to divide in a series of 2 and 5. The output of each device 321a through 321l is fed to one of the terminals 1 through 13, respectively, of a 13-position rotary switch 34 which serves as the speed selector switch for control means 25. As FIG. 2 shows, switch 34 is mounted on cab 52 of crane 40. Speed selector switch 34 comprises a rotatable switch leaf 34a which is movable to a desired one of any of its thirteen terminals to couple the output signal from any one of the dividers (321a through 321l) through an output buffer to an output terminal 322. Each terminal of switch 34, therefore, provides a series of pulses of different frequency. The output buffer comprises a transistor 323 which has its base 323a connected through a resistor 324 and a capacitor 325 in parallel therewith to switch leaf 34a. The emitter-collector circuit of transistor 323 is connected between 5 volt d.c. output terminal 310 of power supply 26 and ground. Resistor 326 and 327 across the emitter-collector circuit of transistor 323 provide an output termination of some desired resistance value for output terminal 322 which matches the impedance of the circuit connection to terminal 322.

The square wave signals or pulses provided at output terminal 322 appear at a rate or frequency determined by the setting of speed selector switch 34. Thirteen selectable times are available in one and five steps of from one millisecond to one second. A pulse generating means suitable for use in accordance with the invention is fully disclosed at page 33 et seq. in the Jan. 1971 issue of Popular Electronics magazine published by the Ziff-Davis Publishing Company.

Output terminal 322 of pulse generating means 28 is connected by a conductor 330, preferably a shielded coaxial cable, to an input terminal 331 counter means 30 which is a predetermining or presettable pulse counter device which can provide up to 1,500 counts per minute, i.e., 25 counts per second, of input pulses fed thereto. Counter means 30 is adjustable to allow a predetermined number of pulses to pass therethrough whereupon it cuts off. Counter means 30 comprises a count coil 332 which is series connected with distance selector switch 36 across signal input terminal 331 and ground. Counter means 30 further comprises a reset coil 333 which is series connected with a reset switch 334 across signal input terminal 331 and ground. A cut-off switch 335 (shown closed in FIG. 4) is connected in series circuit between signal input terminal 331 and a signal output terminal 336 of counter means 30. In operation, reset switch 334 is held closed while a predetermined number of counts are supplied to counter means 30 by distance selector switch 36. In practice, distance selector switch 36 is a digital display counting switch which presets counter means 30 to pass a predetermined number of pulses. Switch 36 serves, therefore, as a distance selector which establishes a certain number of steps of rotation of stepping motor 24. When switch 36 is preset, it effects closure of cut-off switch 335 and counter means 30 is then ready to accept and pass only a predetermined number of counts before cut-off switch 335 opens to disconnect the circuit between signal input terminal 331 and output terminal 336. As FIG. 2 shows, distance selector switch 36 is located on cab 52 of crane 40. A counter means 30 and selector switch 36 suitable for use in accordance with the invention, for example, is a Series 7441 Electric Predetermining Counter manufactured by the Veeder-Root Company and disclosed in that company's Form 231143 2553.

The output pulses from counter means 30 are not in proper form for use directly by stepping motor 24. Therefore, output terminal 336 of pulse counter means 30 is connected by a conductor 337 to a pulse or step input signal terminal 338 of translator means 32 which converts serial pulses into properly sequenced drive currents for 3-phase variable reluctance stepping motor 24. Translator means 32 is understood to comprise integrated circuits and associated outboard components which are arranged in a section generally designated 339 and have connection points designated by the numerals 1 through 22. Translator means 32 further comprises a logic circuit power supply 340 for section 339 and a power supply 341 for stepping motor 24. Power supply 340 is connected to terminals 1 and 9 of section 339. Power supply 341 is connected to terminals 10 and 11 of section 339 and to a return terminal 342 of motor 24. Phase terminals A, B and C of motor 24 are connected to terminals 13-14, 16-17 and 19-20 of section 339. Terminal 342 of motor 24 is also connected to terminals 12, 15 and 18 of section 339. Direction selector switch 38 (mounted on cab 52 of crane 40 as shown in FIG. 2) is connected to direction input terminals 3 and 6 of section 339 of translator means 32. An input signal from selector switch 38 to input terminal 3 effects counterclockwise rotation of motor 24 and an input signal from selector switch 38 to input terminal 6 effects clockwise rotation of the motor. A translator circuit such as 32 is available in the form of a printed circuit board designated as translator card MCS-1807 from Warner Electric Brake & Clutch Company, Beloit, Wis.

OPERATION

The hydraulic-electric control system for controlling the direction, speed and distance traveled by bridge 42 of crane 40 operates as follows:

Assume that pump 12 is being rotated by motor 13 but that servo-stem control 16 is in neutral position and that motors 10 and 11 are at rest. Further assume that brake means 66 and 68 are released and that bridge 42 is free to be moved in either direction along rails 50 and 51. Also assume that sensing motor 20, control pump 22 and stepping motor 24 are at rest and that the electric control means 25 are energized in readiness for operation but that the direction, speed and distance selector switches 38, 34 and 36, respectively, on cab 52 are at their zero or off positions.

Now assume that the crane operator on cab 52 desires to move bridge 42 forward for a predetermined distance at a predetermined rate of speed. To do so, the operator sets speed selector switch 34 to a desired one of 13 speed positions so that pulses at a frequency corresponding to the speed selected appear at output terminal 332 of pulse generating means 28 and at input terminal 331 of pulse counting means 30. The operator then sets distance selector switch 36 for the distance to be traveled by bridge 42. The operator then sets direction selector switch 38 in "forward" position to energize clockwise rotation terminal 6 of section 339 of translator means 32 and cause operation of the control system and bridge movement.

It may be assumed, for example, that one 10.degree. step of rotation of stepping motor 24 and control pump 22 equals one inch of distance on the circumference of the wheels 47 and 48 of bridge 42. Thus, if the counter of distance selector switch 36 is set for 360 steps of 10.degree. each, and if speed selector switch 34 provides pulses at a rate of one pulse per second, the rotary servo-system will cause the bridge wheels 47 and 48 to rotate at 1 inch per second at their peripheries. Thus, for example, at a predetermined wheel size, bridge 42 moves at the rate of 5 feet per minute for a distance of 30 feet. When the counter of distance selector switch 36 finishes its count down from its 360 step setting to zero, bridge movement stops. If the counter was set for 360 steps and speed selector switch 34 was set at a rate of 10 steps per second, bridge 42 would move at a rate of 600 feet per minute.

As is apparent, after the crane operator makes his initial selections of speed, distance and direction, no further effort is required on his part to effect bridge movement because movement is carried out automatically by the rotary servo-system.

As previously explained, the rotary servo-system operates as follows. With main pump 12 in operation but with servo-stem control member 18a in neutral and piston 17 in actuator 14 in spring-centered position, motors 10 and 11 and sensing motor 20 are stationary. Upon rotation of control pump 22 in "forward" (clockwise) direction at a desired rate of speed for a desired angular distance by "forward" (clockwise) rotation of stepping motor 24 in response to control means 25, fluid pressure begins to build up in one of the fluid lines 132 or 133. The pressure differential between fluid lines 132 and 133 effects pressurization of one of the ports 127 and 129 of actuator 14 and results in proportional movement in the "forward" direction of piston 17 and servo-stem control 18a on main pump 12. This, in turn, effects "forward" rotation of motors 10 and 11. Rotation of motor 10 causes corresponding "forward" rotation of sensing motor 20 and, as sensing motor speed increases, a reduction in the pressure differential between fluid lines 132 and 133. When the speed of sensing motor 20 in the forward direction matches that of control pump 22 (i.e., when both are handling the same volume of fluid) the pressure differential between lines 132 and 133 becomes such that the speed of motors 10 and 11 is maintained constant at whatever speed was chosen. When "forward" rotation of control pump 22 stops, motors 10 and 11 also stop. The servo-stem also holds the motors 10 and 11 in power position, even if the braking means 66 and 68 are not applied. The two small hydraulic accumulators 147 and 148 (shown in FIG. 3), one for each direction of rotation of control pump 22, store the pulses of hydraulic fluid in the servo-system and control acceleration of the control system so that no pulse is missed or ignored if stepping motor 24 and control pump 22 are accelerated rapidly.

As is apparent from the foregoing, if direction selector switch 38 is placed in "reverse" position, reverse movement of bridge 42 is carried out by the control system at a speed and for a distance selected by the crane operator.

Claims

I claim:

1. In a control system for a main hydraulic motor which is adapted to be supplied with hydraulic fluid from a main hydraulic pump and is adapted to move a component in at least one direction at some predetermined rate of speed for some predetermined distance;

hydraulic control means, including a fixed volume variable speed hydraulic control pump, for controlling the rate of fluid flow from said main motor to thereby regulate the speed of said main motor and said component,

said hydraulic control means further controlling the duration of fluid flow from said main pump to said main motor to thereby regulate the distance said main motor moves said component;

an electric stepping motor for rotating said control pump at some predetermined rate of speed for some predetermined angular distance to thereby regulate the speed and distance of movement of said component, respectively,

and electric control means for controlling operation of said stepping motor to effect rotation thereof at a predetermined rate of speed for a predetermined angular distance.

2. A control system according to claim 1 wherein said electric control means comprises first means, including a speed selector switch, for effecting rotation of said stepping motor at a predetermined rate of speed, and further comprises second means, including a distance selector switch, for effecting rotation of said stepping motor for a predetermined angular distance.

3. A control system according to claim 2 wherein said first means comprises variable frequency pulse generating means for generating a series of pulses and wherein said second means comprises counter means for providing a predetermined number of said pulses to said stepping motor.

4. A control system according to claim 3 wherein said main hydraulic motor is rotatable in either of two directions; wherein said component is movable in either of two directions; wherein said control pump and said stepping motor are both rotatable in either of two directions; and wherein said electric control means comprises third means, including a direction selector switch, for effecting rotation of said stepping motor in a predetermined direction.

5. In combination:

a main hydraulic motor adapted to move a component in either of two directions at a predetermined rate of speed for a predetermined distance;

a main hydraulic pump for supplying hydraulic fluid to said main motor;

said main pump having a servo-stem control means thereon which is movable to control the direction, rate and duration of fluid flow to said main motor to thereby regulate the direction, rate, and distance of movement of said component;

a hydraulic actuator for operating said servo-stem control means;

a hydraulic speed sensing motor driven by said main motor;

a fixed volume variable speed hydraulic control pump for supplying hydraulic fluid to said speed sensing motor through a pair of hydraulic fluid lines; said hydraulic actuator being connected to said lines and responsive to fluid pressure conditions therein to operate said servo-stem control means on said main pump;

an electric stepping motor for rotating said control pump in a predetermined direction at some predetermined rate of speed for some predetermined angular distance to thereby regulate the direction, speed and distance of movement of said component, respectively;

and electric control means for controlling operation of said stepping motor to effect rotation thereof in a predetermined direction at a predetermined rate of speed for a predetermined angular distance.

6. A combination according to claim 5 wherein said electric control means comprises first means, including a speed selector switch, for effecting rotation of said stepping motor at a predetermined rate of speed; second means, including a distance selector switch, for effecting rotation of said stepping motor through a predetermined angular distance; and third means, including a direction selector switch, for effecting rotation of said stepping motor in a predetermined direction.

7. A combination according to claim 6 wherein said first means comprises variable frequency pulse generating means for generating a series of pulses and wherein said second means comprises counter means for providing a predetermined number of said pulses to said stepping motor.