Dynamic Braking System For Electric Drive

Abstract

A system for dynamically braking an electrically driven vehicle wherein a retarding means is directly coupled to the engine/generator drive and employed to absorb the major portion of the energy produced during power regeneration. Running at near rated engine speed, the retarding means generates substantially constant braking torque during vehicle deceleration and associated control systems automatically operate the retarding means in response to command signals and employ circuits to maintain the excitation of generator below that of the wheel motors during retarding whereby better power transfer is effected.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a dynamic braking system which is uniquely adapted for use on large vehicles that are driven electrically through electrical motors driving their wheels or tracks. In small vehicles wherein the wheels are directly geared to the engine, controlling the speed during power regeneration is normally accomplished by a combination of engine braking and service brakes. While this combination suffices for small, gear driven vehicles, larger vehicles having high gross weights require some means of dynamically braking the vehicle in addition to normal engine braking and service brakes even when a gear drive is employed. Thus, in large vehicles with electric drives where there is no direct mechanical couple between wheels of the vehicle and the engine, the problems become compounded due to a partial loss of engine braking.

Many prior art systems have been suggested for supplying dynamic braking for electrically driven vehicles including the use of hydraulic or electric retarding devices which are mechanically connected in the wheel drives of the vehicle. In some electrically driven vehicles the drive motors are used as generators during dynamically braking and the power generated by the generator is dissipated in a grid of resistors or similar devices, but these systems require expensive high power switching devices. In the unique case of electrically driven vehicles which are powered from external power lines or sources, the overall system can be used as an electric power absorbing means and the electrical power generated by the drive motors during power regeneration can be merely returned to the general power distribution system. This latter arrangement, however, is not suitable in the case of a self-powered work vehicle having an engine driving a large generator electrically connected to wheel drive motors.

While several systems for dynamically braking electrically driven vehicles are known as indicated, they all have problems which have rendered them unsatisfactory, especially for work or earthmoving vehicles having electric drives which are continually decelerated and accelerated. For example, suitable control is severe in the case of earthmoving loaders which are usually required to stop at least four times and then accelerate for each bucket load transported. When hydraulic retarders are employed in low speed operations, it poses a problem in that the torque generated by the retarder is directly related to vehicle ground speed because they are connected to the wheel drive train. Thus, as the drive vehicle slows the dynamic braking provided by the retarder will proportionally decrease which is serious during low speed operations because of high energy dissipation required of the service brakes. Electrical retarding devices have this same characteristic and in large vehicles pose an additional complication because of their large physical size. Further, electric retarders in large sizes are relatively expensive and as indicated like the hydraulic one tend to lose effectiveness as the speed of the vehicle slows.

SUMMARY OF THE INVENTION

The present invention solves many of the above problems by providing a novel dynamic braking system for a vehicle utilizing an electric drive. More particularly, in an electric drive where the prime mover of the vehicle drives the generator at a constant speed so the electrical power output to the wheel drive motors is controlled by excitation of the field of the generator, a hydraulic retarding device is directly coupled to the prime mover and generator drive so that it is driven at the same constant speed which arrangement makes the speed of the retarder independent of the vehicle speed at all times allowing it to generate a substantially constant braking torque during power regeneration regardless of the vehicle speed. Control circuitry maintains the generator voltage below that of the wheel drive motors during power regeneration to insure improved operation at lower vehicle speeds.

Retarder control for the vehicle is provided with two levels of operation, one being based on engine/generator speed and the other being a general throttle position or other operator-initiated condition in which the retarder is armed or partially charged with hydraulic fluid. Thus, as the operator releases the power pedal the retarder will be placed in a semioperating condition. As energy is taken from the vehicle and transferred to the generator, the speed of the generator and retarder will increase thus causing the hydraulic retarder to activate and prevent engine overspeed. The retarder serves to convert generator shaft horsepower (braking) into heat in the retarder cooling system.

The use of the above arrangement provides several advantages over previous retarding systems, such as quicker retarder response since the retarder is always operating in a very efficient speed range. In addition, running the drive at constant speed ensures better cooling of the engine and its related auxiliary equipment. Finally, the use of a constant retarder speed provides a constant dynamic braking torque which in turn provides a faster response in a system which is independent of the vehicle speed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more easily understood from the following detailed description of a preferred embodiment when taken in conjunction with the attached drawings in which:

FIG. 1 is a block diagram arrangement of the mechanical drive components of the present invention;

FIG. 2 is a block diagram showing the electrical arrangement and the electrical control system;

FIG. 3 is a vertical section of the hydraulic retarding device shown in FIG. 1;

FIG. 4 is a flow diagram showing the hydraulic controls for the retarder;

FIG. 5 is a graph illustrating the relationship between the vehicle speed and the generator voltage during retarding;

FIG. 6 graphically shows the relationship between the vehicle speed and the retarding torque when the system is employed;

FIG. 7 illustrates the relationship between the engine r.p.m. and horsepower; and,

FIG. 8 graphically shows the relationship between the power pedal position and available motoring or retarding horsepower for pedal position.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, the overall arrangement of the mechanical drive of the vehicle is illustrated and while no particular vehicle type is shown, the drive system is adapted to any type of electric drive vehicle. The system is particularly suitable for use in an electrically driven earthmoving vehicle such as a large loader.

Both auxiliary and wheel drive power is provided by an engine or prime mover 10 which may be either a conventional internal combustion engine or a gas turbine. As explained above the engine is operated at a constant speed preferably by means of a governor system (not shown). The engine governor may include separate positions so that the engine can be idled or set to run at rated speed. Directly coupled to the engine output shaft is a generator 11 which supplies power over leads 13 to the wheel drive or traction motors (not shown in FIG. 1) of the vehicle. Connected in the drive train with the engine and the generator is a retarder 12 which is preferably a hydraulic retarder, such as shown in detail in FIG. 3 and it should be appreciated that the retarder may be located any place in the engine/generator drive train so long as it is directly mechanically coupled thereto. The engine also drives two auxiliary alternators 14 and 15 which supply electrical power for various electrical systems, as well as electrical power for exciting the field of the main generator 11 and the main drive motors.

The drive train through the generator shaft is also illustrated coupled to the input of a drive box 20 which contains a series of auxiliary shafts for driving auxiliary equipment on the vehicle. More particularly, the auxiliary equipment illustrated consists of two hydraulic pumps 22 and 23. The hydraulic pumps are a steering pump and a retarder charging pump. This drive box is also connected to the input shaft 24 of a second drive box 25. As shown in FIG. 1 the second drive box is arranged for location on an articulated portion of a vehicle, although all of this auxiliary equipment if desired could be mounted on one unit. The second drive box 25 drives three hydraulic pumps 26, 27 and 28, designated as an implement pump No. 1 and implement pump No. 2 and a brake pump. The implement pumps, of course, supply hydraulic fluid for operating the various earthmoving implements on the vehicle while the brake pump supplies fluid for operating the service brakes on the vehicle.

Referring now to FIG. 2 the electrical and hydraulic controls for the dynamic braking system of the present invention are shown. The engine 10 drives the retarder 12 and the generator 11 while the alternators 14 and 15, shown separated from the engine, are driven by the engine as explained above. Normally, the engine is operated at a constant r.p.m., and more particularly, at rated r.p.m. which is at or near a full throttle position. In this arrangement, the engine governor has no mechanical or electrical connection with the remainder of the control system described herein. The actual vehicle speed and motoring condition is controlled by varying the excitation of the generator field and the fields of the drive motors, as is explained more fully below, and also through the employment of the novel retarder system.

The excitation of the generator field is controlled by a control circuit 31 that is connected to rectify the three phase power supplied by the alternator 14 and control the excitation level of the generator field in response to control signals. This alternator is controlled by a regulator 16 (shown in FIG. 2) which regulates its output on a constant volts-per-cycle basis. For example, the alternator may be regulated to supply 400 cycle three-phase alternating current power to the field control 31. The field control circuit 31 also contains signal conditioning and switching circuitry and is responsive to various conditions such as engine speed, the selected vehicle speed, as indicated by the position of the power pedal, and the generator and motor current. More particularly, the field control circuit 31 is supplied with a signal representing engine speed by means of a tachometer generator 41 on lead 40, a signal representing the generator current flow by means of the measuring device 35 on lead 34 and a signal representing the motor current flow by means of a measuring device 37 on lead 36. The generator and motor current measuring devices may be any well known device which is capable of measuring current flow in the generator and motor armature circuits, respectively. The field control circuit 31 is also supplied with a reference voltage via lead 32 from a battery 33.

A signal from the power pedal 44 over a lead 42 is also received by the generator field control circuit for supplying the operator's selected speed in the control loop. In addition, a separate switching signal is supplied from a switch 45 on the power pedal over lead 46 to the field control circuit and over lead 50 to the retarder control circuits 54 and via lead 51 to the motor field control circuits 53. This switching signal is used to condition the controls for either a motoring mode or a retarding mode, including those of the motor control circuits for either a motoring or a retarding mode of operation. More particularly, in the motoring mode the field control circuit 31 excites the field of the generator to supply power to the drive motors while in the retarding mode it limits generator field excitation sufficiently below the level of motor field excitation so that the generator will act as a motor. Of course, the reverse control relationships are provided by the motor field control.

The power pedal 44 is provided with two potentiometers 47 and 48 and a single wiper 43. More particularly, the potentiometer 47 acts to control the motoring mode of operation and supplies a signal to the field control circuit indicating the desired vehicle speed. Similarly, the potentiometer 48 supplies a signal to the field control circuit indicating the desired amount of dynamic braking.

A retarder control 54 includes both electric and hydraulic controls to control the initial filling or arming of the retarder, as well as the actuation of the retarder. The hydraulic fluid for filling and operating the retarder is supplied by the pump 23 shown in FIG. 1. A retarder control receives a signal from an engine speed reference 55 which is preferably a frequency signal related to the engine speed. This signal can be easily provided by using the frequency of the alternator 14 since it is directly related to the engine speed or a tach generator input as illustrated. This retarder control also includes frequency sensitive means as, for example, frequency sensitive relays 144 and 148, one of which operates at a first predetermined engine speed (i.e., relay 144 could operate at 2,120 r.p.m.) to actuate the retarder 12 and a second relay which operates at a second higher preselected speed representative of maximum safe engine speed (i.e. relay 148 could operate at 2,250 r.p.m.) to disconnect and inactivate both the generator and the motor field controls so that serious damage will not occur. In this condition the vehicle must be stopped by service brakes or other means. Disconnecting circuits for the generator and motor field controls 31 and 53 are provided to prevent damage to either the engine or generator due to an overspeeding condition since normally the engine governor will maintain its speed. Disconnect signals from the relay are supplied over lead 52 to the generator and motor controls, respectively.

The motor field control 53 is similar to the generator field control circuit 31 and utilizes rectifying circuits for rectifying the AC power supplied by the alternator 15. More particularly, the alternator 15 is controlled by a regulator 17. Normally, this alternator will supply 400 cycle three phase power to the motor field control 53 over leads 60.

A reference voltage from a battery 62 over the lead 61 is also received by the motor field control which is summed with a signal related to the output voltage of the generator 11 over lead 63 from a sensing unit 64.

Two drive motors 70 and 71 are shown although additional parallel-connected motors could be used. In the case of a four wheel drive vehicle such as an earthmoving loader, the two motors would drive differential units of the several axles which in turn drive wheels mounted thereon. The armatures of the two motors 70 and 71 are coupled in parallel across to leads 13 from the generator 11 while the field windings 72 and 73 are coupled in parallel across the lead 74 from the motor field control 53.

Referring now to FIG. 3, a hydraulic retarder is shown which is suitable for use with this invention. Other similar types of hydraulic retarders could be employed as well. In the illustrated arrangement shaft 80 of the prime mover is coupled to a flywheel 81 which in turn is coupled to the generator shaft 83 by means of a flexible coupling 82. This flexible coupling may be any of several conventional coupling designs.

Mounted on a spline portion of the generator shaft is a rotor 84 of the retarder which is disposed in the housing 85. The housing is securely attached to the main frame structure 86 of the prime mover. Hydraulic fluid is introduced to the retarder through the inlet opening 90 and flows into an annular space 91 surrounding the blades on the outer periphery of the rotor and through a series of axial holes 92 which are provided near the center of the rotor to provide fluid communication to the opposite side of the rotor. Hydraulic fluid is exhausted through an opening in the outer periphery of the housing not shown in FIG. 3. A more complete description of the construction and operation of the hydraulic retarder may be found in U.S. Pat. No. 3,352,385 issued to Johnson.

Referring now to FIG. 4 a detailed schematic diagram of the retarder electric-hydraulic control 54 is shown. More particularly, the hydraulic controls employ a retarder control valve 100 for controlling the supply of hydraulic fluid to the retarder and a two-stage reducing valve 101 which controls the operation of the retarder. Hydraulic fluid from the pump 23 is supplied to several solenoid operated valves 104 and 105 by a supply line 103, and both these valves are connected by supply lines 106 and 110 respectively to the two-stage reducing valve 101. A pump by a drain line 107 is provided for one valve and a similar drain line 111 is provided for the other.

On the output side of the hydraulic pump is a reducing valve 112 which limits the maximum pressure supplied to the retarder control valve 100 via line 113 to a somewhat lower pressure than supplied to valves 104 and 105. The retarder control valve 100 is connected to a heat exchanger or a cooler 115 by means of a line 114 while the outlet of the cooler is connected to circulate the hydraulic fluid back to the control valve via line 116. This control valve supplies hydraulic fluid to the retarder 12 through line 120 which returns to the valve through a line 121. The two-stage reducing valve 101 is connected by a line 122 to the left-hand end of the control valve 100, as illustrated and operates to sequence the retarder control valve.

The reducing valve includes a lower piston or valve spool 123 and an upper piston or valve spool 126 both of which are positioned in its bore 124. A compression spring 125 is positioned between the two valve spools to control their positioning and an outwardly projecting flange 127 on the upper valve spool 126 limits its downward travel by contacting in a shoulder formed on the body of the valve. In this construction spool 123, when pressurized through line 106, will move toward spring 125 until a modulated pressure output of a predetermined value occurs in line 122 due to hydraulic fluid entering chamber 123a. For example, a modulated pressure of 20 p.s.i. may be employed. However, if pressure thereafter occurs in line 110 it will compress the spring 125 causing maximum system pressure to occur in line 122, due to increased spring pressure on spool 123. This valve is somewhat conventional and provides for initial partial filling (arming) of the retarder 12 and subsequent full actuation.

A spool 130 in the retarder control valve 100 is controlled by pressures in line 122 and initially acts as a differential valve partially filling the retarder 12 with hydraulic fluid when line 122 has approximately 20 p.s.i. therein. In the partially filled state the retarder will be absorbing approximately 100 horsepower. If thereafter, pressure in line 122 is increased, the retarder will be filled as spool 130 is shifted by the increased pressure and will circulate hydraulic fluid through heat exchanger 115. An identical retarder control valve is described in U.S. Pat. No. 3,386,540 issued to Horsch and further discussion is not warranted of the valve itself.

The two valves 104 and 105 are supplied with electrical power from a battery 140 over leads 145, 146 and 147. A normally closed speed relay 144 is disposed in leads 145 and 146 and is controlled by the engine speed reference 55, as described below. Another normally closed fill relay 143 is disposed in the lead 147 to control the operation of the valve 104. Relay 143 is opened to cut off power circulation to valve 104 when the power pedal wiper 43 is in the decelerating mode through a signal via lead 50. It may also be opened by an auxiliary, operator-initiated signal means via lead 50'; an example of this mode of control would be appropriate switching means (not shown) which provides a signal via lead 50' whenever a directional transmission shift is made.

OPERATION OF A PREFERRED EMBODIMENT

The operation of the above-described dynamic braking system can be more easily understood by referring to FIGS. 2 and 4 and the operating curves shown in FIGS. 5--7. Operation of the circuit of the control system will be explained by considering the motoring mode followed by the retarding or braking mode. Considering the motoring mode, it is assumed that the governor control is in a run (rated r.p.m.) position so that the electrical controls are energized and the engine is operating on the straight line portion of the power curve shown in FIG. 7. In this condition, an illustrative engine may have a no load engine speed of 2,080 r.p.m. which will then decrease as the horsepower increases along the straight line portion of the horsepower curve to a speed of approximately 1900 at which point the engine will develop its maximum rated horsepower. When the engine governor is in the run position and depressing the power pedal will move the slider 43 along the potentiometer 47 increasing its negative output, the amplitude of the signal generated at the wiper 43 will then be related to the desired power and thus vehicle speed. This signal is fed to the generator field control circuit 31 where it is summed with the engine speed signal as supplied by the tachometer 41. Since the engine is operating on the straight portion of its horsepower curve, its actual speed will be linearly related to its actual developed horsepower which, in turn, is related to the desired power indicated by the position of the power pedal.

The generator field control circuit responds to the two signals by supplying the proper rectified excitation level from the alternator 14 to the generator field 30. This generator field excitation level will cause the generator to supply the necessary voltage output over the lines 13 to the armature circuit of the two drive motors 70 and 71. The generator output voltage which is sensed by sensing device 64 is also used as a feedback to the motor field control 53. The motor field control will compare the generator voltage with a fixed reference voltage 61 to control the full wave rectifier unit in the motor field control for a proper excitation level for the drive motor fields.

If during the motoring operation the power pedal is depressed still more, the excitation level of the generator field and drive motor fields will be controlled and as the power demand is increased, the engine will slow further but will remain on the straight line portion of its power curve. Since the engine speed changes from 2,080 r.p.m. at no load to 1,900 r.p.m. at full load, the above system provides for extremely fast response from no load to full load operation by converting some of the inertia to power. Further, since the generator field control circuit 31 compares the actual engine operating speed which is related to the power developed with the power setting of the power pedal 44 and controls the generator field excitation in response thereto, auxiliary mechanical loads placed on the engine will have priority over the generator load. For example, if a hydraulic implement places a heavy load on the engine and the engine tends to slow, the generator field control circuit will reduce the field excitation so as to reduce the load placed on the engine by the generator to compensate for this auxiliary load. This results from the fact that a particular setting of the power pedal is related to engine speed and the generator control will attempt to adjust the excitation of the generator field to obtain this engine speed.

In normal operation if the vehicle tends to overspeed, the wheels will increase the speed of wheel drive motors so they act as generators. When the wheel drive motors act as generators, they will reverse the flow of power and supply power to the generator 11 causing it to act as a motor and drive the engine. As the generator attempts to drive the engine above its maximum governed speed of 2,080 r.p.m., the governor reduces the fuel flow to the engine and compression braking, along with braking from the electrical losses in the generator and the friction losses in the engine will absorb some of the energy from power regeneration. If this amount of braking is sufficient to permit the engine to maintain a speed below the first predetermined speed of 2,120 r.p.m., the system will tend to stabilize. If, however, the braking requirement is such that the engine speed reference 55 indicates a speed above 2,120 r.p.m., a signal to the retarder control system 54 will activate it.

The retarder electrical controls consist of three relays 143, 144, and 148 as shown in FIG. 4. Relays 144 and 148 are frequency sensitive so that they operate at a frequency representing engine speeds of approximately 2,120 and 2,250 r.p.m. Relay 143 operates as a function of pedal position or other anticipatory, operator-initiated signal means as mentioned above. These relays are usually of the type that open their associated contacts when their operating frequency is exceeded for failsafe reasons. When the relay 143 opens positive battery voltage is interrupted to the valve 104 causing valve 104 to open, which will initiate partial filling of the retarder with hydraulic fluid, as previously indicated, thereby consuming approximately 100 horsepower as a result of valve spool 123 throttling the hydraulic fluid supplied to supply line 122 connected to the retarder control valve. Pressure of the hydraulic fluid acting on the retarder control valve will be sufficient to move the valve spool 130 to the right so that the edge 132 of one of the projecting lands on the valve spool will uncover the inwardly projecting land 133 on the valve body. This will permit hydraulic fluid to flow into the retarder and partially fill it. Normally, the flow of hydraulic fluid will be sufficient to fill the retarder to approximately 80 percent of its maximum capacity as a result of the first stepwise sequence. This partial filling of the retarder will create the additional loading indicated and thus additional braking on the vehicle. If this additional braking load is sufficient to maintain the engine below said first predetermined speed of 2,120 r.p.m., the system will tend to stabilize. If the operator again depresses the power pedal, relay 143 will be reenergized and relay contacts closed to reenergize the valve 104 thus permitting the hydraulic fluid to drain from the left hand of the piston 130 and return the system to its normal operating condition as the retarder is drained of hydraulic fluid.

If the above retarding is no sufficient frequency sensitive relay 144 will open its contacts at 2,120 r.p.m. which will not only cut off battery voltage in lead 146 and open the valve 105 to the hydraulic supply, but also cut off battery voltage to lead 147 and open valve 104. When the valve 105 is opened it will supply hydraulic fluid through the line 110 to the top of the two-stage reducing valve which will force the valve spool 126 downwardly and cause the lower valve spool 123 to travel towards the bottom of the valve. This in turn will permit the full pressure of the hydraulic fluid to be supplied over the line 122 from valve 104 to the left-hand end of the retarder control valve which will cause the retarder to completely fill from supply line 120. As a result the retarder will be charged to 100 percent capacity at the pressure set by the relief valve 112 and the drain line 121 of the retarder will be coupled by the control valve to the inlet 114 of the oil cooler 115. Similarly, the outlet 116 of the oil cooler will be connected to the drain lines at the left-hand end of the control valve so that the hydraulic fluid will drain back to the sump of the pump 23. Thus, the hydraulic retarder 12 will be placed in full operation to absorb the maximum horsepower in a typical case up to 1,200 horsepower. If this is sufficient to hold the engine speed below a second or maximum allowable speed of 2,250 r.p.m., the vehicle will be continuously dynamically braked to a speed at which the drive motors no longer generate sufficient power to drive the generator. Once the retarder is partially full which happens whenever the power pedal is released or a directional change is made, it can provide maximum power absorption in one-half second so the engine cannot overspeed appreciably.

Under typical conditions, the retarder is partially filled as the operator releases the power pedal which causes the slider 43 to move upwardly from the potentiometer 47 to potentiometer 48. This will adjust the generator and motor field controls so drive motors will operate as generators to supply power to the generator, which will then operate as a motor. The switching of the drive motors to generator operation and the generator to motor operation will be provided by the switch 45 that is also operated by the power pedal.

If the above-described dynamic braking consisting of the retarder, engine compression braking, engine friction losses and electrical losses are insufficient to control the speed of the vehicle, the retarder electrical control 54 at the second predetermined engine speed will send a shutdown signal over lines 52 to the generator field control 31 and the motor field control 53, respectively. A warning device such as a light or bell would advise the vehicle operator of this condition. In response to the shutdown signal the generator and motor field controls will cut the excitation from the field of the drive motors and the generator. This will permit the vehicle to freewheel and the vehicle service brakes then must be used to slow and stop the vehicle--but the fully loaded retarder still serves to slow the power plant. When the power plant speed drops below the second predetermined speed by approximately 75 r.p.m. (to allow a deadband) the safety relay disconnects the shutdown signal in lines 52 and excitation of the fields for electrical braking is resumed.

When the vehicle is operating on normal terrain and the operator desires to slow the vehicle to perform a different operation, he releases some of the pressure on the power pedal 44, so the wiper 43 will shift from the accelerating potentiometer 47 to the decelerating or braking potentiometer 48. As the transfer of the wiper takes place from one potentiometer to the second, it also sends a signal to the accelerator switch 45 to the retarder hydraulic controls. This signal will cause fill relay 143 in the lead 147 to open. This in turn will actuate the valve 104 and initially charge the retarder to 80 percent of capacity as described above. The switching signal from the switch 45 is also supplied by a lead to the generator field control circuit 31 and to the motor field control 53. This switching signal will cause the generator engine speed reference from the tachometer to be removed as a reference in the generator field control circuit and adjust the generator field excitation relative to the retarding signal from the potentiometer 48 and the motor current signal 34. The generator field control circuit will then control the generator so that it operates as a motor to absorb the power produced by the drive motors. The switching signal supplied to the motor field control will remove the fixed reference from the control and replace it with a second fixed reference which will be matched with the generator voltage signal 63. This will have the effect of causing the motor field control to reduce the motor field current versus the generator volts to permit the drive motors to act as generators down to a very low vehicle speed since the generator excitation will be reduced relative to that of the motors, to allow constant reverse current flow to the generator, depending on the signal from the retarding potentiometer.

The vehicle will then transfer regenerative power from its drive train to the drive motor and causing them to act as generators and power from the drive motors will drive the main generator as a motor. The retarding rate will be controlled by the position of the wiper along potentiometer 48 although in all cases the generator 11 will be driven as a motor while the drive motors will act as generators and the engine and retarder 12 will absorb the regenerative power, as necessary.

As described above under motoring conditions if this reverse power flow plus initial charging of the retarder is sufficient to control the speed of the vehicle, the system will stabilize. If this amount of power absorption is not sufficient the retarder hydraulic controls will then fully activate at approximately an engine speed of 2,120 r.p.m. At this engine speed speed relay 144 is energized and signal in leads 146 and 147 cut off. This will have the effect of placing the retarder in full operation, as described above, making maximum braking available. If the full retarding is sufficient to maintain the engine speed below its maximum allowable speed of 2,250 r.p.m., the system will stabilize. As explained above, if the engine speed tends to increase beyond this maximum speed, the engine speed reference 55 will supply a shutdown signal to line 52 through safety relay 148.

Referring to FIG. 5 there is shown a relationship between vehicle speed and the generator voltage during retarding operation. As indicated, the voltage will remain relatively constant until the vehicle reaches approximately one-half speed at which time the generator voltage will then proportionally decrease to 0 relative to speed.

Shown in FIG. 6 is the relationship between the retarding torque and the vehicle speed during retarding. As indicated between maximum speed and one-half speed, the retarding torque will increase from a minimum level, while from one-half speed to substantially 0 the retarding torque will remain relatively constant regardless of the speed of the vehicle as long as the excitation levels of the generator and wheel drive motor are adjusted to achieve constant reverse current flow. FIG. 8 shows the relationship between the power pedal position and the horsepower developed, and as indicated, at mid position neither retarding nor motoring horsepower is demanded. In a fully depressed position the maximum generator output will be developed while in the fully released position the maximum retarding horsepower will be absorbed.

While the above-described described embodiment has been with relation to an earthmoving vehicle, obviously the invention can be applied to any vehicle using an electric drive system. Also, it would be possible to adapt the retarder arrangement to other types of drives as, for example, hydraulic drives, wherein the drive motors can act as energy generators in order to feed power back to the normal energy generating source. Also while a particular control system has been described, obviously other types of control systems can be utilized. The important feature of this invention is the use of a constant torque retarding means in combination with a control system which operates to switch the retarder from a normal condition to a full operating condition in a relatively short time. Further, the switching is accomplished in response either to the actual vehicle speed or command of the vehicle operator, and in addition, the excitation of levels of the generator and motor fields are adjusted to achieve substantially constant reverse current flow from the wheel drive motors to the generator down to almost zero vehicle speed in the retarding mode. This is an advantageous feature when compared to conventional full-electric dynamic brake systems where reverse current drops substantially and braking torque reduces proportionally with decreasing vehicle speed.

Claims

What we claim is:

1. In combination with an electric vehicle drive having a prime mover, an electric generator, a mechanical drive connecting the prime mover with the electric generator, electric motors directly connected to drive ground engaging members of the vehicle and leads coupling the electrical output of the generator to the motors, a dynamic control system comprising

a mechanical retarder connected to the mechanical drive connecting the engine to the electric generator,

a retarder control connected to said mechanical retarder, said retarder control having a transducer connected to said drive to monitor its speed which provides an output signal to said retarder control proportional thereto, and

control circuit means coupled to said electric generator and said electric motors operable to cause said motors to convert to generators and said electric generator to convert to a motor when power regeneration occurs whereby energy motorizing said generator will be absorbed by said mechanical retarder means when said mechanical drive overspeeds.

2. The combination as defined in claim 1 wherein the prime mover drive is a constant speed drive design.

3. The combination defined in claim 2 wherein the retarder control means is provided with a control having at least two separate modes of operation, one mode being responsive to the output signal provided by the transducer, and the second mode being disposed to control the actuation of said retarder control means independent of said output signal.

4. The combination defined in claim 2 wherein the retarder control has at least two separate control outputs, one of said control outputs being coupled to place said retarding means in a partial operating state the other of said control outputs being coupled to said retarding control means to place said retarding means in a full operating state.

5. The combination defined in claim 4 wherein the two separate control outputs are responsive to a signal in the control circuit means when power regeneration occurs and said other control output is responsive to the output signal of the transducer.

6. The combination defined in claim 4 wherein said retarder is a hydraulic retarder and one control output from the retarder control initially charges said hydraulic retarder with fluid for partial operation and said other control output supplies fluid to said retarder for full operation.

7. The combination defined in claim 6 wherein the control circuit means includes a generator control means and a motor control means, said generator control being operably coupled to said generator to cause said generator to operate as a generator to supply power and as a motor when power regeneration occurs, said motor control being operably coupled to said motors to cause said motors to operate as motors when said vehicle is being driven and as a generator when power regeneration occurs.

8. The combination defined in claim 7 wherein said generator and motor controls are disposed to control the excitation level of fields of said generator and motors respectively.

9. The combination defined in claim 8 wherein the control circuits include a power pedal control, coupled to the generator control, said power pedal control having two positions, one position being a run position, the other a braking position which provides a signal to the retarder control means to place the retarding means in a partial operating state, said power pedal also including a switch means for switching said generator control from a first mode where it controls the generator as a generator to a second mode where it controls said generator as a motor by controlling its field excitation level relative to the field excitation level of the motors.

10. The combination defined in claim 9 wherein an override means responsive to a maximum mechanical drive speed disables the retarder, circuit means and excitation of the generator and motors.