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ELECTRIC GOLF CART WITH REGENERATIVE BRAKE OF LOW
CROSS REFERENCE SPEED TO RELATED APPLICATION This application claims priority based on the patent application of 5 US. Serial No. 09 / 736,657 filed on December 14, 2000 with the title "Electric Golf Car with Low Speed Regenerative Braking" and based on the provisional application of US Pat. No. 60 / 173,638, entitled "Electric Golf Cart including regenerative braking to zero" (electric golf cart including regenerative braking 10 to zero) filed on December 30, 1999. BACKGROUND OF THE INVENTION 1. Field of Invention invention relates in general to a control system for an electric golf cart and more particularly to a control system for a car
electric golf that includes a regenerative braking system, which employs a semi-bridge rectifier that provides regenerative braking at a lower speed than the base. 2. Discussion of Related Art All electric motors work on the principle that two electric or 2 fields in proximity will have a tendency to align. One way to produce a magnetic field is to take a coil of wire and pass it a current. If two coils with current passing through them are in proximity to each other, their respective magnetic fields will tend to align. If the two coils are between 0 and 180 ° out of alignment, this tendency will create a torque between the two coils. If one of these coils is mechanically fixed between one arrow and the other is fixed to an outer housing, an electric motor is provided. The torque produced between these coils varies with the current in the coils. Unfortunately, this engine will only spin half a revolution before the fields line up. In this way it is necessary to ensure that there is always an angle between the two coils, in order to continue producing torque as the motor shaft rotates by more than 180 °. A device that provides this function is called a switch. The switch disconnects the current from the active moving coil, referred to as the armature coil and reconnects it to a second armature coil, before the angle between the armature coil and the field coil connected to the housing reaches zero. The ends of each of the armature coils have constant surfaces known as commutator bars. Elaborate charcoal contacts called brushes are attached to the motor housing. As the motor shaft rotates, the brushes lose contact with a set of bars and make contact with the next set of bars. This process maintains the angle between the active armature coil and the field coil, relatively constant. This constant angle between the magnetic fields maintains a constant torque through the rotation of the motor. If a coil moves in a magnetic field, a voltage and current are induced in the coil. If the current passes through the field coil and the armature coil is rotated, a voltage and current is induced in the armature coil, effectively converting the motor into a generator. This has two important effects. When the motor is used to energize an electric vehicle, such as an electric golf cart, referred to as motorized, the rotation of the motor induces a voltage across the armature coil called back electromotive force (EMF = Electromotive Force). This voltage goes up with the speed of the motor, and also with the current field. When the back electromotive force is equal to the voltage or voltage across the motor terminals, the top speed has been reached. The other effect is that if an electric charge is placed on the armature coil, and the armature coil is turned, the motor will act as a brake and generate power. This effect is known as regenerative braking. This is an electric motor where the produced torque varies with the current in the armature and field coils, and the speed varies with the applied armature voltage. Examples of this type of regenerative braking for an electric golf cart can be found in U.S. Patents. No. 5,565,760, assigned to Ball et al.; No. 5,814,958 granted to Journey; No. 5,332,954 granted to Lankin; and No. 4,626,750 granted to Post. The speed of an electric vehicle will vary as the voltage applied to the motor varies. With a lower voltage, the counter electromotive force of the motor reaches the applied voltage at a lower speed. There are two different ways to vary this voltage. The first is to insert resistors in series with the motor, to reduce the effective voltage to the motor. This is the way the industry uses to control engine speed. Unfortunately, this method is extremely inefficient at lower speeds. This inefficiency can be explained by Ohm's law and Kirchoff's current and voltage laws. Ohm's law states that: V (voltage) = I (current) x R (resistance) of which:
P (energy) = I (current) x V (voltage) Kirchoff's law simply states that in a circuit, all voltages must be added to zero, and that all currents must be the same in a given loop. 5 By Kirchoff's current law, the current through the battery, the resistor, the armature coil, and the field coil in an electric vehicle motor circuit must all be the same. Also by the voltage law of Kirchoff, the voltages across the receiver, the armature coil and the field coil must all be added to the battery voltage (36 V in one example), so that the sum of all the voltages of the circuit are equal to zero. Considering that certain driving conditions (grade, surface, tire pressure, load on the vehicle and the desired speed) dictate that a current of 100A to 18V be through the motor (armature coils and field). The torque varies with the current and the speed varies with the voltage. The circuit can be analyzed to determine how much energy is lost in the resistor. By Kirchoff's law, the voltage across the resistor is given as: VBATT = VARM + VF | ELD + VREs - 36 = 18 + VRES. VRES = 18 volts The current is 100A, therefore by Ohm's law, the energy lost 20 in the receiver is given by: PRES = 100 x 18 = 1800 watts. Also by Ohm's law, the energy used by the motor is: P = (VA M + VFIELD) x I.
--- • - "-» - "- P = 18 x 100 = 1800 watts. This means that half of the energy that comes out of the batteries is lost in heat in the resistor. Under these conditions, the speed controller system uses half the energy of the resistor system for the same performance. In a resistor system, the resistance decreases as the pedal position increases. In a speed controller system, the service site increases as the pedal position increases. Both forms effectively control the voltage to the motor and therefore the speed of the vehicle. The difference in efficiency is less noticeable the closer you are to complete strangulation or reduction. While conventional electric vehicles operate on the principles previously established, there are different ways to control it. The standard electric golf cart uses a series wound motor. A series coiled motor has field coils wound with a few turns of very thick wire. In order to obtain maximum torque, the armature and field coils are connected in series. Other electric vehicles use wound motors in derivation, where the field coil has many smaller turns of wire. In order to obtain maximum torque, the armature and field coils are connected in a parallel or "bypass" configuration. The magnetic field strength produced by a coil varies with the current passing through the coil and the number of turns in the coil. Therefore, the same field strength can be provided by passing less current through a shunt field winding. For example, the same field strength at 300A in the series-wound motor can be achieved with 15-20 A in the shunt wound motor. There are a couple of notable differences in the controller equally. Since less current is required to obtain the same field in a shunt-wound motor, it gives the opportunity to control the field coil with a separate set of smaller energy components. This is called for control excited separately from the motor. As discussed above, the counter electromotive force varies with the field strength, which varies with the field current. In a series-wound motor, the armature current and the field current are the same, so the relationship between the field strength and the armature current is a straight line. In a separately excited system, any field current can be selected for a given armature current. As the field current decreases, decreases field strength. In this way, the back electromotive force is reduced, which increases the motor speed for a given armor current. This is called field weakening. If the vehicle starts rolling backwards on a hill while the field current is still active in the forward direction, it will generate reverse current directly on the free-running diode. Since the diode looks like a short circuit in that direction, this will cause the motor to act as a brake at very low speed. This type of braking is called stop braking. If the vehicle has been stopped with the accelerator released for more than a predetermined period of time, the controller will deenergize the field coil and continue to monitor the speed sensor. If the vehicle starts moving without the throttle depressed, the controller will re-energize the field in the opposite direction of the vehicle's movement to start stop braking.
COMPENDIUM OF THE INVENTION In accordance with the teachings of the present invention, a control system for an electric vehicle such as a golf cart is described, which includes a plurality of semiconductor field effect metal oxide transistors (MOSFETs) = Metal-oxide semi-conductor field effect transistors). Four of the MOSFETs constitute a complete H-bridge and two of the MOSFETs constitute a semi-H bridge. One of the MOSFETs in the half-bridge is connected in parallel through the armature coil and the other MOSFET in the semi-bridge is connected in series with the armature coil. The parallel MOSFET through the armature winding includes a free-running diode. When the motor operates below its base speed, ie when the back electromotive force is below the power supply voltage, a current flow can be induced by momentarily turning the parallel MOSFET shorting the armature coil. Due to the inductance in the motor armature, a current flow in the counterclockwise direction starts, at which point the serial MOSFET is turned off. Consequently, a large voltage spike occurs through the motor, which results in current flow back to the battery pack. By rapidly rotating the parallel MOSFET on and off using a technique referred to as pulse width modulation (PWM = Pulse Width Modulation), the current flow can be maintained and regenerative braking can occur even at low speeds. Additionally, control of the negative quadrant of the field map is possible due to the fact that the armature current is monitored. By varying the field current (lf) to correspond to a reference current (lref), the curves of the field map can be maintained. In addition, because the motor speed varies with the armature voltage (VARM), the armature signal PWM can be used to control the speed of the vehicle as opposed to the speed control method by field current variation in previous controllers. Also, through a closed control loop, the speed reference value (determined by the pedal position) is compared to the current speed value returned by the speed sensor. Adjustments are made to the armature signal PWM to vary the vehicle speed, according to a throttle position feed signal. And in order to avoid irregular regenerative braking characteristics during low-speed maneuvering, the speed is monitored and the limit of regenerative armature current (lmax) is reduced at low speeds. Additional objects, advantages and features of the present invention will be apparent from the following description and the appended claims, when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of the electrical system for an electric vehicle, employing a semi-bridge architecture according to the invention; Figure 2 is a regenerative / monitoring field map; Figure 3 is a schematic block diagram showing control of the armature current in the electrical system shown in Figure 1; Figure 4 is a schematic block diagram showing a closed control loop, for determining a speed reference value, based on the accelerator pedal position;
Figure 5 is a schematic block diagram showing the speed of the vehicle being monitored and the armature current that is adjusted with respect to the speed; and Figure 6 illustrates the relationship between maximum armature current IAR and vehicle speed. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES The following discussion of the preferred modalities addressed to a regenerative braking system for an electric golf cart, is simply exemplary in nature, and is in no way intended to limit the invention or its applications or uses. Figure 1 is a schematic diagram 10 of the electrical system for an electric vehicle, such as an electric golf cart, including a regenerative braking system, according to one embodiment of the present invention. Diagram 10 includes a battery system 12, a motor 14 and a control system 16. The typical battery system 12 includes a battery pack 18 that includes similar batteries 20. In one embodiment, the battery pack 18 includes six batteries of 6 volts, for a total of 36 volts, where the batteries 20 are lead-acid batteries of deep discharge. The motor 14 includes a motor armature coil 22 and a field coil 24. The coils 22 and 24 in the motor act as inductors. This means that once the current flows through the coils 22 and 24, the current will tend to continue to circulate. A current sensor 44 is provided in the control system 16, to measure the current through the armature coil 22. The current sensor 44 can be a coil of wire wound around an integrated circuit with Hall effect that effectively causes to the Hall effect circuit inside the magnetic bonina loop, and the magnetic field it detects is proportional to the current
'ARM- The control system 16 includes 6 MOSFETs impellers 26-36 and a capacitor 40. The MOSFETs 26-36 in effect, are switches that can be turned on and off 15,000 times per second. Through this discussion, any mention of "MOSFET" can be considered to mean any MOSFET or a plurality of MOSFETs connected in parallel and connected to a single signal. The MOSFET 34 is connected through the motor armature coil 22, and includes a free-running diode 42. The diode 42 acts as a one-way valve for electricity, where the electricity will only pass in the direction of the Arrow of the symbol. In known quarter-bridge armature circuits, the free-running diode 42 is a separate device. However, in the construction of the energy MOSFET 34, the free-running diode 42 is provided as an intrinsic part of the silicon. The combination of the MOSFETs 26-32 constitutes a bridge H, or a complete bridge, and the MOSFETs 34 and 36 constitute half or half bridge. The semi-bridge controller refers to the semi-bridge architecture used for the motor armature coil 22, instead of the full bridge used for the field coil 24. The driving MOSFETs 16-32 and the bridge construction H are conventional . A unique aspect of the system 10 comes from the use of the two power MOSFETs 34 and 36 associated with the motor armature coil 22, and more particularly with the power MOSFET 34 and the diode 42 in parallel, through the coil of motor armor 22, and the way in which they are driven. In conventional DC motor-driven systems employed in conventional electric vehicles, only the series MOSFET 36 is below the motor armature coil 22 and the free-running diode 42. The motor 14 is a shunt-wound motor, that the armature coil 22 and the field coil 24 are in parallel through the two main battery terminals. This operation can be seen, for example, when the series driving MOSFETs 36 of the motor armature 22 is turned on and the two opposing energy MOSFETs 26 and 32 of the field coil 24 are turned on. In conventional separately driven systems, typically the speed of the motor 14 is achieved by varying the field current. In the present invention, the pulse width modulation (PWM = Pulse Width modulation) of the armature current (RM) are used to control the speed of the vehicle. In this system, the logic circuits in the control system 16 have control over whether the MOSFETs 26-36 are turned on or off. To control the effective voltage to the motor 14, the proportion of the amount of time that the MOSFETs 26-36 are turned on against the amount of time that they are off is controlled. This proportion is called the service cycle. When the MOSFET 36 is on, the batteries 20 are connected directly through the armature 22. By Kirchoff's law, this means that the voltage across the batteries 20 and voltage of the armature 22 are both 36V. Also, by Kirchoff's law, the currents through the armature 22 and the battery 20 are the same. Since the current in the circuit is circulating in the opposite direction of the arrow of the free-running diode 42, the diode 42 can be ignored under this condition.
When the MOSFET 36 is off, the circuit with the batteries 20 is interrupted. Since there is current flowing through the armature 22, it does not want to stop. The free-running diode 42 is the only part of the circuit that remains. If the current continues to circulate at the same speed and in the same direction as it did previously, it will circulate through the free-running diode 42. The voltage across both the diode 42 and the armature 22 will be close to zero, but the current will remain same. In the battery circuit there is no comment since the circuit is not complete, but the battery voltage It remains at 36 volts. The voltage across the armature 22 is 36V when the MOSFET 36 is on and zero when the MOSFET 36 is turned off. The average voltage across motor 14 is given as: VARM = (36V x duty cycle) + (OV x (1-duty cycle)) VARM = (36V per duty cycle) The torque required to move the Vehicle determines the motor current. Since the battery current is equal to the motor current when the MOSFET 36 is on and is zero when the MOSFET 36 is turned off, the battery current is: IBATT = (IARM x service cycle) + (OA x (1-duty cycle)). IBATT = IARM service cycle. Using the example of 100A and 18V above with a solid state speed controller, the differences are given as: VARM = 36 X Service Cycle 18 = 36 per Service Cycle. Service Cycle = 50% IBATT = IARM per Service Cycle IBATT = 100A x 50 IBATT = 50A. At this point, the energy leaving the batteries and the energy to the armature 22 can be calculated by the energy formula previously discussed. From the previous example, it is known that the motor power under these conditions is 1800 watts. PBATT = IBATT VBATT PBATT = 50A x 36 V = 1800 watts. Table I below summarizes the differences under these conditions for each of the two systems. Table I
The normal operation of the motor armature coil 22 in a regenerative braking mode, for example when the vehicle moves downhill, exists only when the speed of the motor 14 is above the base speed of the motor 14. The base speed is speed at which the counter electromotive force equals the battery voltage. When the back electromotive force is equal to or is below the battery voltage, it is usually not possible for the battery to be generated by the motor 14 to circulate back to the battery pack 18, because the battery voltage is very high. In order to overcome this limitation, the power MOSFET 34 in combination with the free running diode 42 are added. If the vehicle speed is below its base speed, this will be inefficient to charge the battery pack for previously established reasons, and there would be no regenerative braking. To overcome this limitation, the MOSFET 34 is turned on briefly. Since the voltage drop across the MOSFET 34 when it is fully on, is approximately zero, the motor 14 generates a large current, which runs very briefly through the MOSFET 34. Since this short current can not be stopped instantaneously, it is generated a large voltage that exceeds the normal counter electromotive force, which is above the battery voltage. The armature coil of the motor 22 then pumps energy to the battery pack 18 for a short period of time equal to the period of time required for the high voltage of the motor 14 to go from its peak to the battery voltage. This process is repeated hundreds or thousands of times per second, in order to cause the energy of the motor 14 to produce sufficient high voltage to generate a regenerative braking effect through the battery pack 18 and thus braking the motor 14. It is It is important to note that when turning on the power MOSFET 34, MOSFET 34 has the effect of applying hard braking to motor 14. This is a normal way to brake a CD motor, but it would be objectionable if it is maintained for a long time. Therefore, it is important to extend this braking action in one or more ways. However, the nature of the power MOSFET 34 already requires that it be turned on or off. In this way, in order to produce an extended effect, it is necessary to vary the service cycle PWM of the reinforcement, to achieve extended. Table II below gives the values for different PWM service cycles for VBATT,
IBATT, 'ARM and VARM- Table II
The only time the armature voltage is reversed is in normal motorized mode when the 36-series power MOSFET is turned off. For that brief instant, the current continues to circulate and the armature voltage reverses and pushes the current through the free-running diode 42. For conventional electric golf carts, the base speed of the
engine 14 allows regenerative braking to occur at vehicle speeds over 16 kilometers per hour (10 miles per hour). Under 10 miles per hour, there is no regenerative braking. Also, regenerative braking is used to limit the speed of the vehicle downhill, even with the accelerator pedal fully depressed at 20.8 kilometers per hour (13 miles per hour) for a driving feel
more secure.
"One important advantage of the present invention is that it allows the accelerator pedal to be used to brake the vehicle at speeds as low as 3.2 kilometers per hour (2 miles per hour). In the past, known drivers only limited the speed of the vehicle when the pedal has been fully released. In other words, if the accelerator pedal were to be depressed even slightly, this would allow the vehicle speed to reach the maximum engine speed and the regenerative braking would automatically contribute only on top of that maximum speed. This may be undesirable on particularly steep slopes. In this invention, the accelerator pedal works effectively in the same way, in terms of feeling, like a vehicle with an internal combustion engine that has a manual transmission, where the accelerator pedal is used to directly regulate the low speed of the vehicle , to produce a relatively linear feeling between the position of the pedal and the speed. In other words, when using the accelerator pedal for this invention, the speed can be regulated to 12.8 kilometers per hour (8 miles per hour), 9.6 kilometers per hour (6 miles per hour), 8 kilometers per hour (5 miles per hour) ) or at any point where the operator chooses to put the pedal. The top speed of the vehicle is a parameter to adjust in conventional controllers. This is the speed that the vehicle will get at ground level. When a vehicle reaches the summit on a hill and begins to descend, the typical conventional driver adjusts to maintain that higher speed as the vehicle goes down the hill even though gravity would cause the vehicle to go faster. To summarize, in conventional impulse systems, pushing the pedal simply allows the vehicle to go from the base speed to the maximum speed, but does not give any control below the base speed of the engine. However, in the system of the present invention, the position of the pedal allows the vehicle speed to be controlled at any point just above almost zero speed up to the maximum speed that is established by the control system 16 of the engine 14. This it can be a linear relationship between the position of the pedal and the speed of the vehicle. However, it will be appreciated that this can also be a non-linear relationship if desired, for example to provide greater sensitivity at lower speeds. The counter electromotive force of the motor 14 increases with the speed of the motor and also with an increase in the current of the field coil. In the series-wound motor, the motor armature coil 22 and the field coil 24 are connected in series, thereby ensuring that there is no independent control between the armature current of the motor and the field coil current, which they always have to be the same. In shunt wound motors, the motor armature coil 22 and the field coil 24 are completely separated, so that the currents in these two circuits can be regulated separately. Accordingly, in order to reduce the counter electromotive force of the motor, it is simply necessary to reduce the field coil current. This allows the engine controller designer, more flexibility to achieve the desired engine characteristics. Figure 2 is a typical field map. The field map is a term widely used in the invention to describe the relationship between the field current and armature for an electric motor wound on bypass. Each engine has a specific version in its field map in which its performance is optimal. A far operation outside this region can result in permanent damage to the engine. Each engine has a specific version in its field map where its performance is optimal. An operation far removed from this region can result in permanent damage to the engine. Line 46 in the upper right quadrant of the field map represents the typical performance of the series coiled motor with field current which is a function of the independently controlled armature current. In bypass winding motor systems, the field current and the armature current are independently controllable, thus resulting in a non-linear curve 56, as also illustrated in the upper right quadrant of FIG. 2. Particular field in this way, allows better performance under conditions that can be programmed in conventional controllers. Nevertheless, the conventional controllers using the quarter-bridge armature circuit, with the free-running diode 42, do not provide any control in the upper left quadrant of the field map. The present invention however provides a technique for operating in the upper left quadrant of the field map on line 58. In the known bypass wound motor system, with the bridge quarter controller, the armature current was currently measured due to the known resistance value of a power MOSFET 34. In other words, the armature current is measured by measuring the voltage across the power MOSFET 34. In the present system, the current sensor 44 is capable of measuring the current of armor IARM in the negative quadrant of the field map. Then, according to the IARM value that is detected, the controller chooses the correct value lf of the graph. In the past, this reinforcement current information was simply not available, thus making it impossible to regulate other variables based on this information. Figure 3 is a re-feeding loop diagram 48 that includes a sum joint 50 having an IRES field current reference feed, and produces the feed signal in a field PWM block 42, which then produces a field commentary modulated in pulse width (PWM). The field current modulated in pulse width is then applied to a current source 54 that generates the field current lf. The current lf is then fed back to the sum joint 50. The PWM field current currently ends up controlling an average voltage and this may or may not achieve the desired field current. The problem with the field coil is that it is subject to changes in temperature that change its resistance. In this way, for a given voltage, a different field current may result. By using the re-feeding loop of Figure 3, the desired field current can be obtained independent of temperature consideration. In addition, the field coil 24 operates in a real environment where there may be magnetic fields induced by the armature or whatever. The re-feeding system of Figure 3 also handles these types of auxiliary factors that would otherwise tend to destabilize the desired signal lf. One of the benefits of regulating the negative quadrant operation of the field map of the motor 14 is that the motor 14 can be operated more reliably under all conditions. Typically, shunt-wound DC motors operate under certain conditions or ranges of lf versus IARM. Outside that range, there is the possibility of damage to the motor 14 if the operation in this long-term condition is carried out. Accordingly, the reliability and serviceability of the engine 14 can be increased by ensuring that in all instances, the IAR recommendations are observed, including those that exist in the downhill regenerative conditions or other unusual environments where the engine 14 it may be outside the normal desired IARM operating range. It should be noted that this aspect of the safe regenerative operating area really only comes into play with large vehicles, such as golf carts for 5 people. This is because typically, the smaller two-person golf carts will not be operated in the undesirable lf / lARM region for a long time, to have an adverse effect on the engine 14. This is a possibility in sufficiently mountainous fields, where Larger vehicles tend to strain the engine to a greater degree. Figure 4 is a block diagram 60 showing the control of vehicle speed when using the armor PWM signal. An accelerator pedal position signal 62 is a speed reference signal that is fed to a summing junction 64. The current speed 66 of the vehicle is also detected and fed to the summing junction 64. The comparison of the two signals provides a re-feed signal which is then placed in an armature PWM block 68 as a power signal. This produces an appropriate output PWM signal that is fed to the series MOSFET 36 in the motor armature coil 22, which then regulates the speed of the vehicle. More generally, the armor block PWM 68 operates both the series MOSFET 36 and the MOSFET 34 in parallel, with the armature coil 22.
In order to achieve a desired motor speed, the voltage applied across the motor armature coil 22 must be regulated. This voltage control across the motor armature terminals is determined by operation of both the power MOSFETs 34-36 for the armature coil 22. They can be operated in any desired way to achieve the desired motor voltage. Given the current vehicle speed that determines the speed of the motor 14, and thus the back-electromotive force voltage, the desired voltage to be applied across the motor terminals can be calculated and cause the power MOSFETs 24 and 36 to operate properly. to reach additional voltage to increase the speed of the motor. By taking the terminal armature voltage of the motor below the back electromotive force, regenerative braking will be induced. In this way, the voltage applied to the armature coil of the motor 22 can be adjusted in any desired way through the left or right field quadrants of the field map. The term "loose pedal braking" is now used with respect to motor control systems with conventional bypass winding. Basically, when the vehicle accelerator pedal is released, the control system 16 will actively implement a regenerative braking situation all the way to the base speed of the engine 14, typically 16 kilometers per hour. Beyond that, there is no active control, since the regenerative braking feature of the motor can not provide enough back electromotive force to exceed the battery voltage. In this way, there is no regenerative braking. Since the present invention now provides regenerative braking below the base engine speed 14, the term "loose pedal braking" can be employed at a speed much lower than the base speed of the engine 14. If the control system permits a ratio linear between maximum M and speed during slow speed regenerative braking, the speed of vehicle 5 will brake very quickly when the pedal is released. Basically, this is similar in feel when driving with the parking brake on. Fundamentally, as long as the speed of the vehicle is above the speed indicated by the position of the pedal, the vehicle will normally advance without a motor. In the improved system, if active regenerative braking is left on, that regenerative braking in fact
will brake the vehicle so quickly that this would be objectionable. In such a way that this particular feature inhibits regenerative braking with low speed below a certain fixed point of low speed that is safe to allow the vehicle to simply advance without a motor. This fixed point can be anywhere from 3.2 to 12.8 kilometers per hour (2 to 8 miles per hour) probably with 8 kilometers per
hours (5 miles per hour) that are preferred for mountainous terrain. Figures 5 and 6 show an example of how the armor current IARM can be controlled between some minimum value and some maximum value, as a function of the speed. The maximum armature current typically occurs during the regeneration mode, where the current limit is designed to
2 or protect the control system and switch 16 from overheating. A speed sensor system 32 is shown including a sum joint 74, which receives a speed sensor signal as a power and a current signal from a current sensing circuit 76 representing the current sector 44. The
£ ^ g | Minimum armature current can be zero, or it can be some value that does not produce excessive braking at low speeds. From the foregoing discussion, the control system for the electric vehicle of the invention offers advantages over those known in the art. These advantages include regenerative braking at a lower speed than the base. When the engine 14 operates below its base speed, ie when the back electromotive force is below the power supply voltage, a current flow can be induced by momentarily turning on the MOSFET 34, by shorting the armature coil 22. Due to the residual inductance in the motor 14, a Current flow in the counterclockwise direction starts, at which point the MOSFET 34 is turned off. Consequently, a large voltage spike occurs through the motor 14, which results in current circulating back to the battery pack 18. By quickly turning the MOSFET 34 on and off, the current flow can be maintained, and regenerative braking can occur even at low speeds. Additionally, control of the negative quadrant of the field map is possible due to the fact that the armature current is monitored. By varying lf to correspond to lREF, the field map curves can be maintained. In addition, because the motor speed varies with armature voltages (VARM), the armature PWM can be used to control the vehicle speed as opposed to the speed control method by field current variation in previous controllers. Also, through a closed control loop, the speed reference value (determined by the pedal position) is compared to the current speed value returned by the speed sensor. Adjustments are made to the armor PWM to vary the vehicle speed according to a reduction or throttled position feed signal. And, in order to avoid regenerative braking characteristics with jerking during low speed maneuver, the speed is verified and the regenerative reinforcement limit (IMAX) is reduced at low speeds. The above discussion describes and illustrates simply exemplary embodiments of the invention. A person skilled in the art will readily recognize from this discussion and the accompanying drawings and claims that various changes, modifications and variations may be made without departing from the spirit and scope of the invention, as defined in the following claims.