DE4133722A1 - METHOD FOR CONTROLLING ELECTRIC MOTORS WITH PERMANENT MAGNETIC EXCITATION - Google Patents

Abstract

The present invention proposes a process for controlling electric motors with independent excitation and pulse-length modulated drive in which the efficiency of the motor on parking is relatively high and the point of the deflection manoeuver lies within the operative range of the electric motor. This means that the maximum efficiency and maximum output power coincide as closely as possible. The motor is so laid out that the "parking" operating point lies at the maximum efficiency at constant input power. The constant input power from the motor vehicle's own power system ensures that the motor voltage may be varied at will so that the motor characteristic is no longer a straight line. This provides the facility for uniting the maximum power and efficiency.

Description

The present invention relates to a method for Control of electric motors with permanent magnetic excitation. In particular, a method is presented in which the Interpretation of the engine parameters not as usual for a constant Motor voltage takes place, but for a constant Input power.

Electric motors with permanent magnetic excitation often used for steering in motor vehicles. In particular come almost exclusively when used in motor vehicles permanently magnetically excited DC or three-phase motors to use. These three-phase motors are usually by a power electronics with corresponding Controlled semiconductor devices. Because the semiconductor devices are destructive to overcurrents, it is necessary, current limiting devices to protect the Semiconductor components in the circuit of power electronics to install.

Such current limiting systems are in the DE-A1 33 17 834. The control is in this system the power consumption of a separately excited DC motor using an accelerator pedal that is proportional gives a current control signal to its position to the  Target size of the torque of the externally excited electric motor with Using a microcomputer to determine.

The microcomputer control system monitors the actual motor current and responds to the target motor current signal to determine the actual Set the motor current to the setpoint as long as the setpoint does not exceed a current limit.

Such current limiting measures essentially influence the output power (P _ab ) and the efficiency (η) of the electric motor. In general, it is therefore necessary to coordinate these sizes of the electric motor so that they represent an optimal technical solution for the corresponding application.

In motor vehicles, the on-board voltage is usually 12 Volt. Electric motors with greater output, as for Steering systems are required, therefore, have to handle high currents be charged. Not only does this cause Heat dissipation problems, but the electrical system is under Under certain circumstances also impermissibly charged.

For reasons of the dynamic requirements of such an electric motor, only externally excited direct current or three-phase motors are possible for use in the motor vehicle, whose speed-torque characteristic curve is linear, as is shown in curve 1 of FIG. 1. The maximum power of curve 2 at point a is half the maximum possible moment M, the efficiency (curve 3 ) there being less than 50%. At maximum efficiency at point b of curve 3 , the moment is only a fraction of the possible moment.

One of the main operations of the steering system in the motor vehicle is when parking, so that the electric motor must easily meet the requirements placed on it. To do this, it must deliver a relatively high torque M at a high speed n ( FIG. 2, point a). On the other hand, another operating point is required, which comes into play during evasive maneuvers. Here the required torque is somewhat smaller, but the speed is significantly higher ( Fig. 2, point b). The entire area in which the engine is operated is thus determined ( Fig. 2, area A). The parking process is carried out much more frequently than an evasive maneuver. Only small moments and speeds are required during the journey. The design is usually such that the speed-torque straight line goes through the two operating points a and b ( Fig. 2, curve 4 ). Point a is then close to the maximum output, i.e. less than 50% efficiency. However, this requires very large battery currents, which can place an unacceptably high load on the vehicle electrical system.

The design for maximum efficiency during the parking process ( FIG. 2, curve 5 ) leads to a more favorable parking flow. The theoretically achievable maximum output of such a motor is much higher than the required power, but cannot be used due to currents that are much too high. The operating point for evasive maneuvers is therefore not reached.

When designing for maximum efficiency at the avoidance maneuver operating point ( FIG. 2, curve 6 ), the entire working area A is covered. However, the efficiency during the parking process is then poorer, so that more battery power is required to apply the necessary torque to the motor.

The power amplifier of the electronics is usually equipped with power MOSFETs. These components can only withstand a certain maximum current. To prevent the power amplifier from being destroyed, the motor current is limited to a maximum value. This means that the motor can never deliver more torque than the maximum current control allows. A disadvantage of a design that takes into account both operating points on the torque-speed characteristic curve, that is to say that both points a and b lie on characteristic curve 4 , has the effect that the current required during the parking process becomes very large because of the lower efficiency.

Even with a design for maximum efficiency at Parking is still relative to the battery power required large. Because the moment at maximum efficiency only one Is a fraction of the maximum possible torque Characteristic from the parking operating point to zero very much flat. In other words, the run speed is not significantly above the speed of the corner power. The operating point (b) Evasive maneuvers cannot therefore be achieved. At a  It is designed for maximum efficiency during evasive action the required battery power when parking is again too large.

It is therefore an object of the present invention to provide a To provide methods that provide a technical teaching on optimal design and control of an electric motor provides what results in optimal efficiency and optimal geometric relationships for electric motors for operating Steering systems in motor vehicles leads.

A process must therefore be developed which Engine design and control improved, taking into account must be that the efficiency of the engine when parking is as large as possible and the point of the evasive maneuver within the operating range of the electric motor. It is further to ensure that the maximum efficiency and the maximum the output power coincide as far as possible.

According to the invention, this object is achieved by the characterizing features of main claim 1 such that the voltage applied to the motor is variable and that maximum power and efficiency are combined at the same torque and that at least from this torque the output power of the energy supply source is constant.

It is particularly advantageous with this solution that the Sizing not as usual for a constant motor voltage takes place, but for a constant input power from Vehicle electrical system. The motor voltage is thereby variable, and so the characteristic curve of the motor is not a straight line more. This opens up the possibility of maximum performance and To combine maximum efficiency at the same moment. Since at the Interpretation a size is freely selectable, can also be a additional operating point even at a much higher speed the design are taken into account.

The constant input power results from the constant battery voltage and to a fixed value limited battery power, while usually in the prior art Technology the motor current is limited. The variable motor voltage with the help of a clocked motor output stage realized in a known manner.  

So far, those have been used for the above purpose Electric motors according to the methods to be described now designed.

An electric motor is essentially characterized by its Motor resistance (R) and the torque constant (kt) determined. It is therefore necessary to determine what these engine parameters by the geometry of the engine, such as the Number of turns, diameter, length of the winding can happen. It is important that the desired motor resistance (R) and the torque constant (kt) is guaranteed.

The motor resistance (R) is not the only one Understand the resistance of the motor, but it continues together from the pure motor resistance, the resistance of the Motor supply line, as well as the resistance in the final stage of the Electronics including the power MOSFETs.

In the equations used and described later it is assumed that the friction of the Drive system is known. Furthermore, it is assumed that a Operating point with torque and speed is required from the engine. The torque of the motor, which is referred to below as M, is not equate with the required delivery torque, but is over the friction higher. It is the ent in the engine standing moment.

a) interpretation with working point parking in the maximum of Output performance according to the state of the art

When designing the electric motor, the output power P must _be assumed.

P _ab = (MM _rub ) n · 2 · π (1)

with M _rub = friction of the drive system related to the motor shaft, M = internal motor torque at the working point, n = speed at the working point. If n is replaced by a function of n with the parameters U _bat (battery voltage), kt (torque constant) and R, the result is:

By specifying R and kt of a motor, it is already fully described. In equation (2) there are therefore only known quantities and two unknowns with which the motor is designed to be ready. It is known that the output power of a permanent magnet excited motor has a maximum at a certain moment. The derivation of the equation ( 2 ) after the moment therefore leads - if it is set to zero - to a condition for R and kt so that the engine torque is at the maximum of the output power

This inserted in (2) gives the maximum achievable Output power for the working point, depending on the Frictional torque and battery voltage:

The efficiency achieved there is also interesting. With I = M / kt results:

The equation of (4) and (1) gives the searched Torque constant, so that the operating point in the maximum of Power output is:

The motor is defined by inserting it into equation (3):

b) interpretation with working point parking in the maximum of Efficiency with constant motor voltage in the state of the technology

When designed for maximum efficiency according to State of the art must use the efficiency equation as a basis be taken. The following applies:

with η = efficiency, M = motor torque, M _friction = friction torque of the system related to the motor shaft, n = motor speed, U _bat = battery _voltage , I _bat = battery current.

The speed n can in turn be replaced. With I _bat = M / kt we get:

with kt = torque constant, R = motor resistance (resistance of motor, supply lines, output stage).

It is known that the efficiency of a permanent magnet excited motor at a maximum Moment. Deriving equation (9) after the moment therefore leads to one if it is set to zero Condition for R and kt so that the engine torque at the maximum of Efficiency is:

This condition, used in equation (9), gives the maximum achievable efficiency η _max , which only depends on the engine and friction torque:

This used in equation (8) with I _bat = M / kt gives the torque constant kt so that the motor with its operating point (M, n) is at the maximum efficiency:

After inserting in (10) the motor is fixed:

c) interpretation with working point parking in the maximum of Efficiency with constant input power

Compared to the design methods described above with the help of the given equations, in the design according to the invention with the parking working point at maximum efficiency with constant input power, the procedure is as follows: it can be assumed that the power supplied by the vehicle electrical system remains constant. This can be achieved by regulating the battery current I _bat to a constant value while the battery voltage U _{bat is} also constant. The equation for maximum efficiency then looks like this:

with P _bat = constant power supplied.

The conversion to known sizes analogous to the above leads to

The derivation of equation (15) after M equated with zero again results in a condition for R and kt:

This used in (14) again gives the maximum achievable Efficiency:

The equation of (14) and (17) gives that at the working point necessary input power:

In equations (14) to (18), motor voltage U _mot does not appear anywhere. It is not necessarily the same as the battery voltage U _bat , but can be chosen as desired. So there is still a degree of freedom for the design available. The battery voltage is converted into motor voltage in the clocked power stage. In a first approximation, U _bat · I _bat = U _mot · I _mot .

Determine the torque constant and the motor resistance the motor voltage:

Equation (16) used here leads to a relationship between U _mot and kt. This allows the torque constant to be determined after selecting a motor voltage:

By inserting into equation (16) the motor is then fixed:

In general, when determining the motor voltage, that smaller values of the motor voltage to smaller ones Torque constants, smaller motor resistances and too lead to higher idling speeds. Must now have a second Operating point with a lower torque and higher speed at the Design of an engine can be taken into account, this can be done by suitable choice of motor voltage happen.

In the following, the drawings in comparative with the conventional design methods the invention explained in more detail. It shows the

Fig. 1 is a diagram in which a rotational speed-torque characteristics are an output power characteristic curve 2 and an efficiency characteristic curve 3 Figure 1;

Fig. 2 is a speed-torque diagram with various linear speed-torque characteristics: either through point a (parking), point b (evasive maneuver) or through both points a and b, and area A shows that for one Steering required work area;

Figure 3 is a family of curves with the achievable efficiency depending on the ratio torque / friction with upper curve: const P _to =, middle curve.:. U _mot = U _bat = const, lower curve:. U _mot = U _bat = const, wherein. the operating point is at maximum output;

Fig. 4A shows the design of an engine operating point with the maximum power and a constant motor voltage with and without limiting the motor current;

FIG. 4B shows the design of a motor with operating in the maximum efficiency and a constant motor voltage with and without limitation of the motor current;

Fig. 4C shows the design of a motor with operating in the maximum efficiency and constant power supplied with and without limitation of the battery and motor current;

Fig. 4D shows the design of a motor with operating in the maximum efficiency and constant power supplied with and without limitation of the battery and motor current;

Fig. 5 shows the motor resistance depending on the ratio torque / friction torque; the power generated by the motor was used as a parameter.

If you now compare the conventional design types for maximum power or maximum efficiency with constant motor voltage, as well as the new design described here for maximum efficiency with constant supplied power, the following can be determined: First of all, the maximum achievable efficiency is of interest. Fig. 3 shows a comparison, plotted against the ratio of the torque delivered to the friction torque. If these two quantities are the same, the efficiency must inevitably be zero. As the ratio increases, however, the advantage of the new design method becomes clear. From bottom to top, the characteristic curves correspond to the design types according to section a), b) and c).

FIGS. 4A to 4D show an example motor characteristics, as they result for engines that have been calculated according to the various methods of interpretation.

The characteristic curves are provided with indices to make it easier to assign them to the various vertical axes. It means: n = speed, I = motor current (in FIGS. 4A and 4B the same as the battery current), P = output power, η = efficiency. The dotted areas apply if the motor current is not limited; the solid areas apply with and without limitation of the motor current.

Fig. 4A shows the motor characteristic in design according to section a). As required, the operating point is at the maximum output. The efficiency is below 50%. The motor current is therefore relatively high at the operating point.

FIG. 4B shows the motor characteristic in design according to section b). The efficiency is much better than in Fig. 4a. Therefore the motor current is smaller. As expected, the operating point is maximum efficiency.

Fig. 4C and 4D show the motor characteristics at design according to section c). The following applies: dashed lines = no limitation of the motor and battery current; dash-dotted lines = limitation of the battery current, but no limitation of the motor current; solid lines = limitation of motor and battery current to different values. The battery power is provided with the index I _bat .

In Fig. 4C, the motor voltage equal to the battery voltage equal to 12 V was assumed. As can already be seen from FIG. 3, the efficiency in the operating point is again improved compared to FIG. 4b. The motor current has decreased again. Since the motor voltage is the same as the battery voltage, it is not possible to achieve much higher speeds at a lower torque. The maximum speed is even slightly lower than in Fig. 4B. The motor current was limited to the same value as in Fig. 4B. Since the efficiency is now better, a higher delivery torque can be achieved with this current, in this case it is about 10% more.

In Fig. 4D, the motor voltage was set to 8 V. The efficiency at the operating point has not changed compared to FIG. 4C. The motor current at the operating point is larger compared to FIG. 4C; at the same time, the motor voltage is lower. The operating point is now in the area of the characteristic curve, where the battery current is already limited. In this case, the motor current is only limited at the operating point, so that no greater torque can be delivered. With a smaller engine torque, however, a much higher speed is now possible. The speed is somewhat lower in some areas, even slightly higher in others than in FIG. 4A. The efficiency is much better in any case. It is easy to imagine that by choosing the motor voltage at the operating point, almost any high speed can be achieved. It should be noted, however, that the motor current at the operating point also increases with a higher desired speed.

Fig. 5 shows the calculated motor resistance for the various types of design over the ratio of M to M _friction . The power generated by the motor was used as a parameter. The equations for this are obtained by transforming equations (7), (13) and (21) with the definition F = M / M _friction :

from (7)

from (13)

from (21)

12 V was selected as the battery and motor voltage. When designing according to section c), the motor resistance must be the smallest from a ratio M / M _friction greater than approx. 3. This already applies to the design U _mot = U _bat . If U _mot less than U _{bat is} chosen, then the resistance must become even smaller. This results in a clear criterion for differentiating the different design and operating modes. The characteristic curves are assigned to sections a) -c) as follows: dash-dotted - section a), dashed - section b), solid - section c).

Claims (8)

1. A method for controlling electric motors with external excitation and pulse-length modulated control, characterized in that the voltage applied to the motor is variable and the maximum power and efficiency is combined at the same torque and that at least from this torque to the output power of the energy supply source is constant. 2. The method according to claim 1, characterized records that when operating with the maximum possible Speed and increasing engine torque initially the Battery current is limited. 3. The method according to claim 2, characterized indicates that the motor current from a motor torque is limited, which is above the engine torque from which is limited to the battery current so that the maximum Delivery torque is reached. 4. The method according to claim 3, characterized in that the freely selectable parameters of the motor design are chosen so that the maximum efficiency and the maximum output power of the motor in the torque range between limiting the battery current (I _bat ) and limiting the motor current (I _mot ) lie. 5. The method according to claim 1, characterized records that the maximum speed of the motor is much greater than the speed at maximum Efficiency, or at maximum output. 6. The method according to claim 1, characterized records that the external excitation of the motor by means of a permanent magnet is generated. 7. The method according to claim 1, characterized records that the motor is a three-phase motor with associated rotor position encoder. 8. The method according to claim 1, characterized  records that the engine with power electronics is driven, the output stage equipped with power MOSFETs and is controlled with pulse length modulation.