Method for controlling the current of an inductive load

Abstract

A method for pulse-modulated current control of an inductive load current. A first operating mode is implemented when a change in current setpoint value is effected. An initial duty cycle of the pulse modulation is determined and used to operate the inductive load in a second operating mode. In the first operating mode, a switching state of at least one switching element is set depending on whether a current setpoint value of the load current is higher or lower than a current setpoint value of a directly preceding cycle. The switching state is retained until a current limit value is reached and then switched over. The switching state of the switching element is switched over multiple times to generate a prescribed number of periods, and the duty cycle of the pulse modulation is determined based on at least a portion of the prescribed number of periods.

Claims

1. A method for pulse-modulated (PWM) current control of a load current of an inductive load by at least one switching element for switching the load current, in which a first operating mode for operating the inductive load is implemented when a change in the current setpoint value has been effected, the method comprising: setting, in the first operating mode, during a present cycle, a switching state of the switching element depending on whether a current setpoint value of the load current in the present cycle is higher or lower than a current setpoint value of a directly preceding cycle, wherein when the current setpoint value of the load current in the present cycle is higher than the current setpoint value of the directly preceding cycle: the switching state is closed and retained until the current setpoint value of the load current in the present cycle is reached and is then opened, the opened switching state is retained for a predetermined amount of time, and is then closed, wherein the switching state of the switching element is switched between the opened and closed state multiple times to generate a prescribed number of periods, wherein when the current setpoint value of the load current in the present cycle is lower than the current setpoint value of the directly preceding cycle: the switching state is opened and retained until the current setpoint value of the load current in the present cycle is reached and is then closed, the closed switching state is retained for a predetermined amount of time, and is then opened, wherein the switching state of the switching element is switched between the opened and closed state multiple times to generate a prescribed number of periods, and determining the duty cycle of the pulse modulation based on at least a portion of the prescribed number of periods, said duty cycle being used as the initial duty cycle for the second operating mode.

2. The method as claimed in claim 1, wherein an average duty cycle is determined based on at least the portion of the prescribed number of periods, said average duty cycle being used as the initial duty cycle for the second operating mode.

3. The method as claimed in claim 1, wherein the initial duty cycle for use in the second operating mode is calculated using an average length of time of at least one of the switching states of the switching element over at least a portion of the number of periods.

4. The method as claimed in claim 1, wherein the initial duty cycle is calculated using an average switch-on time of the switching element over at least a portion of the number of periods.

5. The method as claimed in claim 1, wherein a period duration of the periods is prescribed by a timer.

6. The method as claimed in claim 1, wherein proceeding from an initial duty cycle identified in the first operating mode, the duty cycle during the second operating mode is prescribed taking into account the current setpoint value.

7. The method as claimed in claim 1, wherein in the second operating mode, a current actual value is determined, a control deviation from the current actual value and the current setpoint value is detected and the control deviation is used to calculate a duty factor of the pulse-width modulation.

8. An electronic circuit arrangement for the pulse-modulated current control of a load current of an inductive load comprising: at least one switching element for switching the load current; and a control circuit for actuating the switching element, wherein the circuit arrangement is configured to operate the inductive load in a first operating mode when a change in the current setpoint value has been effected, wherein, in the first operating mode, during a present cycle, a switching state of the switching element is set depending on whether a current setpoint value of the load current in the present cycle is higher or lower than a current setpoint value of a directly preceding cycle, wherein when the current setpoint value of the load current in the present cycle is higher than the current setpoint value of the directly preceding cycle: the switching state is closed and retained until the current setpoint value of the load current in the present cycle is reached and is then opened, the opened switching state is retained for a predetermined amount of time, and is then closed, and wherein the switching state of the switching element is switched between the opened and closed state multiple times to generate a prescribed number of periods, wherein when the current setpoint value of the load current in the present cycle is lower than the current setpoint value of the directly preceding cycle: the switching state is opened and retained until the current setpoint value of the load current in the present cycle is reached and is then closed, the closed switching state is retained for a predetermined amount of time, and is then opened, wherein the switching state of the switching element is switched between the opened and closed state multiple times to generate a prescribed number of periods, and the duty cycle of the pulse modulation is determined based on at least a portion of said periods, said duty cycle being used as the initial duty cycle (DC) for the second operating mode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further preferred embodiments emerge from the following description of exemplary embodiments on the basis of figures.

(2) In a basic illustration:

(3) FIG. 1 shows a block circuit diagram of a circuit arrangement for controlling the current of an inductive load according to the method according to an aspect of the invention,

(4) FIG. 2 shows a time/current graph for illustrating the temporal profile of the controlled current in the inductive load,

(5) FIG. 3 shows a flow chart for explaining the calculation of the starting value of the duty cycle for the current control of the inductive load when the current setpoint value of the current changes during an operating mode I and

(6) FIG. 4 shows a flow chart for explaining the current control of the inductive load in an operating mode II.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) In order to enable a short and simple description of the exemplary embodiments, identical elements are provided with the same reference signs.

(8) FIG. 1 shows a schematic illustration of a load circuit 1 having a valve coil for operating an electro-hydraulic or electro-pneumatic valve in a motor vehicle brake installation as inductive load L and a FET transistor T1 as load current switch connected in series with the load L, and a control circuit 2 for actuating the FET transistor T1 and executing the control method according to an aspect of the invention. To this end, a PWM voltage signal U.sub.PWM, generated by the control circuit 2, is fed to the gate electrode of the FET transistor T1, the duty cycle (ratio of the pulse duration to the period duration) of which signal is adjusted according to the current requirement. A measuring resistor R.sub.m arranged in series with the load L is provided in the load current path, wherein the current actual value I.sub.ACTUAL of the load current I.sub.L is determined from the measuring voltage U.sub.m, which is applied across the measuring resistor R.sub.m, by means of a current measuring amplifier 2.2 and control circuit 2. The load circuit 1 comprising the series circuit composed of the FET transistor T1, the inductive load L and the measuring resistor R.sub.m is connected to a voltage supply U, for example a vehicle battery, wherein the FET transistor T1 is present, for example, in low-side arrangement. A further FET transistor T2 is connected in parallel with the inductive load L with respect to ground, said further FET transistor being able to be operated as a freewheeling diode by the control circuit 2.

(9) According to this exemplary embodiment, the control circuit 2 has an A/D converter 2.1, which digitizes the amplified measurement signal output by the current measuring amplifier 2.2 for the purpose of processing by the control circuit 2.

(10) The method according to an aspect of the invention for operating a valve coil L is explained in the following text based on the time/current graph according to FIG. 2 and the block circuit diagram and the flow chart according to FIGS. 3 and 4.

(11) In general, the following holds true:

(12) $I=\frac{U}{R}\mathrm{DC}\mathrm{or}\mathrm{DC}=\frac{R}{U}I$$\mathrm{where}DC=\frac{t_{\mathrm{on}}}{t_{\mathrm{on}}+t_{\mathrm{off}}}$

(13) The duty cycle DC is influenced by the supply voltage U and the overall resistance R of the load circuit, in particular the resistance of the coil L and the measuring resistor R.sub.m. Since these are variable in running operation, for example on account of temperature influences, a clear load current I.sub.L cannot be derived from a prescribed duty cycle (and vice versa).

(14) The valve coil L is therefore energized in at least two phases or operating modes, wherein, upon a change in the current setpoint value I.sub.SETPOINT being determined, the load current I.sub.L is moved closer to the new current setpoint value I.sub.SETPOINT in an operating mode I according to the method steps explained with reference to FIG. 3. According to the exemplary illustration according to FIG. 2, the new current setpoint value I.sub.SETPOINT is higher than a current setpoint value I.sub.Setpoint,t1 of a preceding cycle, with the result that the load current I.sub.L,I is increased in the operating mode I. When an upper current limit value I.sub.SETPOINT,o,I of the operating mode I is reached or exceeded, no further actuation is carried out for a prescribed length of time, with the result that the load current I.sub.L,I decreases, wherein the actuation is subsequently continued until the upper current limit value I.sub.SETPOINT,o,I is reached, whereupon disconnection takes place again. By using the average switch-on time over a number N of periods, an average duty cycle DC.sub.mean is determined, wherein the period duration T can be prescribed by a timer, such as, for example, the maximum value of a counter:

(15) $\begin{array}{cc}{\mathrm{DC}}_{\mathrm{mean}}=\frac{\frac{.Math.t_{\mathrm{on}}}{N}}{T}& \left(1\right)\end{array}$

(16) The load current I.sub.L for the operating mode II is adjusted based on the changed current setpoint value I.sub.SETPOINT by using the average duty cycle DC.sub.mean.

(17) After determination of said starting duty cycle DC.sub.mean for the operating mode II, said operating mode follows the operating mode I, which can be implemented according to the method steps according to FIG. 4 and in which the load current I.sub.L is controlled between the upper current limit value I.sub.SETPOINT,o,II and the lower current limit value I.sub.SETPOINT,u,II. For the operating mode II, FIG. 2 illustrates only an average load current I.sub.L controlled by the PWM.

(18) The upper current limit value I.sub.SETPOINT,o,I of the first operating mode I is determined to be higher than the upper current limit value I.sub.SETPOINT,o,II of the second operating mode II, in particular in order to achieve a situation in which the average load current I.sub.L in the operating mode II runs approximately in the center between the current limit values, wherein the fact that the switch-on and switch-off behavior of the load current I.sub.L has an exponential profile and an average current is therefore lower than an average value between the upper I.sub.SETPOINT,o,II and the lower I.sub.SETPOINT,u,II current limit value is taken into account. The described procedure can likewise be applied in the case of a reduction in the setpoint current value I.sub.SETPOINT, wherein there is preferably no provision for widening of the tolerance range, by appropriate determination of the lower current limit value I.sub.SETPOINT,u,I, in order to prevent too low a load current I.sub.L in the operating mode I.

(19) A more precise explanation of the calculation of the starting value of the duty cycle of the current control during the operating mode I is given below with reference to FIG. 3. According to the flow chart in FIG. 3, after the start, a check is first of all carried out in a first method step S1 to determine whether there is a change in the current setpoint value I.sub.SETPOINT in comparison with the preceding cycle. If no change has taken place, there is a switch to the operating mode II or said operating mode is retained. Instead, if a change has taken place, according to method step S2, a timer is started and a check is carried out to determine whether the current setpoint value I.sub.SETPOINT is greater than the current setpoint value I.sub.SETPOINT,t1 of the preceding cycle (S3).

(20) If the current setpoint value I.sub.SETPOINT during the comparison in method step S3 is not less than or greater than the current setpoint value I.sub.Setpoint,t1 of the preceding cycle, that is to say the load current I.sub.L is intended to be increased, the load circuit 1 is closed in method step S4.1 by means of the FET transistor T1, so that there is an increase in the load current I.sub.L when the valve coil L is energized. The current actual value I.sub.ACTUAL is detected in each case at consecutive sampling times t.sub.Si (i=1, 2, . . . ), which differ by a control pause duration t of, for example, 200 s, and a check is then carried out in step S5.1 to determine whether the upper current limit value I.sub.SETPOINT,o,I has been reached or exceeded, wherein the check is cyclically repeated until an exceeding has been identified and subsequently the load current I.sub.L is switched off by means of corresponding actuation of the FET transistor T1 in method step S6.1 at least until the exceeding of a length of time or period duration set by means of the timer started in step S2, wherein, after the set length of time of the timer has been exceeded, the counting starts again, so that it is run through cyclically. After a length of time (S7.1), a check is carried out to determine whether the prescribed number of PWM periods generated in this way has been reached, wherein there is a return to method step S4.1 when further periods are envisaged. The number of periods generated in this way can preferably be adapted.

(21) The average load current I.sub.L for the operating mode II is adjusted based on the current setpoint value I.sub.SETPOINT by means of said PWM periods. If the preset number N of periods is reached, in method step S9, a defined number of switch-on times t.sub.on1, t.sub.on2, . . . of the periods is used and the corresponding average duty cycle DC.sub.mean is calculated therefrom based on equation (1). Said corresponding average duty cycle is transmitted as the starting duty cycle to the current controller for the execution of the operating mode II. In this case, the lengths of time of the last two switch-on phases t.sub.on1, t.sub.on2 can be used, for example, to form an average value:
t.sub.on.sub._.sub.mean=(t.sub.on1+t.sub.on2)/2.

(22) If the load current I.sub.L is intended to be reduced, that is to say the prescribed current setpoint value I.sub.SETPOINT during the comparison in method step S3 is lower than the current setpoint value I.sub.SETPOINT,t1 of the preceding cycle, the load circuit 1 is interrupted in method step S4.2 by means of the FET transistor T1, whereupon the load current is reduced, and in S5.2 a check is then carried out to determine whether the lower current limit value I.sub.SETPOINT,u,I has been undershot, wherein the check is repeated until an undershoot is identified and subsequently the load current I.sub.L is switched on (S7.2) for a prescribed length of time t.sub.on1, . . . by means of appropriate actuation of the FET transistor T1 in method step S6.2. If the length of time t.sub.on1 has been exceeded, a check (S8.2) is carried out to determine whether the prescribed number of periods generated in this way has been reached, wherein there is a return to method step S4.2 when further periods are provided. If the preset number N of periods has been reached, in method step S9, the duty cycle DC.sub.mean to be used for the operating mode II is calculated and transmitted to the current controller for execution of the operating mode II, as has already been described.

(23) FIG. 4 shows a flow chart for explaining the current control in operating mode II. The manipulated variable that is used for readjustment is a fixed value in operating mode II. If the load current I.sub.L fluctuates within the current limits, the duty cycle is not changed. The time at which a control intervention is performed is linked to the sampling time of the current signal. According to the flow chart in FIG. 4, after the transmission of the starting value of the duty cycle, the current actual value I.sub.ACTUAL is first measured (S10) and in method step S11 a check is carried out to determine whether the load current value identified in this way is located within the lower I.sub.SETPOINT,u,II and upper current limit I.sub.SETPOINT,o,II of the operating mode II. If it is located within the limits, there is a return to step S10 and the current actual value I.sub.ACTUAL is measured again. In the case that the load current I.sub.L or the measured current actual value I.sub.ACTUAL is outside of the limits, in step S12 a check is carried out to determine whether said load current or measured current actual value is greater than the upper current limit I.sub.SETPOINT,o,II and, if this is the case, the duty cycle of the PWM is reduced (S13). If said load current or measured current actual value is not greater, the duty cycle is increased (S14).

(24) The limit values I.sub.SETPOINT,u and I.sub.SETPOINT,o can also be combined according to a preferred configuration of an aspect of the invention to form a single current setpoint value in order to implement the control of the load current I.sub.L,II during the operating mode II with respect to said single current setpoint value, wherein continuous adjustment is performed by the control.