GB2127186A

GB2127186A - Pulsed regulation of current flow in coil circuit

Info

Publication number: GB2127186A
Application number: GB08323934A
Authority: GB
Inventors: Joachim Braunger; Werner Nitschke; Peter Taufer; Dieter Schaller
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1982-09-10
Filing date: 1983-09-07
Publication date: 1984-04-04
Also published as: DE3233536A1; GB8323934D0; DE3233536C2; JPS59139606A; JPH0410722B2; GB2127186B

Abstract

A switch (12) and a measuring resistor (13) are disposed in the current circuit of a coil (10), which is bridged by a freewheel element (11). A signal indicative of the voltage drop across the resistor (13) is compared in a comparator (14) with a freely selectable target value and the switch (12) is opened on attainment of the target value. The switch (12) is closed again after passing through a hysteresis of the coil current. A delay element (15) and a hysteresis switching stage (16) are arranged between the comparator (14) and the switch (12). The circuit is particularly suitable for current regulation in an electro- magnetic valve or coil ignition system. A timing circuit (17) prevents transient disturbance feedback when the switch (12) is closed. <IMAGE>

Description

SPECIFICATION Pulsed regulation of current flow in coil circuit means The present invention relates to coil circuit means, and has particular reference to pulsed regulation of current flow through a coil.

For the regulation of current flowing through a coil it is known to arrange a measuring resistor, across which there is a voltage drop proportional to the coil current, in the coil current circuit. The voltage signal can then be utilised for regulation of the current flowing through the coil. It is also known to provide pulsed regulation of that kind, wherein the coil is bridged over by a freewheel element, usually a diode or an appropriately controlled thyristor. In that case the coil current may rise along an exponential function determined by the coil inductance and the measuring resistance, and a switch arranged in the coil current circuit may be opened on attainment of a predetermined threshold value, so that the coil current again decays according to an exponential function.

After passing through a hysteresis, which in the known equipment is set as fixed time interval, the switch in the coil current circuit is closed again so that a periodic regulating process results. An equipment of that kind is described in, for example, US 3 575 1 54.

A problem of the known equipment is, however, that the target current value to be regulated towards is fixedly predetermined, so that different target values cannot be utilised.

Moreover, the known circuits, insofar as they are composed of few components, tend towards self-oscillation.

According to the present invention there is provided coil circuit means comprising a coil, a freewheel element connected in parallel with the coil, a switch and a measuring resistor arranged in a current supply circuit of the coil, and switching control means to control opening and closing of the switch for pulsation of the current flow through the coil, the switching control means comprising a comparator for comparing a signal value derived from the resistor with a selectable threshold value and a hysteresis switching stage connected to output means of the comparator by way of a delay element and arranged to cause the switch to open in response to the signal value exceeding the threshold value and to close after passing through a predetermined hysteresis.

Circuit means embodying the present invention may have the advantage that current regulation to any desired predetermined threshold value may be possible, so that the coil current can be regulated within a wide range. This is of particular advantage if the circuit means is employed for the driving of an electromagnetic valve in an electronically controlled automatic transmission of a motor vehicle. Such valves are employed for the setting of the main pressure of the transmission, in which case it must be possible to regulate towards a wide pressure range.

In addition, circuit means embodying the present invention may, by reason of its multistage construction, be simple to construct, since only few components are needed, and may provide a high insensitivity to self-oscillations.

Preferably, the cycle time of the switching control means is made settable, so that individual adaptation to different uses is possible.

In addition, the signal from the measuring resistor may be suppressed on renewed switching-on of the coil current, so that disturbances due to the switching-on process of the coil are suppressed.

Embodiments of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of coil circuit means embodying the invention; Figure 2 is a circuit diagram of one particular form of such coil circuit means; and Figure 3 is a circuit diagram of another particular form of such coil circuit means.

Referring now to the drawings, there is shown in Fig. 1 coil circuit means comprising a coil 10 which is connected in parallel with a freewheel diode 11. The coil 11 is connectible through a switch 1 2 between a voltage UO and ground. A measuring resistor 1 3 is also arranged in the coil current circuit and a signal, which can be taken off at the resistor 13, is conducted to the inverting input of a comparator 14, to the non-inverting input of which can be applied voltage at a target value Us The output of the comparator 1 4 is connected through a delay element 1 5 with the input of a hysteresis switch 16, the output of which controls the switch 1 2. The output of the hysteresis switch 1 6 also actuates a time switch 17, which is arranged in or at the connection of the measuring resistor 1 3 to the comparator 14.

When a target value Us is selected and the circuit means according to Fig. 1 is put into operation for the first time through closing of the switch 12, the current in the circuit rises according to an exponential function which is determined essentially by the inductance of the coil 10 and the resistance value of the resistor 1 3. With the time switch 1 7 closed, a signal corresponding to this rising coil current thus passes to the inverting input of the comparator 14, which switches over when the target value Us is reached.The comparator 14 now acts on the delay element 15, which, after a predetermined time, opens the switch 1 2 through the hysteresis switch 1 6. The coil current now falls off and leads to tipping-back of the comparator 1 4 as well as-through the delay element 1 5-to switching-back of the hysteresis switch 16, which again closes the switch 1 2. A regulating cycle thus results, in which the switch 1 2 is switched off for a certain time in dependence on the set target value U5, the delay time of the delay element 1 5 as well as the width of the hysteresis switch 1 6. In order to exclude disturbances in the regulation on renewed closing of the switch 12, the time switch 1 7 is opened for a predetermined time on the tilting-back of the hysteresis switch 16, which effects a closing of the switch 1 2 so that a signal does not pass from the measuring resistor 1 3 to the comparator 14.

A more specific circuit arrangement is shown in Fig. 2, in which components corresponding to those of Fig. 1 are designated by the same reference numerals. The target value Us in the embodiment of Fig. 2 is produced from the operating voltage UB through a voltage divider 20, 21. The signal from the measuring resistor 1 3 is again conducted to the comparator 14 so that the output of the comparator 1 4 carries a positive logic signal when the signal from the resistor 1 3 exceeds the target value. A capacitor 25 connected to the output of the comparator 1 4 can now be charged through a resistor 23 and a diode 22.The voltage across the capacitor 25 determines the switching behaviour of a comparator 26, which is connected after the capacitor and which is provided with negative feedback through a diode 27 and thereby acts as hysteresis switch. The output signal of the comparator 26 is derivable at a terminal 28 and in addition is conducted through an RCmember 30, 31 to a further comparator 29, the output of which is connected to that input of the comparator 1 4 receiving the signal from the resistor 1 3. The current regulation can be switched off, independently of the cycling, at the terminal 28 through a further external switch connected to ground. This is of particular advantage in the driving of the coil through pulse-width-modulated signals controlled by a microprocessor.

When the voltage across the capacitor 25 exceeds the switch-on threshold of the comparator 26, then the output thereof blocks the switch 1 2. After switching-off of the coil current through opening of the switch 12, the output of the comparator 14 switches to ground so that the capacitor 25 is dischargeable through the resistor 24. The diode 22 in, that case serves for decoupling of the resistor 23, which serves for the discharging of the capacitor 25.When the voltage across the capacitor 25 again falls below the lower switching threshold of the comparator 26, the switch 1 2 is again switched on and the signal of the measuring resistor 1 3 is switched for a short time to ground simultaneously through the RC-member 30, 31 and the comparator 29, so that no disturbances can be transmitted to the input of the comparator 14.

Such disturbances are possible because, in the case of high coil currents to be switched, the freewheel diode 11 must be of relatively large area so that a brief current peak occurs on the renewed switching-on of the coil 10, since the charges present in the pn-transition must initially flow away before the blocking of the diode 11.

A further embodiment is illustrated in Fig.

3, in which the functions are more closely combined than is the case in the circuit of Fig.

2, so that a smaller number of components is required. The circuit according to Fig. 3 also comprises an input comparator 14, which is connected through an input voltage divider 20, 21 with a target value Us The output of the comparator 1 4 is connected through a resistor 41 to the non-inverting input of an operational amplifier 42 and also through a resistor 40 to the operating voltage UB. The amplifier 42 is provided with negative feedback through a capacitor 43 and a reference voltage UR is connected to its inverting input.

The output of the amplifier 42 controls the switch 1 2.

When the switch 1 2 is conductive, the current through the coil 10 rises as described above. The voltage dropping across the measuring resistor 1 3 is now compared in the comparator 14 with the target value Us.

When the voltage drop across the resistor 1 3 exceeds the target value Us then the comparator 14 switches to ground. The voltages at the inverting input of the amplifier 42 then falls according to an exponential function which is essentially determined by the resistor 41 and the capacitor 43. When the voltage at the inverting input of the amplifier 42 reaches the reference voltage UR, the amplifier 42 switches through and the voltage at the noninverting input is suddenly displaced in negative direction because of the charged capacitor 43. The voltage at the non-inverting input of the amplifier 42 now rises again from this new starting value along an exponential function which is determined by the capacitor 43 as well as the sum of the resistors 40 and 41.

When the voltage at the non-inverting input of the amplifier 42 again reaches the reference voltage from below, the amplifier 42 again tips into its initial position. Through both the tipping operations, the voltage at the output of the amplifier 42 and thereby the switch 1 2 is switched over. A switching operation thus results, in which the switch-off phase of the switch 1 2 is determined substantially through component values and the switchon phase through the current rise in the coil 10. The circuit is proof against short-circuit, as the flowing current is limited to a low value in the case of a coil short-circuit.

It is self-evident that a time switch 1 7 can also be employed in the embodiment according to Fig. 3.

Claims

1. Coil circuit means comprising a coil, a freewheel element connected in parallel with the coil, a switch and a measuring resistor arranged in a current supply circuit of the coil, and switching control means to control opening and closing of the switch for pulsation of the current flow through the coil, the switching control means comprising a comparator for comparing a signal value derived from the resistor with a selectable threshold value and a hysteresis switching stage connected to output means of the comparator by way of a delay element and arranged to cause the switch to open in response to the signal value exceeding the threshold value and to close after passing through a predetermined hysteresis.

2. Circuit means as claimed in claim 1, wherein the hysteresis switching stage comprises a comparison element provided with negative feedback through a diode.

3. Circuit means as claimed in claim 1, wherein the hysteresis switching stage comprises an operational amplifier provided with negative feedback through a capacitor, the capacitor being chargeable and dischargeable in dependence on an output signal from the comparator indicative of the comparison result.

4. Circuit means as claimed in any one of the preceding claims, the switching control means further comprising a time switch controllable by the hysteresis switching stage to prevent application of the signal value to the comparator on the closing of the switch.

5. Circuit means as claimed in any one of the preceding claims, wherein the coil is part of an electromagnetic control valve for a motor vehicle automatic transmission.

6. Circuit as claimed in any one of claims 1 to 4, wherein the coil is an ignition coil for a motor vehicle ignition system.

7. Coil circuit means substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

8. Coil circuit means substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

9. Coil circuit means substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.