GB2152305A

GB2152305A - Circuit arrangement for controlling and monitoring electrical loads

Info

Publication number: GB2152305A
Application number: GB08431766A
Authority: GB
Inventors: Karlheinz Arnold; Georg Haubner; Kurt Neuffer
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1983-12-22
Filing date: 1984-12-17
Publication date: 1985-07-31
Also published as: GB2152305B; DE3346435A1; JPS60153522A; GB8431766D0

Abstract

A micro computer 15 applies variable width control pulses to a switch 12 in series with a load 10 and a current measuring resistor 16 which is connected to two threshold value switches 17, 18 set to different response values. The switching state of the first threshold value switch (17), operating as a short-circuit sensor, is interrogated at a first instant of each control pulse, and the control pulse is immediately switched off upon response of the said threshold value switch. The switching state of the second threshold value switch 18, operating as an overload current sensor, is interrogated at a second instant of each control pulse, and the width of following control pulses is reduced by a fixed amount upon response of the said threshold value switch. Micro computer 15 may keep the switch 12 turned off if a short circuit persists offer a predetermined number of control pulses, a short-circuit indicating signal being also produced then. The load 10 may be a solenoid controlling fuel flow in an internal combustion engine. The microcomputer 15 adjusts the width of the control pulses applied to the transistor 12 in response to an engine speed signal at input 15e. <IMAGE>

Description

SPECIFICATION Circuit Arrangement for the Switching-on and Switching-off and Monitoring of Electrical Loads The invention relates to a circuit arrangement for the switching-on and switching-off and monitoring of electrical loads.

In a circuit arrangement known from German Offenlegungsschrift No. 31 35 805, electrical loads are switched by way of a high-power field effect transistor which is at the same time used as a current-measuring element for the current flowing in the load circuit. In this circuit arrangement, the field effect transistor is intermittently switched by a control circuit in dependence upon the intensity of the current, the "on" phase being reduced by the control circuit during increasing current strength in the load circuit. In the event of a short-circuit the "on" phase is chosen to be sufficiently short to avoid damage to the field effect transistor. In this manner, in the known circuit arrangement, the average current strength is kept at a predetermined value by way of the control circuit.Hence, the circuit arrangement can only be used for loads which ensure up to a predetermined average value of current when switched on or which are fully switched off. The known circuit arrangement cannot be used for loads (such as in actuating solenoids) in which a continuously variable "on" state is desired irrespective of the average value of the current.

The present solution seeks to provide a switching arrangement for the switching-on and switching-off and monitoring of electrical loads which can be switched over between a working position and a normal position and which can also be operated in an optional number of switching positions between the "on" and "off" positions. It is already known from German Offenlegungsschrift No. 32 23 168 to trigger an adjusting member, controllable in this manner, for the idling speed regulator of an internal combustion engine byway of a switching stage, by a microcomputer whose control pulses have a constant period, and to influence the operating position of the adjusting member by a variable pulse width.

A circuit arrangement, in accordance with the invention, for the switching-on and switching-off and monitoring of electrical loads has a switching stage, a microcomputer for triggering the switching stage by control pulses of constant period and variable width, two threshold value switches, and a measuring resistor in the load circuit for deriving a measuring voltage dependent upon the instantaneous value of the load current which voltage is fed to the two threshold value switches, one of the threshold value switches being set as an overload switching stage to a maximum admissible average value of the current, and the other of the threshold value switches being set as a short-circuit switching stage to an admissible switching-on current value, the microcomputer being adapted to interrogate the output of the short-circuit switching stage at a first instant during each control pulse, and immediately to switch off the control pulse upon the response of the short-circuit switching stage, and the microcomputer being also adapted to interrogate the output of the overload switching stage at a second instant during a control pulse, and, upon response of the overload switching stage to reduce the width of following control pulses.

A circuit arrangement in accordance with the invention has the advantage that short-circuit monitoring as well as current regulation in the event of too high an average value of the current are ensured, irrespective of the switching position of the load to be influenced by way of the switching stage at any given time. A further advantage is that the control of the load as well as the monitoring of excess current and short-circuits can be performed by one and the same microcomputer, hence ensuring an inexpensive solution.

It is particularly advantageous to process the switching-off signal of the short-circuit switching stage by the microcomputer upon the occurrence of a short-circuit in the load circuit in such a way that only the load is immediately switched off by way of the switching stage, although a monitoring signal (watchdog) of the microcomputer, occurring together with the control pulses, appears across the original full pulse width of the control pulses for the purpose of monitoring the program sequence.

The invention wili now be further described by way of example with reference to the accompanying drawings, in which: Figure 1 shows the circuit arrangement for switching-on and switching-off and controlling and monitoring an actuating solenoid by means of a microcomputer; and Figure 2 is a pulse graph for explaining the mode of operation of the circuit arrangement of Figure 1.

In the embodiment illustrated in Figure 1, an actuating solenoid 10 for controlling the idling speed of a diesel internal combustion engine (not illustrated) is provided as an electrical load and is shown symbolically as an inductor. The actuating solenoid 10 is connected to the positive terminal of the electrical system of the motor vehicle. A diode 11 for the free-running of the current upon the switching-off of the actuating solenoid 10 is connected in parallel with the actuating solenoid 10.

Atransistorized switching stage 12 is connected in series with the actuating solenoid 10 and its control terminal 12a is connected to the control output 15a of a microcomputer 15 by way of a series resistor 13 and an amplifier stage 14. A monitoring signal (watchdog) appears at a further control output 15b and is to be used in a known manner (see German Offenlegungsschrift No. 29 03 638) to monitor the sequence of the program in the microcomputer.

Furthermore, a measuring resistor 16 is connected to the output of the switching stage 12 and its terminal remote from the switching stage 12 is connected to earth. For the purpose of monitoring the current intensity of the actuating solenoid 10, the measuring voltage which drops across the measuring resistor 16, and which is dependent upon the instantaneous value of the load current, is fed by way of a terminal 16a to two comparators 17 and 18 which operate as threshold value switches and whose positive inputs are connected to a predetermined reference voltage. The two reference voltages are obtained by means of a voltage divider 19, a component resistor 19a being connected to a positive potential, a second component resistor 19b being connected between the two comparators 17 and 18, and a third component resistor 19c being connected to earth.In order to protect the comparators 17 and 18 against overvoltage, that terminal 1 6a of the measuring resistor 16 which carries the measuring voltage is limited to 5.1 V by way of a series resistor 20 and a Zener diode 21 connected to earth. Furthermore, the negative inputs of the two comparators 17 and 18 are each provided with an RC suppressor stage 22 for the purpose of decoupling high-frequency interference voltages and for smoothing the measuring voltage.

The first comparator 17 serves as a short-circuit switching stage. Its reference voltage at the positive input is set to 1.5 V. The second comparator 18 serves as an overload switching stage and the reference voltage at its positive input is set correspondingly lower at 1.0 V. The outputs of the comparators 17 and 18 are each connected to a signal input 15e and 15d respectively of the microcomputer 15. Speed signals for measuring the speed of the internal combustion engine are fed to the microcomputer 15 by way of a further signal input 15e. In order to produce at the negative input of the comparator 18 a measuring voltage corresponding to the average value of the current, a series resistor 24 and a capacitor 25, connected to earth, for voltage smoothing are connected to said input of the comparator 18.

The mode of operation of the circuit arrangement of Figure 1 will be further explained hereinafter with reference to the signals illustrated in Figure 2.

Control signals Is, which appear at the output 15a of the microcomputer 15, are plotted on the first time axis a. The control signals /5 have a constant frequency of f=60 Hz predetermined by the microcomputer 15. Their period To comprises 256 time increments which are to be continuously counted off by the microcomputer 15, time incrementsforthe pulse width being in each case continuously recalculated by the microcomputer in dependence upon the measured speed.On the basis of the frequency and the width of the control pulses /5, the actuating solenoid assumes a position which is determined by the average value of the current and by which the quantity of fuel required for idling ofthe internal combustion engine is metered in such a way that the internal combustion engine is governed to an idling speed of 600 min~l. If the idling speed drops below 600 min~l as a result of, for example, loading of the internal combustion engine, this is detected by the microcomputer 15, the width of the control pulses I5is increased, the average value of the current is consequently increased, and the actuating solenoid 10 opens the fuel passage to a slightly greater extent, so that the quantity of fuel made available to the internal combustion engine increases for the purpose of increasing the engine speed. Conversely, the width of the control pulses I is reduced when the idling speed is too high, and hence the supply of fuel is correspondingly throttled.

The currentiflowing in the actuating magnet 10, and which produces across the measuring resistor 16 a voltage drop having a corresponding characteristic, is plotted on the second time axis b.

The monitoring signal wd (watchdog) at the output 15b of the microcomputer 15 is plotted on the third time axis c. The circuit arrangement of Figure 1 operates in a trouble-free manner in the first interval of time A. The corresponding voltage drop across the measuring resistor 16 is applied to the negative inputs of the two comparators 17 and 18by way of the series resistor 20 and the RC suppressor stages 22. However, the increase in the measuring voltage is attenuated on the comparator 18 by the charging of the capacitor 25. Since the average value 1m of the current lies below a critical value for overloading of the actuating solenoid 10 during normal operation, the two comparators 17 and 18 do not respond.

Since the microcomputer interrogates the output of the comparator 17 during each control pulse 1s at a first instant tl 40 us after commencement of the control pulse, an 0-signal is consequently read into the input 15c of the microcomputer 15 in the normal state. In the same manner, the output of the second comparator 18 is interrogated during each control pulse 15at a second instant t2 800 us after commencement of the control pulse, and an 0signal is also consequently read into the input 15d of the microcomputer 15 in the normal case.Provided that the comparators 17 and 18 have not responded at the instants t1 and t2, the microcomputer 15 outputs the control pulses /5 in dependence upon the speed of the internal combustion engine. The calculated value for the pulse widths can possibly also be influenced by way of the computer 15 by further parameters such as the air temperature or the temperature of the engine.

The case in which a short-circuit occurs in the circuit of the actuating solenoid 10 is shown in the interval oftime B. The currentlassumes a very high value and a correspondingly high voltage drop occurs across the measuring resistor 16 and is applied to the negative input of comparators 17 and 18 by way of the series resistor 20 and the suppressor stage 22. The comparator 17 thereby responds in the first instance.A 1-signal is read into the input 15c of the microcomputer 15 at the instant tl. The program of the microcomputer 15 is designed in such a way that the control pulse 4 at the output 15a of the microcomputer 15 is immediately switched off upon reading the 1-signal into the input 1 sic at the instant t1. Consequently, the switching stage 12 and hence the actuating solenoid 10 are also switched off by way of the amplifier 14.

However, the monitoring signal wd produced at the output 15b of the microcomputer 15 in synchronism with the control pulses 4 will continue to run over the original full pulse width even when the comparator 17 responds, so that it is ensured that only a fault in the sequence of the program of the microcomputer 15 is detected by the monitoring signal wd. The operation described is repeated at the commencement of each further control pulse 4.

However, by appropriate programming, the switching stage 12 can be finally switched off by the microcomputer 15 after a predetermined number of control pulses upon the occurence of a short-circuit, and the short-circuit can be indicated by way of a further signal output (not illustrated) of the microcomputer 15. By virtue of the short "on" period in the event of a short-circuit, and recharging of the capacitor 25 at the commencement of each control pulse, the second comparator 18 is no longer switched over.

In the interval of time C, it is assumed that the average value 1m of the current is exceeding an admissible maximum limit. The corresponding voltage drop across the measuring resistor 16 does not cause the comparator 17 to respond, since the comparator's reference voltage is higher. On the other hand, the comparator 18 having the lower reference voltage at its positive input has responded at the instant t2 of the control pulse 15, and hence a 1-signal is read into the input 15b of the microcomputer 15. As a result of this signal, a correspondingly designed program of the microcomputer 15 shortens the following control pulses by a time increment At during the next run through the program. The average value 1m of the current is thereby also slightly reduced.If the average value /m of the current is even then still so high that the comparator 18 continues to supply a signal at the instant t2 of the control pulses 15, the control pulses are again shortened by a time increment At during the next run through the program. In this manner, the average value of the current of the actuating solenoid 10 is to be gradually reduced again to an admissible value.

Since a large number of control pulses 15 are output by the output 15a of the microcomputer 15 during a run through the program, the width of the control pulses 15 is, upon response of the comparator 18, in each case reduced by a time increment At only at the commencement of a new run through the program. However, for the sake of simplicity, only one control pulse 15 for one run through the program is shown in each of the time intervals B and C in Figure 2. It may possibly also be advantageous to reduce the pulse width by the microcomputer 15 only when the comparator 18 has responded in the case of severai successive control pulses 15. In the present embodiment, the average value of the current is decreased only when the microcomputer 15 has ascertained during 10 measuring cycles that the admissible value of 2.3 A has been exceeded. In this manner, an undesired rapid decrease in the average value 1m of the current across the actuating solenoid 10 is prevented. The short-circuit interruption is set to a current value of 10 A on the comparator 17.

Claims

1. A circuit arrangement for the switching-on and switching-off and monitoring of electrical loads in a load circuit comprising, a switching stage, a microcomputerfortriggering the switching stage by control pulses of constant period and variable width, two threshold value switches, and a measuring resistor in the load circuit for deriving a measuring voltage dependent upon the instantaneous value of the load current, which voltage is fed to the two threshold value switches, one of the threshold value switches being set as an overload switching stage to a maximum admissible average value of the current, and the other of the threshold value switched being set as a short-circuit switching stage to an admissible switching-on current value, the microcomputer being adapted to interrogate the output of the short-circuit switching stage at a first instant during each control pulse, and immediately to switch-off the control pulse upon the response of the short-circuit switching stage, and the microcomputer being also adapted to interrogate the output of the overload switching stage at a second instant during a control pulse, and, upon resonse of the overload switching stage, to reduce the width of following control pulses.

2. A circuit arrangement as claimed in claim 1, wherein, upon response of the overload switching stage, the microcomputer reduces the width of the control pulses only upon the commencement of a fresh run through the program.

3. A circuit arrangement as claimed in any of the preceding Claims, wherein a monitoring signal of the microcomputer, produced in synchronism with the control pulses for the purpose of monitoring the program sequence, appears over the original full width of the pulse even upon response of the short-circuit switching stage.

4. A circuit arrangement constructed and arranged substantially as herein described, with reference to, and as illustrated, in the accompanying drawings.