Apparatus and method for driving and controlling electric consumers, in particular heat plugs

An apparatus for driving and controlling electrical loads, in particular glow plugs, is proposed which includes semiconductor switches which are assigned to the glow plugs and can be driven by a microprocessor, and also includes at least one measuring resistor and is characterized in that the microprocessor (17) is so designed that the glow plugs (RK) are switched on and/or off sequentially with time displacement for such a short time that a virtually continuous current rise or decrease is produced and/or in that, in order to detect an open circuit or a short circuit in any of the glow plugs (RK), the glow plugs (RK) are driven sequentially at any desired time interval for a very short time, preferably for 1 ms and the current flowing through the glow plugs (RK) is measured with the aid of the measuring resistor (R) and/or in that one or more glow plugs (RK) are driven simultaneously if a high-energy overvoltage occurs in the voltage supply of this apparatus.

BACKGROUND OF THE INVENTION 
The invention is based on an apparatus for driving and controlling 
electrical loads, in particular glow plugs. In a known apparatus of this 
type, glow plugs of an internal combustion engine of a motor vehicle are 
driven sequentially with a phase displacement. However, this type of 
driving has the disadvantage that each time a glow plug is switched on, 
the current rise can substantially decay before the next plug is switched 
on. With short pulse lengths, it is also possible that a plug is already 
switched off again before the next plug is switched on. This produces 
high-frequency interference in the vehicle supply system. 
SUMMARY OF THE INVENTION 
The apparatus according to the invention for driving and controlling 
electrical loads and the method for driving and monitoring electrical 
loads by means of the apparatus have, on the other hand, the advantage 
that negative effects on the voltage supply, when the electrical loads or 
glow plugs are driven, are avoided by sequentially switching the loads on 
and/or off at short time displacements so that a virtually continuous 
current rise or fall is produced. A particular advantage is that the 
electrical loads or glow plugs are tested for open circuit or short 
circuit by driving them in sequence at any desired time interval with 
measurement pulses of preferably 1 ms duration and determining the current 
flowing through the glow plugs with the aid of the measuring resistor. It 
is particularly advantageous that high-energy interference voltages of the 
voltage supply or of the vehicle supply system are reduced by driving one 
or more glow plugs simultaneously for a certain time. 
It is particularly advantageous that the power of the individual loads or 
glow plugs can be controlled.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
In principle, the apparatus is suitable for driving and controlling any 
electrical loads. Particularly advantageous, however, is the use for 
driving and controlling glow plugs in motor vehicles having an 
automatically controlled internal combustion engine. An exemplary 
embodiment with four glow plugs is explained below. 
For simplicity, FIG. 1 only shows the internal resistances RK of the four 
glow plugs whose first end is connected to a first conductor 1 connected 
to ground. Their second end is connected to a semiconductor switch 3 which 
is connected via a shunt or resistor R, which acts as a measuring 
resistor, to a second conductor 5. The conductor 5 is connected to the 
voltage supply or the vehicle supply system, for example to terminal 15, 
to which a voltage of, for example, approximately 12 to 14 V is applied 
during operation. 
In the present case n-channel enhancement MOSFETs have been selected as 
semiconductor switches. Other semiconductor power switches can also be 
used. Source S and substrate or bulk B of the FETs are connected to each 
other and are connected to the second end of the internal resistance RK of 
the glow plug which is situated opposite the ground connection. The drain 
electrode D of the FETs is connected to the node 7 at which the 
semiconductor switches are connected to the measuring resistor R. The gate 
electrode G is connected to a multistage sequential logic circuit which is 
shown here as a shift register 9. The subdivision of the shift register 9 
into four sections indicates that each stage, that is, each flip-flop, of 
the shift register is assigned to an FET 3. A measuring line 11 is 
connected from the nodes 7 to a signal evaluation or an 
undercurrent/overcurrent detection circuit 13 which determines the 
potential present at the node 7 and compares it with the potential present 
on line 5 and/or on line 1 by means of undercurrent/overcurrent 
comparators. A signal line 15 is connected from the detection circuit 13 
to a microprocessor 17. The microprocessor 17 is connected via a driving 
line 19 to the shift register 9. 
FIG. 2 shows a further exemplary embodiment of the apparatus. In FIGS. 1 
and 2, corresponding elements are provided with identical reference 
symbols. 
FIG. 2 shows series connections of glow plugs, of which only the internal 
resistance RK is shown for simplicity, and semiconductor switches which 
are configured as n-channel enhancement MOSFETs 3. The drain electrodes D 
of all the FETs 3 are connected to each other at the node 7. Between this 
node and the second conductor 5 there is, in this embodiment, only one 
shunt or resistor R serving as measuring resistor. Owing to the change in 
the circuit, only one connecting line 11 is connected to the 
undercurrent/overcurrent detection circuit 13. 
FIGS. 3a to 3d show in separate diagrams the time-dependent course of the 
currents I.sub.K1 to I.sub.K4 flowing through the four glow plugs. In 
addition, in FIG. 3e, the course of the voltage U.sub.R across the 
resistor R shown in FIG. 2 is shown. Finally, it is also shown at what 
point in time the voltage is measured across the shunt or resistor. The 
voltage measurement is not required while the glow plugs are being 
switched off. This is made clear by the dotted representation. 
The operation of the apparatus is explained in more detail below with 
reference to the FIGS. 
During preheating, all the glow plugs are brought to a temperature of 
approximately 800.degree. to 1000.degree. C. For this purpose, the voltage 
supply, that is, the vehicle supply system has to deliver a high voltage. 
This causes the vehicle supply system voltage to drop considerably if all 
the glow plugs are driven at the same time. High-frequency interference 
voltages occur in the vehicle supply system during phase-displaced driving 
as described above. In the embodiments shown, the glow plugs are therefore 
driven by the microprocessor 17 with time displacement. This can be done 
by a suitable program stored in the microprocessor or by the 
microprocessor having a multistage sequential circuit which, in the 
present case, is configured as shift register 9 and shown in FIG. 1a . 
FIG. 2a corresponds to FIG. 1a but shows the apparatus thereof with only 
one measuring resistor. 
Each stage of the shift register 9 is assigned to an FET 3 which serves as 
a semiconductor switch. That is to say, the gate G of the FETs 3 is driven 
by signals from the shift register 9 in such a manner that the FETs go 
over to the conducting state and thereby connect the glow plugs RK with 
the voltage-carrying conductor 5. The FETs 3 are driven in such a manner 
that the glow plugs are sequentially switched on so rapidly that, during 
switch-on, the current rise in one glow plug is still not entirely 
completed when the next glow plug is switched on. 
In this way, a quasi-steadystate current rise is produced. 
The process of switching off the glow plugs is controlled in a 
corresponding manner, that is, before the current decrease of a glow plug 
has decayed, the next one is switched off so that a virtually continuous 
current decrease is produced. This results in a "damped" switch-off 
operation. 
The preheating operation is consequently initiated and terminated in such a 
manner that no high-frequency interference signals can be produced in the 
vehicle supply system. 
Faults in the glow plugs, for example, a short circuit or open circuit, can 
be detected by measuring the plug currents. The four resistors R connected 
in series with the FETs 3 and the internal resistances RK of the plugs 
serve this purpose according to FIG. 1. The voltage drop across the 
resistors R is measured via the measuring lines 11 by the 
undercurrent/overcurrent detection circuit 13. This circuit evaluates the 
measured values preferably with undercurrent or overcurrent comparators 
designed as individual comparators and delivers a corresponding output 
signal via the signal line 15 to the microprocessor 17. The measuring 
lines 11 may also be connected to an OR-circuit whose output signal is 
conducted to the detection circuit 13. The OR-circuit may also be 
incorporated in the detection circuit 13. 
FIG. 2 shows a simplification of the apparatus in which only a shunt or 
measuring resistor R is provided which is assigned to the parallel circuit 
of all the plugs with the FETs 3. This likewise reduces the number of 
measuring lines 11 to one. Correspondingly, only one comparator is 
provided in the detection circuit 13. 
To detect open circuits, the plugs are switched on during vehicle operation 
in sequence without heating at any desired time interval for a very short 
time, preferably for 1 ms. The current flowing through the plugs is 
measured by measuring the voltage drop across the shunt or resistor R. At 
the same time, it is not necessary to sample the voltage drops across the 
resistors R individually in the detection circuit 13 and to feed them to 
the individual comparators configured as undercurrent comparators; an 
OR-logic operation of the signals is sufficient to determine whether a 
particular current threshold has been exceeded or not. Both the 
embodiments in FIGS. 1 and 2 are suitable for the undercurrent detection. 
Because the plugs are driven by means of the microprocessor 17 via the 
control line 19, it is known which plug has just been driven. In this way, 
an open circuit, that is, an excessively low voltage or current value, can 
be assigned to a plug without an identification occurring from the 
OR-logic operation. 
During vehicle operation without glowing it is also possible to detect the 
short-circuiting of a plug by measuring the voltage drop across the 
resistor R by means of individual comparators in the detection circuit 13 
configured as overcurrent comparators. As in the case of undercurrent 
detection, the plugs are switched on sequentially at any desired time 
interval for a very short time, preferably 1 ms. Because of the known 
assignment of the driving with respect to time by the microprocessor 17, 
an OR-logic operation of the measuring signals is also sufficient in this 
case so that both exemplary embodiments can be used for overcurrent 
detection. However, a higher current threshold should be chosen in this 
case than for the undercurrent detection. 
The short-circuiting of plugs can also be detected during preheating while 
the plugs are being switched on sequentially with time displacement. Owing 
to the assignment of the switch-on process with respect to time, the 
defective plug can be identified if an overcurrent occurs. 
If short-circuiting of a plug only occurs when all the plugs have been 
switched on, an overcurrent or a short circuit can only be assigned to a 
particular plug if an individual shunt is assigned to all the plugs 
according to FIG. 1. 
If the measuring lines 11 in FIG. 1 are interconnected by an OR-element, 
the detection circuit 13 cannot detect which of the plugs is 
short-circuited. In this case, all the plugs are first switched off and in 
a time-displaced switch-on process, a determination is then made as to 
which of the plugs is defective. 
In the circuit according to FIG. 2, it is at first not possible to 
determine which of the plugs is defective if the fault occurs after all 
the plugs have been switched on. 
Here, too, all the plugs are first switched off if an overcurrent occurs 
and then the plugs are driven at any desired time interval with pulses of 
preferably 1 ms duration with only one FET 3 being brought to the 
conducting state in each case. Since it is known which branch has just 
been energized when an overcurrent occurs, the defective plug can be 
identified. 
In the embodiment of FIG. 1, instead of the resistor R which serves as 
measuring resistor, the bulk resistance of the semiconductor switch can 
also be used to measure the current flowing through the glow plugs. In 
that case, the potential present at the source electrode S has to be 
measured. Any other desired current measuring method can, however, also be 
used, for example, also Hall sensors. 
The fault detection and identification of a defective plug can be combined 
with a visual and/or acoustic fault indication. 
Defective plugs can be switched off selectively if a freely settable 
sequential circuit is used. In this way, interference in the vehicle 
supply system can be avoided without it being necessary to shut off the 
engine immediately. 
The apparatus according to FIGS. 1 and 2 are also suitable for reducing 
interference voltages. In motor vehicles high-energy interference 
voltages, for example, so-called load-dump pulses may occur which assume a 
voltage of up to 120 V over several hundred milliseconds for an internal 
resistance of 0.5 to 4 .OMEGA.. To suppress such pulses, which may result 
in the destruction of electronic control equipment, protective Zener 
diodes have been used up to now which convert the energy of the 
interference signal source into heat. Large and expensive diodes are 
necessary for this purpose. 
The energy of these interference signals can also be reduced or converted 
into heat with suitable driving via the glow plugs. 
For this purpose, the microprocessor 17 determines in any desired way 
whether a fairly high interference voltage of, for example, 50 V and over 
is present. If this is the case, one or more glow plugs are switched on 
simultaneously by a control signal delivered via the drive line 19, for 
example after 1 ms, preferably for 200 to 300 ms, to ensure the reduction 
of the dangerous energy. The parallel-connected glow plugs have a total 
resistance of approximately 100 m.OMEGA., so that the interference source 
is so heavily loaded that the interference voltage drops to a value which 
is safe for electronic control equipment. 
In this way, interference voltages can only occur for approximately 1 ms 
before the microprocessor 17 responds. These voltages can be reduced with 
substantially smaller and less expensive protective Zener diodes. 
The driving apparatus explained in more detail with reference to the 
figures can also be used, as is evident from FIG. 3, to control the power 
delivered by the glow plugs. When the glow plugs are switched on 
sequentially, the voltage dropping across the shunt or resistor R (compare 
with FIG. 2) common to all the plugs is measured. The sequential driving 
of the plugs can be seen in FIG. 3 from the variation with time of the 
currents I.sub.K1 to I.sub.K4 assigned to the individual plugs. Since a 
common shunt is assigned to all the plugs, the voltage U.sub.R dropping 
across this resistor R, whose variation with time is also shown in FIG. 3, 
is proportional to the total current. According to FIG. 3, the measurement 
of the voltage is shown in a separate diagram. 
The instantaneous electrical power associated with each individual plug is 
calculated with the aid of the microprocessor 17 from the voltage changes 
corresponding to the particular plug current and from the instantaneous 
operating voltage. 
A predetermined mean power can be set on the basis of this calculation for 
each individual plug. This takes place because the switch-on time can be 
lengthened or shortened by .DELTA.t. In FIG. 3, the switch-on time of 
I.sub.K2 is shortened and that of I.sub.K3 is lengthened. In this way, 
variations in the tolerances of the plugs, which may lead to the current 
level varying by .DELTA.I, can be compensated for, as can the variations 
in the vehicle supply system voltage and different cylinder performance. 
Finally, it should further be pointed out that the driving apparatus 
described can also be used for controlling the temperature of the glow 
plugs. For this purpose, for example, temperature-dependent resistors 
whose measurement signals are fed to the microprocessor 17 are assigned to 
the glow plugs. The microprocessor 17 then drives the glow plugs with 
short switch-on pulses approximately 1 s long in order to maintain the 
desired temperature.