Circuit arrangement for controlling the electromagnetic drive of a switching device

A reduction in contact bounce in an electromagnetic switch is accomplished by optimizing armature speed over its travel path with a circuit arrangement for controlling a drive current in the coil of the electromagnetic switch. A superposed speed loop including a speed sensor produces a measured voltage in response to speed of the armature. A converter coupled to the speed sensor converts the measured voltage into a value corresponding to an actual speed of the armature. A first summer receives a constant reference value corresponding to a desired speed for the armature and the value corresponding to the actual speed of the armature, and produces a difference voltage corresponding to a difference between the desired speed and the actual speed of the armature. A proportional element amplifies the difference voltage and produces a desired current value corresponding to the amplified difference voltage. An underlying current control loop including a current sensor produces a measured current value corresponding to a current in the coil. A second summer receiving the measured current value and the desired current value produces an output current corresponding to a difference between the desired current value and the measured current value. A chopper coupled to the output current of the second summer operates with hysteresis for conducting a pulsed control voltage to the coil and is interrupted from doing so when the measured current value is greater than the desired current value plus an hysteresis value.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application claims priority with respect to application No. P 44 30 
867.1 filed in Germany on Aug. 31, 1994, the disclosure of which is 
incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
The invention relates to an electromagnetic switching device, such as a 
contactor, solenoid, or relay, in which an armature moves in response to a 
drive current in a coil and in particular to a circuit arrangement for 
controlling the drive current in the coil for reducing contact bounce of a 
contact member attached to the armature. 
Electromagnetic switching devices are used in automation and drive 
technology, where they serve, for example, as relays which in cooperation 
with other components to ensure safe control of different electrical 
devices. For optimum adaptation of these switching devices to their 
switching task, while taking into consideration different operating 
conditions and specific device characteristics, it can be desirable to 
adhere to a predetermined speed/distance profile of the contact movement. 
In this way, special switching principles can be applied so that the 
contact bounce at the time of actuation can be minimized. This leads to a 
reduction in burn-up and mechanical wear of the contact, which can be 
translated into an increase in service life and/or maximum switching 
capability of the device. The more successfully the necessary ideal course 
of the speed of the switching device is assured over the course of the 
contact travel, the less wear takes place and the better the adaptation of 
the device to the switching task. This type of speed/distance profile for 
reducing bounce can essentially be attributed an optimum speed during the 
making of contact and a reduction in speed when the core halves impact. 
This optimum speed during the making of contact is usually smaller than 
the speed of the uncontrolled switching device, which varies in a wide 
range. The increase of the contact travel up to the making of contact due 
to burn-up is a particular hindrance, because the ideal course of the 
speed/distance characteristic curve is consequently changed over the 
service life of the switching device. 
A reduction in bounce can partially be accomplished by a better matchup 
between the contact, transmission and drive systems of the switching 
device. This matchup is only possible for certain conditions, mostly 
nominal or rated operating conditions, but not for the whole range of 
allowed conditions. In contrast, maintaining a certain speed/distance 
profile assures a reduction of bounce under all acceptable conditions of 
use over the entire service life of the switching device, with the 
consideration of the manufacturing tolerances of the device. The effective 
maintenance of this ideal curve can be realized by circuit arrangements 
that are suitable for controlling the course of movement. 
European patent application No. EP 0 376 493 A1 discloses a control circuit 
with which the movement process of electromagnetic relays is influenced in 
order to reduce the incidence of bounce. In this case, a very high current 
is permitted in the first phase of movement for the purpose of rapid 
acceleration. Before the relay is closed, the current is reduced to a 
relatively small value, and the speed of movement of the armature/contact 
correspondingly assumes a smaller value, which leads to reduced bouncing. 
The objective of the known circuit arrangements for electronic switching 
drives is to reduce armature speed, without a special contact-making speed 
optimized to minimum bounce being achieved at the same time. Further, only 
fluctuations in the control voltage and, to a certain extent, the 
temperature, are compensated or taken into consideration. Likewise, 
disturbances of desired armature motion such as burn-up, friction and 
tolerances are not considered. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a circuit arrangement 
for controlling the drive of an electromagnetic switching device, by means 
of which the maintenance of optimum contact-making speeds and the 
limitation of the armature core impact speed are assured, with the 
simplest means, over the entire service life of the switching device, and 
in spite of disturbances caused by burn-up, friction and tolerance the 
permissible ranges for control voltage and temperature are even expanded, 
and greater tolerances can be permitted. 
The above and other objects are accomplished according to the invention by 
the provision of a circuit arrangement for controlling a drive current in 
a coil of an electromagnetic switching device having an armature that 
moves in dependence of the drive current, including: a superposed speed 
loop including a speed sensor for producing a measured voltage in response 
to speed of the armature; a converter coupled to the speed sensor for 
converting the measured voltage into a value corresponding to an actual 
speed of the armature; a first summer receiving a constant reference value 
corresponding to a desired speed for the armature and the value 
corresponding to the actual speed of armature, and producing a difference 
voltage corresponding to a difference between the actual speed and the 
desired speed of the armature; a proportional element for amplifying the 
difference voltage and producing a desired current value corresponding to 
the amplified difference voltage; an underlying current control loop 
including a current sensor for producing a measured current value 
corresponding to an actual current in the coil; a second summer receiving 
the measured current value and the desired current value and producing an 
output current corresponding to a difference between the desired current 
value and the measured current value; and a chopper, operating with 
hysteresis, coupled to the output current of the second summer for 
conducting a pulsed control voltage to the coil when the measured current 
value is greater than the desired current value plus an hysteresis value. 
The circuit arrangement of the invention can be used in electromagnetic 
switching devices that are operated both with direct and alternating 
current. Furthermore, their effectiveness is independent of the turn-on 
phase position of the control voltage, and the switching process begins 
without delay initiated by a control circuit, so the closing delay time is 
scarcely increased compared to a non-controlled switching device. 
The circuit arrangement is distinguished by a simple design, which does not 
require a memory for desired curves or a microcontroller for controlling 
the drive. The use of a simple speed sensor also permits suppression of 
the influence of disturbances such as fluctuations of control voltage, 
burn-up of the contacts, temperature, friction and/or assembly and 
manufacturing tolerances, within a wide range. 
Further advantageous embodiments and features of the invention will become 
apparent from the following detailed description and the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a block diagram of a circuit 
arrangement for controlling the movement of an armature 1 in an 
electromagnetic switching device, not shown in detail, particularly in a 
contactor, solenoid, or relay having a coil 3, which is connected to a 
chopper 19 for generating pulsed control voltages. A superposed speed loop 
is provided which includes a speed sensor 7 that measures the speed of 
armature 1 and supplies a measuring voltage V.sub.m proportional to speed 
to a converter 9. Speed sensor 7 can have a variety of configurations, for 
example, it can be inductive or optical, as will be appreciated by those 
skilled in the art. The measurement voltage from sensor 7 is converted in 
converter 9 into voltage a V.sub.a value that corresponds to actual speed 
of the armature, and is fed to a summing device 11 for determining a 
difference between the actual armature speed V.sub.a and a desired speed 
value v.sub.d fed to the positive input of summing device 11 as a constant 
reference value. 
This desired speed value v.sub.d is a desired value that remains constant 
during the entire control process. Its value corresponds approximately to 
the desired armature speed at a time that contact is made. 
An output signal from summing device 11 that corresponds to a difference 
voltage .DELTA.v is then conducted to a proportional element 13 for 
conversion and amplification in order to form a desired current value 
I.sub.d. The signals of the desired current value I.sub.d and a measured 
current value I.sub.m in coil 3 are fed to a summing device 15, in which 
the difference current .DELTA.I between the desired current value I.sub.d 
and the measured current value I.sub.m is determined. The measured current 
value I.sub.m results from the measured voltage determined, for example, 
by means of a measuring resistor 17. 
With a positive current control deviation .DELTA.I=I.sub.d -I.sub.m, i.e., 
the desired value of the current is greater than the measured current 
value, chopper 19 is closed and a rectified supply voltage is conducted 
from a full-wave rectifier 18 to coil 3. 
With a negative control deviation .DELTA.I, the operation of chopper 19 is 
interrupted, and the coil current then flows via the measuring resistor 17 
and a free-wheeling circuit having a free-wheeling diode 21 as better 
illustrated in FIG. 2 discussed below. Thus, the current in the coil 3 is 
maintained up to the next turn-on pulse of the chopper 19. The full-wave 
rectifier 18 can be charged with direct or alternating current. 
In an advantageous embodiment, chopper 19 operates with hysteresis. For 
this purpose, chopper 19 does not interrupt the circuit until the measured 
current value I.sub.m lies above a desired value by a fixed hysteresis 
value I.sub.Hysteresis. The underlying current control loop can be used in 
connection with chopper 19 operating with hysteresis for holding pulses 
after the pick-up process in that a fixed holding current limiting value 
is fed to the summing device 15. Switching from derived current value 
I.sub.d to such a constant holding current is advantageously carried out 
by means of a constant time element for the change-over-time whose time 
constant is clearly greater than the maximum possible total closing time. 
In accordance with the invention, a superposed speed-control loop and a 
dynamically faster, underlying current-control loop form a circuit 
arrangement for an electro-magnetic switching device, with which a 
reduction in contact bounce and thus a reduction in burn-up is 
accomplished by an optimum contact-making speed and a limited armature 
core impact speed. This lengthens the service life of the switching device 
and/or increases switching capability, while the speeds under the 
influence of fluctuations in control voltage, permissible ambient 
temperatures, tolerances, contact burn-up and friction are held relatively 
constant for the duration of use. 
FIG. 2 shows a circuit schematic for implementing the block diagram in FIG. 
1. A subtracter 23 is provided that forms a difference between the desired 
speed value V.sub.d and the actual speed value V.sub.a resulting from the 
measured speed V.sub.m measured with speed sensor 7 according to FIG. 1. 
The desired speed value is proportional to a reference voltage value 
V.sub.Ref which remains constant. The speed difference is amplified in an 
operational amplifier 12 by the resistance ratio R.sub.N /R.sub.V of 
resistors 25, 27, 29, 31, so that the desired value U.sub.i-des for the 
current is present at the output of subtracter 23. A possibly necessary 
calibration factor of the speed sensor can also be considered in the 
amplification of subtracter 23. The desired value of the current is fed to 
a comparator 16 as a reference or threshold value. As long as the measured 
value of the current U.sub.i-means is less than the reference value, a 
high potential is present at the output of comparator 16. An n-channel 
power MOSFET 39 is controlled by a charge pump 37 for conducting current 
from full-bridge rectifier 18 to coil 3. As soon as the measured value 
U.sub.i-mess becomes greater than the reference value U.sub.i-des plus a 
switching hysteresis that can be adjusted by means of a resistor 33 
connected in parallel by way of the comparator 16, a low potential is 
present at the output of the comparator 16, and the semiconductor switch 
20 is blocked. The current of the coil 3 then flows via the free-wheeling 
diode 21. The semiconductor switch 20 can also comprise a p-channel power 
MOSFET. 
FIGS. 3a-3c illustrate a pick-up process controlled in accordance with the 
invention, in which the time units are the same in each figure. FIG. 3a 
shows the temporal course of the pulsed control voltage, wherein the 
control voltage is a rectified AC voltage which is controllably 
interrupted by semiconductor switch 20 in accordance with the invention. 
FIG. 3b shows the constant desired value for speed and the actual value 
for speed during the pick-up process. The times at which contact is made 
and of impact of armature cores, as the core halves are closed, are shown. 
The desired and measured values for the current are illustrated in FIG. 
3c. The desired value of the current results from the difference between 
the desired and actual speed, which can be seen in FIG. 3b, and is 
amplified by a factor K. Only when the speed of the armature approximates 
its desired value, and the speed difference is thus small enough, is the 
control supply voltage shut off by the chopper 19. Up to this point, the 
available energy is consumed completely in order to accelerate the 
armature. Consequently, an advantage of the circuit arrangement of the 
invention is the shortest possible pick-up times and, as a function of the 
switching hysteresis, only a few switching cycles. This low switching 
frequency leads to good EMC (Electromagnetic Compatibility) properties and 
a lower stress on the semiconductor components. 
FIG. 4 illustrates three speed curves of the armature under special 
conditions. The dashed line 3 shows the worst case at maximum excess 
energy, where the highest control voltage, the lowest temperature, the 
least friction, the least load spring force and the smallest air gap 
during the making of contact at maximum burn-up are present. The opposite 
extreme case, at minimum energy for pick-up, is represented by the solid 
line 1. The speed curve under normal conditions (when the device is new 
and operating under nominal conditions) is represented by the dotted line 
2. The more excess energy that is available, the sooner the pick-up 
process is completed. The speeds, particularly at the time contact is 
made, deviate only slightly from one another because of the circuit 
arrangement according to the invention. 
In a modification of the foregoing, the superposed speed control loop and 
the underlying current control loop may be realized, in part, by 
algorithms in a microprocessor. 
FIG. 5 shows a microprocessor 43 with at least two 
analog-digital-converters for measured speed V.sub.measure and measured 
current I.sub.measure. The current through the coil is measured by a 
contactless current transducer 17. The function of the superposed 
speed-control-loop and the underlying current-control-loop are converted 
into algorithms. A digital output of the microprocessor charges an 
optocoupler 41 which controls the semiconductor switch. This switch is for 
example carried out as an charge pump 37 and a n-channel power MOSFET 39. 
The invention has been described in detail with respect to preferred 
embodiments, and it will now be apparent from the foregoing to those 
skilled in the art that changes and modifications may be made without 
departing from the invention in its broader aspects, and the invention, 
therefore, as defined in the appended claims is intended to cover all such 
changes and modifications as fall within the true spirit of the invention.