Method and device for enlarging a stoppage safety function for electric motors

In order to obtain a stoppage safety function for electric motors, in particular such intended for driving power-driven screwing machines with intermediate stepping-down gears, it is proposed that the system be prevented from dropping once more below the cut-off threshold determined by the control circuit (U211B) once the electric motor has been switched-off for the first time due to the stoppage-safety function, in that an electric storage (C1) is provided for holding the stored voltage even when, after the system has been switched off, a new tachogenerator signal should be produced due to a momentary return motion resulting from the mechanical release and the stresses that have built up in the mechanical drive train.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and/or a device permitting, in 
connection with the control of electric motors, to extend to a certain 
degree, in the sense of improving, the stoppage safety function of such 
motors. 
The invention is based on a monolithic integrated circuit which has long 
been known, corresponding for example to a U211B circuit chip of the type 
produced and sold by the Telefunken Company, which chip is used especially 
in modern phase control systems employed for controlling electric motors 
of any application. 
So, it has been known for example (DE-OS 32, 22, 065) to equip an electric 
hand-held tool, especially a hand-held drilling or screwing machine, with 
a torque-limiting feature for the speed control, and to effect a speed 
adjustment in connection therewith. The torque-limiting feature acts in 
one sense of rotation only, for example in the screwing-in sense, and is 
automatically switched off in the other sense of rotation. 
It has further been known to provide an electric motor of a hand-held tool 
intended for tightening nuts (U.S. Pat. No. 3,892,146) with an electric 
drive circuit which drives the motor, for the purpose of tightening of the 
nut, until the armature current of the motor exceeds a predetermined 
value; the drive circuit responds when this threshold value is exceeded 
and reverses the sense of rotation of the motor, thereby releasing the nut 
via an adapter. 
In the case of another known power-driven screwing machine with 
torque-limiting feature (EU-OS 0,187,353; DE-PS 35,00,714) the electric 
motor is driven by means of a phase control comprising a setting 
potentiometer for the desired torque, with a usual triac being connected 
into the load circuit of the electric motor, whose angle of current flow 
is released by the phase control in response to the setting. 
As has been mentioned before, such known phase control systems are always 
provided with incorporated speed safety functions which will respond when 
the electric motor is further excited, when it is already in a stopped or 
almost stopped condition under load. 
The before-mentioned phase control chip model U211B from Telefunken, for 
example, is equipped with an input connection (pin 8) through which a 
frequency-to-voltage converter can be controlled. Due to a corresponding 
internal wiring system, each drive signal will result in recharging 
conditions at this input connection, which may be connected for example to 
a tachogenerator of the electric machine being monitored and driven, which 
recharging conditions are utilized for releasing the excitation of the 
triac. 
To this end, an additional input connection (pin 18) is provided which 
offers a pulse-blocking feature which becomes effective when no recharging 
conditions are encountered any more in the frequency-to-voltage converter, 
as a result of incoming speed-responsive pulses. In this case, a capacitor 
connected to the input connection, corresponding to pin 18, is charged up, 
via a resistor, until a cut-off threshold is reached at which point the 
triac triggering pulses will be interrupted automatically by the control 
module corresponding to U211B. 
In the case of such a circuit, which has been known as such and also in 
connection with driving systems for electric motors, considerable problems 
may be encountered in special applications, as a result of the 
tacho-monitoring and speed stoppage function, as follows: 
As has been mentioned before, the output signal blocking function will not 
become active due to recharging processes in the f/v converter, the 
control circuit U211B being continuously supplied with speed-responsive 
pulses of any shape and magnitude. The ordinary and, insofar, welcome 
cut-off function (i.e. interruption of the triac triggering pulses) must 
however lead to faulty operation when such an electric motor is used for 
driving a power-driven screwing machine, which sometimes performs screwing 
operations at extremely high torques and with the aid of a greatly 
stepped-down intermediate transmission. Until the electric motor of such a 
power-driven screwing machine, in particular if the latter is designed for 
high and extremely high torques, is switched off by the speed-responsive 
stoppage monitoring function, the entire transmission train will be 
subjected to considerable stresses, including torsion of shafts, or the 
like, so that when the motor current is switched off via the triac of the 
phase control in the control module U211B, which is used here as control 
circuit by way of example, a certain return motion will occur in the 
transmission and motor area, and this will of course give rise again to 
the generation of new tachogenerator pulses by the existing tachogenerator 
or other means generating speed-responsive pulses. 
Now, it is exactly one of the properties of the before-mentioned control 
module, and practically of any other modern phase control circuit as well, 
that when speed-responsive pulses are received, certain processes, in the 
present case the recharging processes in the converter will be resumed and 
the values will drop below the cut-out threshold so that the electric 
motor will be driven again at full motor current. Consequently, the motor 
will be started again and run up as far as possible. But given the fact 
that the screw had been tightened before, the motor will be blocked and 
stopped again so that no speed-responsive pulses will be received any more 
and the circuit will switch the system off; but the high mechanical 
stresses, which are again encountered, result again in a return motion so 
that the whole system gets into pulsating operation. However, such a 
pulsating behavior is particularly undesirable for the power-driven 
screwing machines discussed here as it results in just that type of 
malfunction which was to be excluded by the torque-detection and setting 
features. In fact, all electronic control modules containing modern phase 
control systems are designed in such a way as to permit presetting of a 
torque so that when a given, maybe even very high torque has been selected 
power-driven screwing machines are capable of performing screwing 
operations perfectly, in particular smoothly, until the maximum torque has 
been reached, without there occurring any torque peaks, abrupt vibrations, 
or other disturbances. However, the malfunction just described, which is 
the result of the speed monitoring in the stopped condition, leads to 
quite the contrary condition so that it is no longer possible to preset 
defined torques, the mechanical kinematics in the system giving rise to 
sudden, abrupt torque peaks when the system starts running again, 
following a return motion, which torque peaks have particularly 
undesirable effects on the part to be screwed in or to be tightened, due 
to the high stepping-up ratio. 
Now, it is the object of the present invention to remedy these 
disadvantages and to extend the stoppage safety function, in particular 
for an electric motor used in power-driven screwing machines, in such a 
way that the electric motor will not start running up again, i.e. will not 
be fully switched on by the existing monolithic integrated phase control 
circuit, even when, after the system has been switched off for the first 
time, stresses produced in the system will give rise to new 
speed-responsive pulses, during the return motion, due to a reversing 
process. 
SUMMARY OF THE INVENTION 
The invention solves this object and offers the advantage that when the 
electric motor driving the power-driven screwing machine has been switched 
off for the first time because the electric motor has come or almost come 
to a stoppage, repeated switching-on will be impossible in the event the 
release of stresses in the mechanical components participating in the 
screwing operation give rise to a return motion of the electric motor and, 
consequently, to the generation of new tachogenerator pulses. 
Another advantage of the invention is seen in the fact that the 
switched-off condition of the electric motor will be maintained under all 
circumstances so long as the entire circuit is connected to mains voltage; 
to say it in other words: The invention provides simultaneously a locking 
function which will be released only when the machine is switched off by 
an operator, i.e. disconnected from the mains, and switched on again, for 
example for the purpose of carrying out another screwing operation at a 
different place. 
In this connection, the decisive aspect is seen in the fact that the 
invention does not use an approach proposing to change the existing 
kinematic and mechanical conditions, which would necessarily require a 
much greater input, and does not--this solution would also be 
possible--seek to prevent any return motion so that no tachogenerator 
pulses can be produced under this aspect, but takes recourse instead to an 
electric switching function which is ensured by suitable external circuit 
elements of the U211B circuit chip used, which ensures that the output 
pulse blocking condition rendered possible by the circuit chip and 
occurring when the recharging processes in the converter have for the 
first time ceased to appear, will be maintained, and this even in the 
event such recharging processes should start again at some later time due 
to tachogenerator pulses or signals being received. 
According to a particularly advantageous embodiment of the invention, the 
disconnection and locking module is implemented in the form of a bistable 
flip-flop which, when the device is switched on, occupies a first initial 
position in which a stoppage safety function provided by the phase control 
chip can become active, but which will then change over to its second 
locking condition that, once the circuit has responded and interrupted the 
current supply to the motor because of a detected no-speed condition, can 
be released only by disconnecting the supply voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It is the basic idea of the present invention to ensure for a known IC 
drive chip in the form of a phase control for electric motors, and here 
especially of the U211B control module from Telefunken, that in the case 
of the particular application of a power-driven screwing machine, a return 
movement provoked by mechanical stresses in the mechanical transmission 
train cannot lead to repeated switching-on of the electric motor. This is 
achieved by interrupting this protective function on the IC circuit by 
connecting the latter to a predetermined potential (frame potential), and 
locking the circuit simultaneously in that position. 
As can be seen in the drawing, the motor M is connected in series to the 
switching member, a TR1 triac, as usual with phase control circuits. The 
connection to the two mains terminals N1, N2 is completed by a current 
sensor resistor R1 whose output is connected to the input 14 of the U211B 
circuit, which comprises the entire control chip. 
In addition to that part of the arrangement which is of interest for the 
invention and which is marked by the dashed line, the drawing illustrates 
only a few additional elements required for the understanding of the 
invention. Most of the remaining connections (pin 1 to pin 18) of the 
control module U211B are shown without the external circuit elements as 
these areas are no part of the present invention and, besides, any man 
skilled in the art will be capable, without any problem, of finding the 
necessary circuit components, and the recommended handling of this IC 
control module as well, in the published literature, based on the detailed 
description of the Telefunken U211B IC control circuit. 
The essential point of the invention lies in the fact that the output of a 
tachometer device T is connected, maybe via an RC member consisting of R 
and C, to the input pin 8 of the IC circuit which, internally, has 
assigned to its input connection corresponding to pin 8 an input 
frequency/voltage converter (f/v converter) which discharges a capacitor 
C1 every time a speed-responsive signal is received from the tachometer 
device T, the capacitor C1 coacting with a resistor R1 and forming with 
the latter an RC element which is connected across the connection inputs 
corresponding to pin 16 and pin 18. The IC connection pin 16 carries a 
constant voltage potential relative to which the capacitor C1 is then 
charged up, i.e. in the described particular embodiment to positive 
voltage, when recharging processes resulting from incoming 
speed-responsive signals are no longer encountered in the f/v converter. 
The system then operates, via the connection pin 18 of the U211B IC 
circuit, in the manner known as such: When the discharging processes cease 
to occur, the capacitor C will exceed a cut-off threshold value, as has 
been described before, and the further supply of triac triggering pulses 
from the connection pin 4 via the resistor R2 to the triac TR1 will be 
interrupted so that the triac will block the current supply to the motor M 
and the latter will be made dead. 
In addition, a further active circuit component corresponding to TR4, which 
may be a transistor, supplies to the connection corresponding to pin 12 of 
the U211B IC circuit a control input signal for the phase control which 
permits to determine the torque to be produced by the electric motor M at 
any time. Arrangements of this type are well known in the art so that they 
need not be described here in more detail. 
The circuit arrangement described heretofore then operates basically in 
such a way that when voltage is applied to the electric motor, the latter 
will start running and, if the motor is used to drive a screwing machine, 
tighten a screw with a torque threshold value which has been predetermined 
via the circuit element TR4. In the further course of this operation, the 
electric motor M will necessarily come to stop, as the screw can of course 
be screwed in only a given length and will be definitely screwed home, at 
a predetermined point in time, giving regard to a predetermined torque 
threshold value. These conditions will result in gradual blocking of the 
motor, through the stepping-down gear, which means that the motor will 
rotate at ever decreasing speed until, when the motor has come to stop or 
nearly come to stop, the speed-responsive pulses supplied by the 
tachometer device T, in some way or other, will cease to appear at the 
connection pin 8 of the IC circuit and the capacitor C1 will no longer be 
discharged via the connection pin 18. The potential rising at the 
capacitor C1 then interrupts, via the operation of the IC circuit, the 
further activation of the motor by rendering the triac TR1 dead. 
According to one feature of the present invention, the capacitor C1 coacts 
with a circuit which ensures quite generally that once the electric motor 
M has been switched off for the first time, this capacitor is prevented 
from discharging once more and, consequently, from dropping below the 
cut-off threshold set internally in the IC chip, as this would lead to 
repeated triggering of the triac and restarting of the motor. 
The circuit coacting with the capacitor C1 comprises two semiconductor 
circuit elements connected in the way of a bistable flip-flop, namely 
transistors TR2 and TR3; the latter are connected to the constant voltage 
line S1 via their interconnected emitters and a common emitter resistor 
R3, and have their other main electrode connections (collectors) connected 
to a common frame potential line S2 which, in the present case, carries 
positive potential, which is worth noting for the better understanding of 
the circuit. 
There is further provided a nominal value voltage divider circuit 
consisting of the resistors R4 and R5, which is connected to the two 
voltage lines S1 and S2 and which supplies to the one transistor TR2 of 
the flip-flop K, at the junction P1, a biassing potential (nominal value) 
which, during normal operation, is always above an actual-value potential 
present at the capacitor C1 (junction P2) which latter, being continuously 
discharged, remains at a potential below the before-mentioned cut-off 
threshold potential, in spite of being continuously charged up via the 
series resistor R1, as has already been explained in full detail. The 
transistor TR2 of the flip-flop K is then conductive and keeps its 
parallel transistor TR3 blocked via the common coupling resistor R3. The 
flip-flop, therefore, acts in the manner of a comparator which changes 
over at the moment when the potential at the junction P2, i.e. the 
potential across the capacitor C1, gets more positive than the potential 
present at the junction P1, because of the non-appearance of discharging 
pulses produced by the tachogenerator device; to say it in other 
words--the electric motor comes to stop and will be switched off as soon 
as the cut-off threshold is exceeded at the connection pin 18. In this 
case, the flip-flop K will assume its second, simultaneously blocked 
state, and the transistor TR2 will be disabled; the transistor TR3 becomes 
conductive and pulls the energizing potential of another transistor TR5 
down, via its collector resistor R6, so that TR5 becomes conductive as 
well and the junction P2, i.e. the potential at the connection pin 18, is 
practically connected to frame potential (full positive voltage). It is 
thus ensured under all circumstances that even if a return motion should 
occur and tachogenerator pulses should arrive again, the capacitor C1 
cannot be discharged below the cut-off threshold so that the electric 
motor M will remain disconnected and a perfectly smooth screwing 
operation, free from shocks, will be ensured, without there being any risk 
that undesirable torque peaks and a pulsating or oscillating behavior of 
the system may develop. 
Given the fact that the through-connection of the transistor TR5 ensures 
that the junction P2 will always remain above the potential at the 
junction P1, which has been divided to R4/R5 by the voltage divider, this 
circuit also assumes a blocked condition which can be released only when 
the mains voltage is switched off, i.e. when the motor is disconnected 
from the mains. 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained and, 
since certain changes may be made in carrying out the above method and in 
the constructions set forth, without departing from the spirit and scope 
of the invention, it is intended that all matter contained in the above 
description and shown in the accompanying drawings shall be interpreted as 
illustrative and not in a limiting sense.