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.