Abstract:
In a hand-held electric power tool, a combination including an electromotor, and an electric circuit for controlling the speed of the motor, including a pressure switch for controlling the motor speed, a first control potentiometer operatively associated with said pressure switch for adjusting the motor speed from zero up to a predetermined motor operating speed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a speed control circuit for the motor of an electric power tool, particularly a hand-held electric drill with an adjustable drilling speed. Various types of drilling machines are already known in which the fixed drilling speed is maintained independent of the load by means of an electronic regulating circuit. In a second switch position such circuits switch the motor over to the full operating speed. With such switches, a sudden variation or change in rotational speed is possible, perhaps resulting in a sudden reversal of the rotation force, occurring while the user is completely unaware and unprepared for any change, and consequently the machine could even be torn from his hands by the counter-rotational moment. Such a sudden reaction of the drilling machine to a change in direction could result in a high risk of damage or breakage to the mechanism, or even of injury to the user. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a drilling machine which overcomes the disadvantages of the prior art. 
     Another object of the invention is to provide an electric drill in which the drilling speed is variable by the user in a simple manner without hampering the operation. 
     Another object of the invention is to provide a control circuit for an electric drill which permits the drilling speed to be adjusted stepwise from zero up to a preselected operating speed by means of a control potentiometer, with the drilling speed being controlled by a pressure-sensitive switch. Thus whenever the motor is turned on, for every selected operating speed, full operating speed could be reached by a gentle increase in speed of the motor, so that the sudden, full impact of the counter-rotational moment would be avoided when the drill is first switched on. 
     The invention provides that the drilling speed may be adjustable from zero up to a predetermined operating speed by means of a pressure switch. The predetermined operating speed is set on a control potentiometer. Thus with the switching on of the motor, a gradual increase of speed is possible, without encountering the large counter-rotational reaction force characteristic of prior-art devices when the motor is started at its full operational speed. Preferably the drilling speed can be continuously adjustable by means of the pressure switch being connected to a slidable wiper of a potentiometer. 
     According to the present invention, the voltage to the motor of the drilling machine is controlled by an electrical circuit including a thyristor and two field coils. The thyristor is connected in series with the rotor of the motor in the primary circuit, which is in turn connected to the wiper of the control potentiometer in a voltage divider circuit. It is expedient if the wiper of the control potentiometer is connected to a second potentiometer whose wiper in turn is connected to the control electrode of the thyristor. 
     The construction of the circuit is particularly simple if the voltage divider circuit is a series connection of a diode with a first fixed resistor, the control potentiometer, and a second fixed resistor. Parallel to the first fixed resistor and the control potentiometer is a condenser. Furthermore, parallel to the first fixed resistor and the control potentiometer through the slide of the control potentiometer is the slide or wiper of the second potentiometer is then connected to the thyristor as noted above. An accurate speed control of the motor is achieved if this slide of the potentiometer is connected through two diodes with the control electrode of the thyristor. Between both diodes a condenser is connected with the output, or the cathode, of the thyristor. 
     For full high-speed operation, the control potentiometer is shifted to its end position, thereby short-circuiting the thyristor. Thus the full voltage, without any phase change, will flow through the motor. Through the arrangement of a short-circuiting contact on the control potentiometer the motor will reach the highest operating speed without preselection on the control potentiometer, so that operational error is largely prevented. At the same time the predetermined position of the control potentiometer is easily usable, so that the usually provided on-off switch for controlling operation of the motor may be eliminated. 
     Using an impulse or spring-free pressure switch, one may preferably provide a switch having three contacts which may be connected by means of a bridge. The bridge operates by means of the pressure switch so that in the first or off position all three contacts are seperated from each other, in the second position the first and the middle contact are connected with one another, and in the last position all three contacts are connected with one another. The first contact is electrically connected to a field coil, and, in turn, to one of the poles of the voltage source; the middle contact is connected with a voltage divider circuit, and the end contact is connected to a switch for short-circuiting the thyristor. In a simple manner it is possible to connect the pressure switch with the wiper of the potentiometer in a mechanical coupling, so that the moving of the wiper to the end position will correspond to the closing of the short-circuiting switch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The single FIGURE shows a highly simplified schematic diagram of the control circuit for an electric drill according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The single FIGURE indicates poles 1 and 2 of a voltage source (not shown) connected to field coils 3 and 4 respectively, and thereby to a pressure switch 5 which connects to a primary circuit 6 and a potentiometer circuit 7. The pressure switch 5 comprises a first contact 8, a middle contact 9, and an end contact 10, which are connected together by means of a conducting bridge 11. In the first position of the pressure switch 5 the contacts 8, 9 and 10 are disconnected from one another; in the second position the first contact 8 and the middle contact 9 are connected to one another; and in the third position all three contacts 8, 9 and 10 are connected with one another. The pressure switch 5 is operated by means of pressure switch in the conventional pistol grip of the hand-held electric drill. (not shown). 
     In the primary circuit 6, the middle contact 9 of the pressure switch is connected through a thyristor 12 to a brush 13, which in turn contacts the rotor 14, and thereby another brush 15 of the operational motor of the drill. The brush 15 is in turn connected to the field coil 4. At the same time the third contact 10 of the pressure switch 5 is connected to a normally open switch 16, which is in turn connected to the output 17 of the thyristor 12, so that in the third position of the pressure switch 5 the contact 8 is connected with the control 10, the switch 16 is closed, the thyrister 12 is short circuited, and the circuit is completed from pole 1 through the field coil 3, the brush 13, the rotor 14, the brush 15 and field coil 4 directly to the other pole 2, so that the motor is driven at maximum power. 
     The regulation of the rotational speed is controlled by a potentiometer 7 which is connected with the control electrode 18 of the thyristor 12. The phase angle control of the thyristor 12 results from the equalization of the back electromotive force of the rotor 14. The potentiometer circuit 7 comprises a diode 19 in series connection with a first fixed resistor 20, a variable resistor 21, and a second fixed resistor 22, connected between the field coil 4 and the middle contact 9. Between the first fixed resistor 20 and the potentiometer 21 a condenser 23 is connected in parallel. Between the wiper or tap 24 of the potentiometer 21 and the diode 19 another potentiometer 25 is connected, whose tap 26 is connected via two diodes 27 and 28 with the control electrode 18 of the thyristor 12. Between the two diodes 27 and 28 a condenser 29 is connected to the output 17 of the thyristor 12. 
     The control potentiometer 21 is mechanically coupled with the switch 16, so that in one end position of the wiper 24 the switch 16 is open, and in the other end position of the wiper 24 of the potentiometer 21 the switch 16 is closed, and the thyristor 12 is short circuited. Likewise the wiper 26 of the potentiometer 25 is mechanically connected with the bridge 11 of the pressure switch 5, so that the third position of the pressure switch 5 corresponds to the end position of the wiper 26 of the potentiometer 25. 
     By sliding the bridge 11 of the pressure switch 5 the contacts 8 and 9 will be connected with one another, so that the current can flow both through the potentiometer circuit 7 and the primary circuit 6. The firing voltage for the control electrode 18 of the thyristor 12 will therefore in a known manner equalize the reference voltage from the potentiometer circuit 7 with the back electromotive force from the rotor 14. With increasing pressure on the pressure switch 5 the potentiometer 25 will increase the reference voltage according to the position of the bridge 11, adjustable up to the end position of the full voltage range of the potentiometer 25, and therefore finally attaining the full voltage at the end position. From this it follows that an increase in rotational speed of the rotor 14 is gradual and is not abrupt as the pressure switch 5 gradually increases the voltage through the potentiometer 25. The wiper 26 of the potentiometer 25 is ajustable for every point of the control potentiometer 21 which may be selected. The control potentiometer 21 is especially sensitive and may be adjusted to low operating speeds, and may be controlled by the position or pressure on the pressure switch 5, so that the rotational speed is adjustable between zero and the desired operating speed. 
     With switch 16 closed, the shaft rotation will result according to the position of the pressure trigger of the pressure switch 5, with a slow increase of rotational speed. In the end position of switch 5, with all three contacts 8 through 10 connected with one another, the thyristor 12 is short-circuited and a direct connection to the motor is made, so that no sudden changes in rotational speed and therefore sudden and unavoidable reactive rotational forces can occur. 
     It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of controrl circuits for electric drills differing from the types described above. 
     While the invention has been illustrated and described as embodied in electric drill speed control, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.