METHOD AND DEVICE FOR CONTROLLING AN ELECTRIC MOTOR

Abstract: A method and a device for controlling an electric motor arranged to serve as drive source for electrically driven hand tools or stationary machines, which during operation are made subject to varying loads. According to the invention are limit values, determined by experience and/or calculations, relating to supplied power with regard to different rotary speeds stored in one or a number of memory circuits and compared to present actual rotary speed and supplied power with use of a microprocessor. An output signal from the microprocessor is used to adjust supplied power within preset limits for each rotary speed as compensation for reductions in rotary speed due to increased load and increases in rotary speed due to reduced load. Over- or understepping stored limit values during a predetermined time period set for each group of limit values being arranged to cause disconnection of the drive voltage fed to the electric motor. Furthermore, the start operation comprises preferably of two or more during which the microprocessor step by step increases supplied power and performs comparison between to the microprocessor supplied information relating to obtained rotary speed and stored information relating to expected rotary speed for each step. A rotary speed lower than stored speed information during a predetermined time period being arranged to cause disconnection of the drive voltage fed to the electric motor.

Description: Method and device for controlling an electric motor The present invention relates to a method and a device for controlling an electric motor arranged to serve as a drive source for electrically driven hand tools or stationary machinery, which are made subject to varying loads during operation. The invention is in this connection particularly well adapted for circular saws having two adjacent saw blades rotating in an opposed relationship, but it is in no way restricted to such use. With regard to, for example, electrically driven hand tools, such as circular saws or similar, it is desirable to maintain a suitable rotary speed and output power adapted to each operation, in order to secure that utilized drive motor is not overloaded and that a substantially constant rotary speed is maintained during a sawing operation, adapted to the present operation and the power of the drive motor. A method to monitor whether or not there is a risk for overloading the drive motor is obviously surveillance of the temperature of the drive motor, and to interrupt the drive voltage when the temperature exceeds a predetermined maximum value. However, such a solution can only be used to avoid that the drive motor is being damaged by overheating, and does not cause any adaption of supplied power to the drive motor based on the different operating conditions and rotary speeds which occur during use, e.g. due to the material and thickness of the workpiece, as well as the pressure applied/feeding speed. The object of the present invention is to disclose a method and a device for controlling an electric drive motor with adaption of the drive motor power to required rotary speed for different operating conditions, whereby the drive motor power can be used to a maximum level without risk of overloading. A further object is to secure a substantially constant rotary speed adapted to actual operative load. Furthermore, the present invention is also intended to secure that the drive motor is not damaged when the rotation of the drive motor is prevented, during startup as well as during operation. The method according to the present invention for control of an electric motor arranged to serve as drive source for electrically driven hand tools or stationary machines, which during operation are made subject to varying loads, is mainly characterized in that limit values, determined by experience and/or calculations, relating to supplied power with regard to different rotary speeds stored in one or a number of memory circuits are compared to present actual rotary speed and supplied power with use of a microprocessor, that an output signal from the microprocessor is used to adjust supplied power within preset limits for each rotary speed as compensation for reductions in rotary speed due to increased load and increases in rotary speed due to reduced load, and that overstepping orunderstepping stored limit values during a predetermined time period set for each group of limit values being arranged to cause disconnection of the drive voltage fed to the electric motor. Advantageously includes the method according to the present invention a start operation including two or more steps with time surveillance during which the microprocessor step by step increases supplied power and performs comparison between to the microprocessor supplied information relating to obtained rotary speed and stored information relating to expected rotary speed for each step, a rotary speed lower than stored speed information during a predetermined time period being arranged to cause disconnection of the drive voltage fed to the electric motor. The device for utilization of the method according to the present invention is defined by the characteristic features disclosed in claim 6 and related subclaims. A number of non-restricting examples disclosing how the method according to the present invention can be used, and a device for utilization of the method, will be more fully described below with reference to the accompanying drawings, in which:- Fig. 1 is a diagram intended to serve as an example of an operating case for an electrically driven portable circular saw; Fig. 2 is a flow chart intended to illustrate an example of how a starting operation is carried out according to the method of the present invention; and Fig. 3 is a flow chart intended to illustrate how the method according to the present invention is utilized during an operation. As an example of a field of use has an electrically driven portable circular saw been chosen, having two saw blades rotating in opposed directions and arranged in an adjacent relationship, driven by a common electric motor. For this type of circular saws, a problem may initially occur when the saw blades are mounted in an incorrect fashion, i.e. when mounted in such a way that they take up contact against each other and thereby brake or prevent rotation for the drive motor when started. Another problem is that this type of rotary saws often are used in the same fashion as an angle grinder, i.e. for operations which are not suitable for a circular saw, which has resulted in overload for the drive motor and other related parts in the circular saw. Substantially similar problems may obviously also occur with regard to other electrically driven hand tools or machines, which means that the present invention should in no way be regarded as restricted to the example of an embodiment which has been chosen for descriptive purposes. With reference to Fig. 1 , a diagram is shown intended to illustrate an example of an operative case for a circular saw of above stated type. A manually operable switching means is influenced in a known fashion, whereby a voltage is fed to the motor of the circular saw, an operation performed at 1. The circular saw is arranged with a sensor reading present rotary speed, e.g. a magnet located in a hole extending crosswise through the motor shaft, arranged to influence an adjacently located stationary Hall-element. As a result, one pulse is generated for each revolution performed by the motor shaft, and the pulse train obtained is used as described later. Other means for sensing the rotary speed can obviously also be used, as well as means giving two or more pulses per completed revolution. In addition thereto, the method according to the present invention also uses a microprocessor with associated storage means for data, which advantageously may be one or a number of memory circuits of a memory type facilitating reprogramming, e.g. flash memory. A control means is also used for controlling supplied power, preferably of semiconductor type, e.g. triacs. The above mention memory circuit is programmed with a table based on values obtained through tests or values calculated for the actual type of saw, i.e. relating to allowed rotary speeds, and for such speeds acceptable levels of supplied power. Also time values for a starting operation are advantageously decided and stored, as well as time values with regard to expected response to changes in power. At previously mentioned voltage feed to the motor (at 1 in Fig. 1 ), a clocked period starts (time period 1 in Fig. 2), e.g. 0.05 seconds, during which restricted power is being supplied to the motor and surveillance is carried out in order to establish if a pulse train is received as a reply confirming rotation of the motor shaft (the saw blades). Should no such reply be obtained, a second clock period is advantageously initiated (time period 2 in Fig. 2), and a further control whether or not rotation has commenced is carried out. Should this not be the case, the current supply to the motor is interrupted, and the problem is advantageously indicated acoustically or optically to the operator, e.g. by means of buzzer, display unit or similar. In this case, the saw blades are probably incorrectly mounted or by any other reason prevented from rotating. However, should a correct start be indicated during time period 1 , or time period 2, by means of a pulse train indicating rotation supplied to the microprocessor, the power supplied to the drive motor is increased in steps (from point 1 to point 7 in Fig. 1 ), which for simplicity reasons have been reduced to three steps in Fig. 2 (time periods A, B and C). For each time period, the microprocessor thus compares stored table data for a correct start with received rotation information for the successively increased supplied power, and divergence outside allowed table values results in interruption of the voltage fed to the drive motor. A further variable which can be monitored by this method is obviously the total time period for startup, i.e. by adding used time periods and comparison with the present rotary speed. Accordingly, should not an expected rotary speed be obtained within a stored maximum time period, this may also be used as a further reason to interrupt the upstart. At point 7 in Fig. 1 , the starting operation has been completed, and the control is now directed to maintaining a stable and constant rotary speed. A small reduction of rotary speed and power is indicated between points 8 and 9 in Fig. 1 , e.g. when sawing thin sheet metal, and new table data are fetched which result in a suitably strong machine, suited for sawing in such a material and utilizing a relatively high rotary speed. Between points 9, 10 and 11 in Fig. 1 , the load on the machine is increased, and as a result, the rotary speed is drastically reduced, e.g. when sawing heavy materials. Also in this case are new table data fetched, and supplied power is corrected in order to increase the strength of the motor. This takes place between points 11 and 12 in Fig. 1 , and the rotary speed of the machine increases towards the intended rotary speed by means of values obtained from the stored table. The steps between the values can in this respect be relatively large, in order to obtain intended rotary speed in shortest possible time. The range between points 12 to 15 in Fig. 1 is intended to serve as an example of conventional sawing with a finally increased load. Since this increased load eventually as a result means that stored table values are exceeded (within the range 15 to 16), the voltage fed to the motor is interrupted at 16, since stored information relating to maximum supplied power (Pmax) is exceeded during a predetermined time period. The above described action after completed and accepted start operation can also be described with reference to the flow chart in Fig. 3. The rotary speed after completed start operation (indicated by the downwardly directed arrow) is a first present rotary speed (N). After a clock ed period X the rotary speed (N) is compared with the new rotary speed (Nx) obtained, If (N) is not larger than (Nx), the operation is repeated. In case that a sawing operation has been initiated, i.e. (N) is larger than (Nx), a checkup operation comparing stored table values is performed in order to ascertain that supplied power to the motor does not exceed a predetermined maximum value (Pmax), a power increase is performed based on rotary speed data in a stored table, and the change in rotary speed during a clock period (Z) is measured and the result is denominated (Nz). The value (Nz) is hereafter a new input value (N), and provided that (N) is not less than a predetermined minimum rotary speed (Nmin), (N) will serve as a new input value to be monitored during the period x and to be compared with the rotary speed (Nx). However, should (N) be less than (Nmin), it is checked if supplied power exceeds (Pmax), and if so, the drive voltage to the motor may be immediately interrupted, or alternatively, as shown, checked again after a time period Y, and thereafter alternatively cause disconnection (if supplied power is to large in relation to the rotary speed) or otherwise result in a repeated checkup operation in previously described fashion. Accordingly, Fig. 3 shows primarily the method according to the present invention within the range 7 - 16 in Fig. 1 , whereas Fig. 2 discloses the start operation according to the range 1 - 7 in said figure. Discussed actions are thus based on stored table values relating to supplied power and rotary speeds, as well as stored information of time intervals, primarily for a start operation but also with regard to surveillance intervals. An increase/decrease of supplied power is controlled by previously mentioned micropro- cessor, which can increase and decrease the opening angle for utilized triacs or similar semiconductors on basis of stored table values for different rotary speeds, which is obtained by the microprocessor as a pulse train with a frequency related to present rotary speed. This means that rotary speed/supplied power always are related to each other, and that a reduction of the rotary speed during a sawing operation is interpreted as an increased load which automatically results in an increase of supplied power, and also that supply in excess of a predetermined power limit (determined by, for example, the present opening angle for utilized triacs or similar semiconductor components) for existing rotary speed (overload) results in disconnection of the feed voltage to the motor. As previously mentioned include stored table values also a number of time monitoring functions, arranged to start a clock period during which surveillance is carried out with regard to changes in rotary speed, if a predetermined maximum power value is maintained exceeded during more than one or a number of accepted clock periods (time periods), and if such parameters are exceeded, this would also result in disconnection of the voltage fed to the motor in order to protect the motor and other included components against damage caused by overload. Disconnection of the voltage fed to the motor is advantageously arranged to cause a fast acting braking action, a possibility existing if disconnection of the drive voltage is caused to feed a reverse braking voltage to the motor, which is interrupted when the pulse train supplied to the microprocessor indicating rotation has substantially completely ceased, a solution as disclosed in WO 97/43821. As an example of a field of use, a circular saw including two adjacently located saw blades rotating in opposed directions has been described above, but it is obvious that the method according to the present invention in no way is restricted to this example of use. The method according to the present invention can thus advantageously be used for other electrically driven hand tools, such as, for example, drilling machines, grinding machines, compass saws and circular saws, as well as stationary machines of different types, and electrically driven feeding devices and other devices which are made subject to varying loads during use, which could result in overload for utilized drive motor. By using memory circuits of reprogramable type, it is also possible to upgrade stored table data in a simple fashion, as well as possible to perform programming in order to adapt to predetermined fields of use. The method according to the present invention makes it thus possible to obtain complete security against overload and thereby caused damage to an electric drive motor with associated components, and secures also that a start operation is performed in a controlled fashion, as well as that a start operation is interrupted in case that rotation is not obtained within a predetermined time period as from voltage feed to the motor is established. Shown and described examples of embodiments are only intended to serve as examples of embodiments within the scope of the inventive thought and the following claims, and should thus not be regarded in any way as restricting the scope of protection. Accordingly, the start operation shown in Fig. 1 (between points 1 and 7) may, for example, include any desired number of steps, and supplied voltage feed can be interrupted already after one "time period 1" in Fig. 2. Alternatively, "time period 1" and "time period 2" can also be increased by a further time period or further time periods, provided that the motor used can accept this without any damage. With regard to the flow chart shown in Fig. 3, same is primarily intended to disclose how an increased load is compensated, and this is shown in a simplified fashion by reduced power when N<N(x). Such a situation occurs when load is reduced, whereby the rotary speed of the motor is increased, and a comparison with stored table values causes a corresponding reduction in supplied power to stored power value (e.g. stored value for the opening angle of utilized triacs).

Claims: C L A I M S 1. Method to control an electric motor arranged to serve as drive source for electrically driven hand tools or stationary machines, which during operation are made subject to varying loads, c h a r a c t e r i z e d i n, that limit values, determined by experience and/or calculations, relating to supplied power with regard to different rotary speeds stored in one or a number of memory circuits are compared to present actual rotary speed and supplied power with use of a microprocessor, that an output signal from the microprocessor is used to adjust supplied power within preset limits for each rotary speed as compensation for reductions in rotary speed due to increased load and increases in rotary speed due to reduced load, and that overstepping or understepping stored limit values during a predetermined time period set for each group of limit values being arranged to cause disconnection of the drive voltage fed to the electric motor. 2. Method according to claim .. c h a r a c t e r i z e d i n, that a start operation includes two or more steps with time surveillance during which the microprocessor step by step increases supplied power and performs comparison between to the microprocessor supplied information relating to obtained rotary speed and stored information relating to expected rotary speed for each step, a rotary speed lower than stored speed information during a predetermined time period being arranged to cause disconnection of the drive voltage fed to the electric motor. 3. Method according to anyone of claims 1 and 2, c h a r a c t e r i z e d i n, that the microprocessor is arranged to control power supplied to the electric motor in such a way, that a predetermined rotary speed with associated power is desirable, and when the rotary speed is lower than the predetermined speed cause an increase of the power supplied to the electric motor intended to cause a larger output power without overstepping maximum allowed supplied power for the particular rotary speed, and if such overstepping should occur, preferably maintained during a predetermined time period, cause disconnection of the drive voltage fed to the electric motor. 4. Method according to anyone of claims 1 - 3, c h a r a c t e r i z e d i n, that the power supplied to the electric motor is controlled by the microprocessor which increases or reduces the opening angle for one or more power controlling semiconductor elements, and that information relating to present rotary speed is supplied to the microprocessor from a rotary speed sensor as a pulse train having a frequency related to the rotary speed. 5. Method according to anyone of claims 1 -4, characterized in, that the disconnection of the drive voltage fed to the electric motor is performed in combination with application of a braking force to the electric motor. 6. Device for controlling an electric motor arranged to serve as a drive source for electrically driven hand tools or stationary machines, which during operation are made subject to varying loads with utilization of the method according to anyone of claims 1-5, characterized in, that it includes one or a number of memory circuits with stored limit values, determined by experience and/or calculations, relating to supplied power with regard to different rotary speeds, a sensor for determination of present actual rotary speed and with an output signal supplied to a microprocessor, and a power controlling device, preferably of semiconductor type, for controlling the power supplied to the electric motor and controlled by the microprocessor, said microprocessor being arranged to compare present actual rotary speed and supplied power with stored limit values, and at divergence adjust supplied power within preset limiting areas for each rotary speed in order to compensate for reductions in rotary speed due to increased load and increases in rotary speed due to reduced load, and when overstepping or understepping stored limit values during a predetermined time period set for each group of limit values being arranged to cause disconnection of the drive voltage fed to the electric motor. 7. Device according to claim 6, characterized in, that utilized memory circuits include table values for a step by step start operation with successively increased supplied power to the motor, and that no response indicating rotation during a first and/or later step in the start sequence being arranged to cause disconnection of the drive voltage fed to the electric motor, and preferably initiate transmission of an acoustical or optical signal to the operator. 8. Device according to anyone of claims 6 and 7, characterized in, that the sensor for determination of present actual rotary speed comprises of a magnet means arranged by a rotary shaft and an adjacently located Hall-element. 9. Device according to anyone of claims 6 - 8, characterized in, that the stored values for power comprises of values relating to opening angle or corresponding type of values for utilized power controlling semiconductor elements, such as triacs or similar. 10. Device according to anyone of claims 6-8, characterized in, that predetermined time periods for surveillance are accomplished by start of counting sequences(clocked periods under surveillance of the microprocessor.

Inventors: ERICSSON HANS BIRGER, ERICSSON, HANS, BIRGER
Applicants: ERICSSON HANS BIRGER, NORD 2000 INTRESSENTER AKTIEBO, ERICSSON, HANS, BIRGER, NORD 2000 INTRESSENTER AKTIEBOLAG
Application year: 1999
Main classification: H02P5
IPC codes: H02P1, H02P27