Abstract:
A control device for soft starting and protecting overload of a motor in a power tool is disclosed. The control device comprises a power switch, a varistor, several diodes, several resistors, several capacitors, a relay, a transistor, a triac, and a microcontroller. The microcontroller is embedded with program such that, after the power switch is turned on, the microcontroller generates a smooth soft start voltage to drive the power tool. With the characteristics of power-control device, the voltage sent to motor is increased from low to full range. When the speed of motor is increased from low to the maximum, the microcontroller turns off the power-control device and turns on a relay.

Description:
BACKGROUND OF THE INVENTION 
   1) Field of the Invention 
   The present invention relates to a control device for soft starting and protecting overload of a motor, and more particularly to a control device for soft starting and protecting overload of a motor in a power tool capable of reducing the risk of physical damage to user&#39;s hand caused by a torque reaction. 
   2) Description of the Related Art 
   With rapid development of industrial technology, manual labors are gradually being replaced by power tools. However, the safety of using power tools is a concern. Since the rotation speed of the power-driven motor varies abruptly in a short time and the large current generates start torque when the motor is started, the user who holds the power tool may react intuitively. For example, for a user who uses a hand-held grinder, he will hold the tool firmly to prevent the tool from flying away when the power tool is started. 
   Further, the main deficiency of the power tool stated above is that, when the power tool is started, there exists risk of physical damage to the hand of user caused by the starting torque. Besides, although there are table-top power tools that are fixed on the table to ameliorate damage to the user&#39;s, such tools still have problems of strong vibration and power consumption. 
   In view of the deficiencies of power tool mentioned above, the present inventor proposes a control device for soft starting and overload protection of motor in a power tool such that the safety and convenience of using power tool is ensured. 
   SUMMARY OF THE INVENTION 
   The primary objective of the present invention is to provide a control device for soft starting and protecting overload of a motor in a power tool capable of reducing the risk of physical damage to the hand of user caused by the torque reaction. 
   Another objective of the present invention is to provide a control device for soft starting and overload protection of motor in a power tool, such that the motor damage caused by large instantaneous current and resultant over heat is prevented. 
   In accordance with the above objectives, the present invention a control device comprising a power switch, a varistor, several diodes, a plurality of resistors, a plurality of capacitors, a relay, a transistor, a triac and a microcontroller. The microcontroller is embedded with program for generating a smooth soft start signal to drive the power tool after power switch is turned on. According to an aspect of the present invention, the voltage sent to motor is increased from low to full range. When the rotation speed of motor is increased from low to the maximum, the microcontroller turns off the power-control device and turns on a relay. The power is supplied to the motor through relay. Becaused the power-control device stops working, and therefore the generation of heat can avoided and thus the service life of the peripheral electronic devices can be effectively increased. 
   The other features and specific embodiments of the present invention together with the accompanying drawings are described in the following. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic diagram of a control device according to a first embodiment of the present invention. 
       FIG. 2  shows a schematic diagram of a control device according to a second embodiment of the present invention. 
       FIG. 3  shows a schematic diagram of a control device according to a third embodiment of the present invention. 
       FIG. 4  shows a flow chart for describing the operation of the embodiments according to the present invention. 
   

   SYMBOL DESCRIPTION OF THE MAIN DEVICES 
   
       
       A: Start Switch 
         1 . Varistor 
         2 . Diode 
         3 . Resistor 
         4 . Zener Diode 
         5 . Capacitor 
         6 . Capacitor 
         7 ˜ 8 . Resistor 
         9 . Microcontroller 
         10 ˜ 11 . Capacitor 
         12 . Resistor 
         13 . Transistor 
         14 . Triac 
         15 . Relay 
         16 . Resistor 
         17 . Capacitor 
         18 . Variable Resistor 
         19 . Capacitor 
         20 ˜ 23 . Resistor 
         24 . Variable Resistor 
         25 . Comparator 
         26 . Amplifier 
         27 ˜ 28 . Resistor 
         29 . Capacitor 
         30 . Protection Switch 
         40 . Motor 
     
  
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a schematic diagram of a control device according to a first embodiment of the present invention. Referring to  FIG. 1 , the circuit includes a start switch A, a varistor  1 , a motor  40 , a microcontroller  9 , a transistor  13 , a triac  14 , a relay  15 , a diode  2 , a Zener diode  4 , multiple resistors  3 ,  7 ,  8 ,  12 ,  16 , and multiple capacitors  5 ,  6 ,  10 ,  11 ,  17 . A fundamental AC power input of 120V/240V and 50/60 Hz frequency is installed adjacent to the start switch A. 
   The power is supplied from an external power source. When start switch A is turned on, varistor  1  retrains instantaneous surge generated by main power. The sine wave signal of power passes diode  2  to generate rectified half-wave signal. Resistor  3  retrains current, and the resistance value depends on the input voltage (AC 120V/240V). Zener diode  4  restrains voltage such that voltage is decreased to 5V. The power passing through capacitor  5 , smoothens and filters the voltage, and bypass capacitor  6  is delivered to microcontroller  9  and peripheral devices. Thus, it is possible to reduce the operating voltage to 5V and the operating current to a few mA so that the objective of low power consumption may be achieved. 
     FIG. 2  shows a schematic diagram of a control device according to a second embodiment of the present invention. Referring to  FIG. 2 , the circuit includes a start switch A, a varistor  1 , a motor  40 , a microcontroller  9 , a transistor  13 , a triac  14 , a relay  15 , a diode  2 , a Zener diode  4 , multiple resistors  3 ,  7 ,  8 ,  12 ,  16 ,  20 ,  21 ,  22 ,  23 , two variable resistors  18 ,  24 , multiple capacitors  5 ,  6 ,  10 ,  11 ,  17 ,  19 , and a comparator  25 . A fundamental AC power input of 120V/240V and 50/60 Hz frequency is installed adjacent to the start switch A. 
   The power is supplied from an external power source. When start switch A is turned on, varistor  1  retrains instantaneous surge generated by main power. The sine wave signal of power passes diode  2  to generate rectified half-wave signal. Resistor  3  retrains current, and the resistance value depends on the input voltage (AC 120V/240V). Zener diode  4  restrains voltage such that voltage is decreased to 5V. The power passing through capacitor  5  smoothens and filters the voltage, and bypass capacitor  6  is delivered to microcontroller  9  and peripheral devices. Thus, the power consumption can be effectively reduced. 
   A resistor  20  is connected to one end of the motor  40 . Power is supplied to the motor  40  via the resistor  20 . The resistance value of resistor  20  is a few mΩ. When the motor  40  is operated, current flows through the resistor  20 , the resistor  20  generates a load-sensing voltage, and then the load-sensing voltage is sent to comparator  25  via current-limit resistor  21 . The load-sensing voltage is compared with a reference voltage at the comparator  25 , and a output voltage is sent to microcontroller  9 . The setting of reference overload current depends on the fundamental parameters of motor  40 . The safety is the main concern. For example, the rated current of circuit breaker is commonly within a range of 15 A˜20 A, the current of a power tool is set around 17 A, and the overload current of motor  40  is not more than 20 A by default. The voltage Vcc is adjusted through current-limit resistor  22 , resistor  23 , and variable resistor  24 . When the load-sensing voltage of motor  40  is ≧reference voltage, comparator  25  generates a signal to notify microcontroller  9 , the microcontroller  9  determines that the motor  40  is at a status of overload exception, and the motor  40  is stopped. 
     FIG. 3  shows a schematic diagram of a control device according to a third embodiment of the present invention. Referring to  FIG. 3 , the circuit includes the start switch A, the varistor  1 , the motor  40 , the microcontroller  9 , the transistor  13 , the triac  14 , the relay  15 , the diode  2 , the Zener diode  4 , multiple resistors  3 ,  7 ,  8 ,  12 ,  16 ,  20 ,  21 ,  22 ,  23 ,  27 ,  28 , two variable resistors  18 ,  24 , multiple capacitors  5 ,  6 ,  10 ,  11 ,  17 ,  19 ,  29 , the comparator  25 , an amplifier  26 , and a protection switch  30 . A fundamental AC power input of 120V/240V and 50/60 Hz frequency is installed beside the start switch A. 
   The power is supplied from an external power source. When start switch A is turned on, varistor  1  retrains instantaneous surge generated by the main power. The sine wave signal of power passes through the diode  2  to generate rectified half-wave signal. Resistor  3  retrains current, and the resistance value depends on the input voltage (AC 120V/240V). Zener diode  4  restrains voltage such that voltage is reduced to 5V. The power passing through the capacitor  5  and bypass capacitor  6  smoothen and filter the voltage is delivered to microcontroller  9  and peripheral devices. The power consumption is thereby reduced. 
   A resistor  20  is connected to one terminal of the motor  40 . Power is supplied to the motor  40  via the resistor  20 ,. The resistance value of resistor  20  is a few mΩ. When the motor  40  is operated, current flows through resistor  20 , the resistor  20  generates a load-sensing voltage, and then the load-sensing voltage is sent to comparator  25  via current-limit resistor  21 . The load-sensing voltage is compared with reference voltage at the comparator  25 , and then an output voltage is sent to microcontroller  9 . The setting of reference overload current depends on the fundamental parameters of motor  40 . The safety is the main concern. For example, the rated current of circuit breaker is commonly in a range of about 15 A˜20 A, the current of a power tool is set around 17 A, and the overload current of motor  40  is not more than 20 A by default. The supplied Vcc is adjusted through current-limit resistor  22 , resistor  23 , and variable resistor  24 . When the load-sensing voltage of motor  40  is ≧reference voltage, the comparator  25  generates a signal to notify microcontroller  9  that the motor  40  is at a status of overload exception, and the motor  40  is stopped. 
     FIG. 4  shows a flow chart for describing the operation of the embodiments according to the present invention. Referring to  FIG. 3  and  FIG. 4 , if the voltage at a point between current-limit resistor  28  and capacitor  29  is a positive voltage, the protection switch is determined to be normal. On the other hand, if the external signal is open, i.e., the resistor  27  provides a negative voltage, the microcontroller  9  concludes that there exists a signal exception, and stops the motor  40 . If protect switch  30  has an input signal when the motor  40  is running normally, the motor  40  is stopped. Even if the input signal is recovered, the motor  40  remains off mode. Accordingly, the present invention is capable of reducing the risk when the power supply to the tool is on. Therefore, whenever microcontroller  9  detects an exception, the motor  40  has to be restarted. 
   When the power of microcontroller  9  remains in the normal status, the control signal is sent to capacitor  10  through amplifier  26  to form a gate driver to control the soft start signal. Since the triac  14  allows bi-directional current flow, it is suitable for AC power control. The control signal output from microcontroller  9  controls the soft start of voltage such that the voltage rises smoothly and the motor  40  runs from low speed to high speed smoothly. 
   When motor  40  works in soft start mode and after the motor  40  runs from low speed to high speed, the control signal sent out from microcontroller  9  transmits through current-limit resistor  12  and transistor  13  to control relay  15 . Thus, the contact of relay  15  is connected. The function of the triac  14  for controlling the voltage of motor  40  is then transferred to the main contact of relay  15 . Meanwhile, the main contact of relay  15  maintains the normal running of motor. The heat source caused by using triac  14  is completely avoided. Thus, problem due to high temperature of the device is resolved, and therefore the lifespans of the device and peripheral devices can be effectively increased. 
   Moreover, in the embodiments according to the present invention, if the power switch A used is UNLOCK type, the microcontroller  9  determines an exception when the switch is repeatedly turned on and off during a short time period. For the power switch A of LOCK ON/LOCK OFF type, even if the motor  40  runs normally when the switch is turned on, the microcontroller  9  will determine an exception and stop the motor  40  when the power is shut down abruptly and turned on immediately. Besides, when the motor  40  works normally, the problem caused by the tool or other external factors may stop the motor  40  (e.g., jammed by the wood when sawing a lumber). On this occasion, resistor  21  connected to motor  40  will generate a voltage signal, which is compared with the reference voltage, if the generated voltage is larger than the setting, the comparator  25  will send a signal to the microcontroller  9 . The microcontroller determines such a condition as an exception and stops the motor  40 . 
   Furthermore, some power tools are equipped with external safety mechanism, such as protection mask, emergency switch, or limit switch, to prevent from arbitrarily starting the motor  40 . Taking the protection mask as an example, if the protection mask is not closed or opened while the motor  40  is working normally, the microcontroller  9  will determine such conditions as exceptions and stop the motor  40 . 
   In summary, the present invention provides a practical control device for soft starting and overload protection of motor in a power tool. 
   Although the preferred embodiment of the present invention has been described, various modifications and enhancements may be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not limited except as by the appended claims.