1. Field of the Invention
The present invention relates to a variable speed power tool, more particularly to a power tool (such as an electric drill, a screwdriver or an impact drill) having a control system for automatically varying the output speed in accordance with a load applied to the power tool.
2. Description of the Related Art
A variable speed power tool (such as for example an electric drill, a screwdriver or an impact drill) usually employs a gear transmission system to selectively output various speeds to adapt to different work conditions. The gear transmission system generally includes a gear transmission mechanism connected between an electric motor and an output shaft and a control system operable by the user to change the gear transmission mechanism so as to vary the gear reduction ratio to output various speeds.
U.S. Pat. No. 6,431,289 discloses a gear transmission mechanism including two ring gears being adjustable forwards and backwards via an actuating member operated by the user outside the housing. By this means, the ring gears can shift between a rotating position where they connect both planet gears and an adjacent planet carrier and a fixed position where they connect to the planet gears and the housing thereby varying the output speed. However by such manual operation, the operator has to monitor the running bit (such as a fastening screw) to decide when to vary the output speed or torque to achieve better output efficiency. This operation is not user-friendly. Furthermore, when the user actuates the actuating member to shift the ring gear from the rotating position to the fixed position, the rotating ring gear is unable to engage a fixed structure in the housing smoothly. In these circumstances, a gear clash between the ring gear and the rotationally fixed structure frequently occurs which results in damage to the ring gear or the fixed structure and may reduce its working life.
U.S. Pat. No. 6,824,491 discloses an automatic variable speed system in which a control system is able to automatically vary the gear reduction ratio by varying the direction of movement of the ring gear according to variation of the torque load. EP-A-0787931 discloses a similar automatic variable speed system. However, these automatic variable speed systems are performed mechanically and the mechanical structure of the systems are complex and difficult to manufacture. Neither system is able to eliminate the occurrence of gear clash.
P-A-3221381 discloses a solenoid mounted on a motor which is able to actuate linear movement of a switching lever via an actuation lever and a transmitting lever to change the gear reduction ratio of the gear transmission mechanism.
JP-A-4262151 discloses a solenoid adapted to change the gear reduction ratio of the gear transmission mechanism.
JP-A-8068461 discloses a keep solenoid used to drive an internal gear from a high speed position to a low speed position and retain the internal gear at the low speed position. A spring is used to pull the internal gear back to the high speed position.
P-A-9057639 discloses an auto speed change mode which will not be activated if the motor does not reach the highest output. It is undesirable that auto speed changing is actuated when the trigger switch is being depressed to increase the motor speed. A motor speed detect module is provided to detect motor speed and feed a signal representing motor speed to the control circuit.
JP-A-9057640 discloses a mode selection switch provided to activate or inactivate an automatic speed changing function. The switch is used to select the auto speed change mode and the fixed speed mode. The switch can select between auto mode, high speed mode and low speed mode.
JP-A-10103462 discloses a curve ηh indicating the tool output efficiency at a high speed state. ηl is a curve indicating the tool output efficiency at a low speed state. The tool first works in the high speed state and when the load torque reaches CP, the auto speed change is activated so that the tool then works in the low speed state.
JP-A-9014433 discloses a keep solenoid which is used only to change from high speed to low speed and various means for returning to high speed.
JP-A-1109845 discloses a battery pack voltage detector VP which will detect BP voltage. When the voltage drops below a predetermined value, the auto speed mode is activated directly to vary from high speed to low speed. The activation of auto speed mode is not controlled by control circuit C. The control circuit C detects the load torque by monitoring the motor speed. If the power drops rapidly and the motor speed reduces sharply, the control circuit C cannot activate auto speed changing before the motor has stalled.