GB2314980A

GB2314980A - Power tool speed controller

Info

Publication number: GB2314980A
Application number: GB9613835A
Authority: GB
Inventors: Hin Ming Lam
Original assignee: Defond Manufacturing Ltd
Current assignee: Defond Manufacturing Ltd
Priority date: 1996-07-02
Filing date: 1996-07-02
Publication date: 1998-01-14
Anticipated expiration: 2016-07-02
Also published as: HK1006243A1; GB9613835D0; GB2314980B

Abstract

A hand holdable electric power tool speed controller has a manually movable slider 31 which has two contact areas (32,Fig 3B) that can make selective contact with separated discrete contact surfaces X1 to X7. The surfaces X1 to X7 are connected to separate series-connected resistors whose combined effective resistance in a circuit serves to adjust the speed of a motor. The contact areas (32) are so arranged with respect to the contact surfaces that one of the areas is always in electrical contact with at least one of the surfaces during motor speed adjustments. The tool may be an electric drill.

Description

POWER TOOL SPEED CONTROLLER The present invention relates to a speed controller for power tools such as electric drills or the like.

The operating speed of such power tools are normally adjustable by users. Conventional speed control is achieved by means of a variable resister provided in an electrical circuit. The variable resistor makes use of a carbon film which is adhered directly on a printed circuit board to provide a resistance that is varied by moving a sliding contact across the film. As the carbon film is subject to frequent sliding engagement by the contact, the variable resistor is vulnerable to wear and tear leading to inaccurate or erratic control unless it is fabricated specially which is however expensive.

The present invention seeks to mitigate or to at least alleviate such a disadvantage.

According to the invention, there is provided a hand holdable electric power tool speed controller incorporating a variable resistor element having a bank of series connected resistor components and a manually movable slider to make contact with the resistor element and adjust its effective resistance, the resistor element comprising an array of separated discrete contact surfaces each electrically connected to one or more resistors in the bank and the slider having two separated contact areas arranged so that when the slider is moved the areas slide to contact different of the contact surfaces and the areas can contact adjacent contact surfaces to bridge separations between the contact surfaces to ensure that the slider is in electrical contact with at least one contact surface at all times during speed control adjustments.

Each of the contact surfaces may comprise an elongate parallel sided strip of conductive material and the strips arranged side by side with their longitudinal axes at about 450 to a direction of movement of the slider, and the contact areas separated in a direction at right angles to the direction of movement of the slider.

Each of the contact surfaces may be an elongate strip of conductive material which is non-uniform in width along its length, such that in the array separations between the contact surfaces are stepped in plan.

The controller may include an integrated circuit for controlling the charging and discharging of a capacitor via the resistor element.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a side view of an electric drill incorporating a speed controller in accordance with the invention; Figure 2 is an enlarged side view of the speed controller of Figure 1; Figure 3A is a schematic circuit diagram of an electronic circuit of the speed controller of Figure 2, incorporating a slider in one operating position; Figure 3B shows a layout of a printed circuit board of the controller circuit of Figure 3A, with the slider in the one operating position; Figure 4A is a circuit diagram corresponding to Figure 3A, showing the slider resistor in a second operating position; Figure 4B shows a layout of the printed circuit board corresponding to Figure 3B, with the slider in the second operating position; Figures 5A and 5B show layouts of similar printed circuit boards, incorporating a different variable resistor element with the slider in the first and second operating conditions positions respectively; and Figures 6A and 6B show another layout of a similar printed circuit board, incorporating another different variable resistor element with the slider in the first and second operating positions respectively.

Referring to Figures 1 and 2 of the drawings, there is shown a pistol-like electric drill 10 which incorporates a speed controller 20 embodying the invention. The drill 10 has a body 11, including a hand grip 12, which houses an electric motor 13 for rotating a chuck 14. The hand grip 12 houses a rechargeable battery pack (not shown). The speed controller 20 has a body 21 located inside an upper end of the hand grip 12 and includes a trigger 22 and a lever 23 which extends below the drill body 11. The trigger 22 is used to switch the motor 13 ON and OFF and to adjust the speed of the motor. The lever 23 is used to select the direction of rotation of the motor 13.

Reference is now made to Figures 3A and 3B of the drawings.

The speed controller 20 has an electronic circuit 30 which is formed by, inter alia, an integrated circuit chip IC1, a variable resistor element VR1, and a MOSFET power transistor TR1, which are all mounted on a printed circuit board 40. The transistor TR1 is connected in series with the motor 13. The speed of the motor 13 increases or decreases with the ratio of the turn-on duration to the turn-off duration of the transistor TR1. The transistor TR1 is turned on and off by a control signal provided at pin 3 of the chip IC1.

The variable resistor element VR1 is formed by a bank of eight series connected resistors R1 to R8, with the seven intermediate junctions connected to respective parallelsided straight contact strips X1 to X7 provided on the circuit board 40. The contact strips X1 to X7 form an array of separated discrete contact surfaces disposed as shown in Figure 3B. The contact surfaces X1 to X7 are inclined at about 45 to the direction of movement of a slider 31. The slider 31 has respective pairs of separated contact areas 32 and 33.

The trigger 22 has a rear part 24 which engages the slider 31 for moving the slider as required to adjust the effective resistance of the resistor element, and to change the speed of the motor. It will be noted that the contact areas 32 are separated such that at any time the slider is in electrical contact with at least one of the contact surfaces X1 to X7 during speed control adjustments. This can be seen for example by comparing Figures 3B and 4B. In Figure 3B the contact areas 32 are both in contact with the strip or surface X2, whereas in Figure 4B the contact areas are in contact with surfaces X2 and X3 respectively; in other words, the slider 31 is electrically bridging the separation between the surfaces X2 and X3. In particular, it can be seen and noted in Figure 4B that one of the areas has contacted surface X3 before the other area has moved out of contact with surface X2. This ensures that the slider 31 is always in electrical contact with one of the surfaces X1 to X7 during speed changing movements of the slider. The trigger 22 is spring-biassed out of the body 11 towards a rest position in which both the areas 32 may be arranged to be out of contact with all of the surfaces X1 to X7. The motor will then remain at rest. Upon pressing the trigger 22, the slider 32 will be moved to electrically contact strips X1 to X7 in sequence to increase the speed of the motor as required.

The resistors R1 and R8 are connected in series to a pin 7 of the chip IC1 via diodes D1 and D2. A current-limiting resistor R9 is also provided. The circuit includes a capacitor C1 connected between the chip IC1 and ground, and a pull-down resistor R10 connected at the IC pin 7. The chip IC1 is programmed to perform periodical charging and discharging of the capacitor C1. In the operating position of the slider shown in Figures 3A and 3B, the IC pin 6 is effectively connected to the contact surface X6 and so the resistor element is divided in to an upper branch provided by the resistors R7 and R8 and a lower branch provided by the resistors R1 to R6. During each charging cycle, the current path extends from the supply V+ via the resistor R10, the diode D1 and the lower branch resistors R1 to R6 to the capacitor C1. During each discharging cycle, the current path extends from the capacitor C1 via the upper branch resistors R7 and R8, the resistor R9, the diode D2 and the IC pin 7. The charging and discharging time periods are altered by changing the effective resistance of the resistor element for each of these paths, or in other words by moving the slider 31.

In Figure 5A and 5B and 6A and 6B different layouts of the contact surfaces are shown for the resistor element with the slider 31 in the first and second illustrative operating positions. In each case, separated contact areas of the slider 31 are provided and contact surface arrays of the resistor element are configured and arranged so that the slider 31 always makes electrical contact with at least one of the contact surfaces at any time. This remains the case even though the contact surfaces are non-uniform so that separations between the contact surfaces are stepped in plan. The effective resistance of the contact elements can be changed, and the speed of the motor altered, in a step-by-step manner and without any control discontinuity occurring. Further, the speed control depends much less than in earlier controllers, using their film single resistor elements for example, on contact pressure and rubbing resistance between the parts. Also, the practical or effective resistance of the resistor element is not much dependent if at all on these factors.

Claims

1. A hand holdable electric power tool speed controller incorporating a variable resistor element having a bank of series connected resistor components and a manually movable slider to make contact with the resistor element and adjust its effective resistance, the resistor element comprising an array of separated discrete contact surfaces each electrically connected to one or more resistors in the bank and the slider having two separated contact areas arranged so that when the slider is moved the areas slide to contact different of the contact surfaces and the areas can contact adjacent contact surfaces to bridge separations between the contact surfaces to ensure that the slider is in electrical contact with at least one contact surface at all times during speed control adjustments.

2. A controller according to claim 1, in which the each of the contact surfaces comprises an elongate parallel sided strip of conductive material and the strips are arranged side by side with their longitudinal axes at about 450 to a direction of movement of the slider, and in which the contact areas are separated in a direction at right angles to the direction of movement of the slider.

3. A controller according to claim 1, in which each of the contact surfaces is an elongate strip of conductive material which is non-uniform in width along its length, such that in the array separations between the contact surfaces are stepped in plan.

4. A controller according to any of claim 1 to 3, including an integrated circuit for controlling the charging and discharging of a capacitor via the resistor element.

5. A controller substantially as herein described with reference to any one or more of the drawings.

6. An electric power tool having a speed controller according to any of claims 1 to 5.