Abstract:
A hand tool RFI suppression system comprising a DC motor having a conductive housing, a power supply and a suppression circuit board supporting at least one suppression assembly and at least one ground circuit electrically coupled to the suppression assembly. The suppression circuit board is mounted to the motor housing with the at least one suppression assembly electrically coupled in parallel between the power supply and the DC motor and the ground circuit is electrically coupled to the motor housing. A method of suppressing RFI in a power hand tool is also provided.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to electric motors and more particularly to power circuits in electric motors and still more particularly to a direct current electric motor having an assembly for suppressing radio frequency interference. 
     Electric motors include a stator and a rotor which rotates about a central axis in response to opposing magnetic fields between the stator and the rotor. The opposing magnetic fields are typically induced by electric currents which flow in wire windings in the stator and rotor. Electric motors thereby convert electrical energy to mechanical energy in a rotating shaft which is attached to the rotor. 
     The electrical energy is supplied to a motor by electrical conductors which are connected to an external supply of electricity such as a battery or alternating current power lines. It is advantageous to provide a stable electrical power source to certain electric motors. For example, direct current (DC) motors require a stable direct current input having substantially no alternating current component. Electrical power circuits typically provide electricity including noise such as voltage surges and transient components. Electrical noise may be caused by any number of phenomena including, for example, power drains and surges and radio frequency disturbances which may be induced by alternating electromagnetic fields caused by near by electrical equipment. 
     Electrical power that is transmitted even a short distance through conductors acquires induced noise that was radiated through space. It is therefore impossible to provide a perfectly clean power signal over any substantial distance. Devices such as some electric motors which require clean power employ filter circuits proximate to their power input connections to remove most unwanted interference components in the electrical power signal. 
     Radio frequency interference (RFI) is typically filtered from power signals by filters comprising inductors and capacitors. It has been common practice in the motor industry to use common axial inductors in conjunction with common capacitors in the motor input circuit connected together as filter circuits which are well known in the electrical arts. 
     The electrical parameters or values of inductors and capacitors are related to their physical size. The effectiveness of a filter is related to the circuit parameters and therefore related to the physical size of the inductors and capacitors. 
     Typically, RFI suppression components are mounted on the outside of the motor or alternatively a motor package size is increased to accommodate the additional components. The amount of RFI suppression achieved by known designs is often limited by size constraints which are imposed upon the inductor and capacitor components. 
     Typical RFI suppression circuits are constructed by assembling the inductor and capacitor components to a motor assembly and electrically connecting them together to form the proper filter circuit. There is a need in the art for an RFI suppression package which reduces the size and complexity of the RFI suppression components and incorporates them into a package to speed production and maximize RFI suppression. 
     SUMMARY OF THE INVENTION 
     The present invention provides in one aspect a hand tool RFI suppression system comprising a DC motor having a conductive housing, a power supply and a suppression circuit board supporting at least one suppression assembly and at least one ground circuit electrically coupled to the suppression assembly. The suppression circuit board is mounted to the motor housing with the at least one suppression assembly electrically coupled in parallel between the power supply and the DC motor and the ground circuit is electrically coupled to the motor housing. 
     The present invention provides in another aspect a method of suppressing RFI in a power hand tool that includes a power supply and a DC motor having a conductive housing. The method includes mounting at least one suppression assembly on a PCB board with a ground circuit electrically coupled to the at least one suppression assembly; electrically coupling the at least one suppression assembly in parallel between the power supply and the motor; and electrically coupling the ground circuit to the motor housing. 
     In at least one embodiment, the present invention replaces the use of ferrite beads and multiple components such as inductors and capacitors used to filter RFI. By reducing the amount of hard wired components, the assembly time and cost will be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded isometric view of a cordless power tool incorporating an exemplary embodiment of the suppression circuit board of the present invention. 
         FIG. 2  is an elevation view of the suppression circuit board of  FIG. 1 . 
         FIG. 3  is an elevation view of the suppression circuit board of  FIG. 1  attached to the motor of the cordless power tool. 
         FIG. 4  is an schematic circuit diagram of the attenuator of the suppression circuit board of the present exemplary embodiment. 
         FIG. 5A  is a graph of the radiated emissions of a tested impact wrench without a suppression circuit board of the present invention. 
         FIG. 5B  is a graph of the radiated emissions of a tested impact wrench with a suppression circuit board of the present invention. 
         FIG. 6  is an exploded isometric view of an alternative exemplary suppression circuit board and motor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention. 
     Referring to  FIGS. 1-4 , a suppression circuit board  10  in accordance with a first exemplary embodiment of the invention will be described. In  FIG. 1 , an illustrative battery powered hand tool  100 , for example, an impact wrench, is illustrated with its clamshell housing components  110  removed to show the DC motor  113  configured to drive the tool  100 . While an impact wrench is illustrated, the present invention may be utilized with any other hand power tool. A switch  112  is electrically connected between a power source  111 , for example, a battery (not shown), and the motor  113 . In the present embodiment, connection is made through a motor voltage terminal  114 A and a power voltage terminal  114 B and a motor ground terminal  116 A and a power ground terminal  116 B. 
     The suppression circuit board  10  of the current embodiment of the present invention is configured to be connected in parallel between the motor terminals  114 A and  116 A and the power terminals  114 B and  116 B. Referring to  FIG. 2 , the suppression circuit board  10  includes a PCB board  12  with various components mounted thereon. A voltage contact  25  is positioned along an edge of the board  12  and is positioned to align adjacent to the motor voltage terminal  114 A. Similarly, a ground contact  27  is positioned along an edge of the board  12  and is positioned to align adjacent to the motor ground terminal  116 A. As shown in  FIG. 3 , the close positioning allows the motor voltage terminal  114 A to be soldered directly to the voltage contact  25  at  115  and the motor ground terminal  116 A to be soldered directly to the ground contact  27  at  117 . While the contacts  25 ,  27  are shown adjacent to the motor terminals  114 A and  116 A, they can alternatively be positioned adjacent to the power contacts  114 B and  116 B or a combination thereof. Minimal distance between the contacts and the terminals prevents noise from bypassing/coupling around the suppression assembly  20 . 
     The voltage contact  25  is electrically connected to a first terminal of the suppression assembly  20  while the ground contact  27  is connected to a second terminal of the suppression assembly  20  such that the suppression assembly  20  is connected in parallel between the power source  111  and the motor  113  as illustrated in  FIG. 4 . In the present embodiment, the suppression assembly  20  is an X2Y attenuator as described at http://www.X2Y.com. The X2Y attenuator provides a pair of terminals along with a pair of ground terminals. Other components, including various capacitors and inductors, and other configurations may alternatively be utilized as the suppression assembly  20 . 
     The suppression assembly  20  of the present embodiment provides a pair of ground circuits  24  and  26 , each with one or more ground circuit contacts  28 ,  29 . Each of the ground contacts  28 ,  29  extends to a ground plane  14  on the underside of the PCB board  12 . The ground plane  14  may be a continuous surface or may define discrete contact points The PCB board  12  is configured to be mounted directly to an end cap  125  of the conductive motor housing  123  with the ground plane  14  in direct contact with the motor housing  123 . The PCB board  12  may be soldered to the end cap  125 , soldered to the terminals  114 ,  116 , connected via screws (not shown) passing through screw holes  30 , or otherwise connected. If screws are utilized, it is preferable that the screw holes  30  includes a conductive contact area  32  such that connection of the screws provides a conductive ground path between the suppression assembly  20  and the motor housing  123 . With such a configuration, the ground path may be provided via the contact of the ground plane  14  with the motor housing  123 , the screw contact with the motor housing  123  or a combination thereof. With the PCB board  12 , and thereby the suppression assembly  20 , electrically coupled to the motor housing  123 , the motor frame provides a virtual ground for the suppression assembly  20  to direct energy captured by the suppression assembly  20 . 
     In an alternative embodiment, the PCB board  12  may additionally be used to support the brushes of the motor  113  and perform the end cap  125  function. In such an embodiment, the ground plane  14  may be configured to contact a conductive surface within the motor housing  123  and/or screws may be utilized to provide the ground path between the between the suppression assembly  20  and the motor housing  123 . 
     Referring to  FIGS. 5A and 5B , the improved RFI suppression of the suppression circuit board  10  is shown.  FIG. 5A  shows the detected radiated emissions from an impact wrench not including a suppression circuit board in accordance with the invention.  FIG. 5B  shows the detected radiated emissions from an impact wrench that includes a suppression circuit board  10  in accordance with the invention. The amount of detected radiated emissions is significantly less with the hand tool utilizing a suppression circuit board  10  in accordance with the invention. 
     The suppression circuit board  10  improves RFI suppression and eliminates the use of multiple components wired together to form an RFI filter. Furthermore, the suppression circuit board  10  is smaller, more conveniently packaged and is less costly than competitive techniques. 
     Referring to  FIG. 6 , a suppression circuit board  10 ′ in accordance with an alternative embodiment of the invention is shown. The suppression circuit board  10 ′ is shown configured for connection to a brushless DC motor  113 ′ having a motor voltage terminal  114 A′ and a motor ground terminal  116 A′. The terminals  114 A′ and  116 A′ are configured for direct connection to wires from the power supply  111 . 
     The suppression circuit board  10 ′ is similar to the previous embodiment and includes a PCB board  12 ′. In this embodiment, the PCB board  12 ′ has a circular configuration. The configuration of the PCB board  12 ,  12 ′ in each embodiment may be selected based on the configuration of the motor and other components of the hand tool. As in the previous embodiment, a rear surface of the PCB board  12 ′ will define a ground plane  14 . Again, the ground plane  14  may be a continuous surface or may define discrete contact points. 
     As in the previous embodiment, the PCB board  12 ′ includes a voltage contact  25  configured to align adjacent to the motor voltage terminal  114 A′ and a ground contact  27  configured to align adjacent to the motor ground terminal  116 A. In the present embodiment, the contacts  25 ,  27  are positioned adjacent through holes  45  and  47 , respectively, such that the terminals  114 A′ and  116 A′ pass therethrough. Again, the close positioning allows the motor voltage terminal  114 A′ to be soldered directly to the voltage contact  25  and the motor ground terminal  116 A′ to be soldered directly to the ground contact  27  at  117 . Minimal distance between the contacts and the terminals prevents noise from bypassing/coupling around the suppression assembly  20 . 
     The voltage contact  25  is electrically connected to the first terminals of a pair of suppression assemblies  20  while the ground contact  27  is connected to second terminals of the suppression assemblies  20  such that the suppression assemblies  20  are connected in parallel between the power source  111  and the motor  113 . Again, the suppression assemblies  20  may take the form of an X2Y attenuator, but other components, including various capacitors and inductors, and other configurations may alternatively be utilized as the suppression assemblies  20 . 
     The suppression assemblies  20  of the present embodiment provide a pair of ground circuits  24  and  26 , each with one or more ground circuit contacts  28 ,  29 . Each of the ground contacts  28 ,  29  extends to a ground plane  14  on the underside of the PCB board  12 ′. The ground plane  14  may be a continuous surface or may define discrete contact points The PCB board  12 ′ is configured to be mounted directly to the end cap  125 ′ of the conductive motor housing  123 ′ with the ground plane  14  in direct contact with the motor housing  123 ′. The PCB board  12 ′ may be soldered to the end cap  125 ′, soldered to the terminals  114 A′,  116 A′, connected via screws (not shown) passing through screw holes, or otherwise connected. As in the previous embodiment, a ground path may be provided via the contact of the ground plane  14  with the motor housing  123 ′, the screw contact with the motor housing  123 ′ or a combination thereof. With the PCB board  12 ′, and thereby the suppression assemblies  20 , electrically coupled to the motor housing  123 ′, the motor frame provides a virtual ground for the suppression assemblies  20  to direct energy captured by the suppression assemblies  20 . 
     While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.