Abstract:
A thermal control circuit ( 49 ) for inexpensively protecting the power interface of cordless and dual-mode powered devices ( 12 ), such as hand-held power tools and appliances. The powered device includes a motor ( 11 ) that is operable in a preselected voltage range. A switch assembly ( 110 ) controls the flow of electrical energy to the motor ( 11 ). A power module ( 14, 16 ) is configured to supply electric power and to mate with the low-voltage DC power tool ( 12 ). The power module ( 14, 16 ) is adapted to provide a DC voltage in the preselected voltage range suitable for powering the low-voltage DC power tool ( 12 ). A case ( 91 ) for the power tool ( 12 ) has a pre-defined envelope for electrically and mechanically mating with the power module ( 14, 16 ). A power interface that includes at least two terminals, electrically couples the power module ( 14, 16 ) to the motor ( 11 ). The thermal control circuit ( 49 ) protects the power interface from damage caused by an overtemperature level. The thermal control circuit ( 49 ), in response to detecting an operating temperature that exceeds the overtemperature level, permanently interrupts the flow of electricity from the power module ( 14, 16 ) to the motor ( 11 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Non-Provisional Application No. 09/458,285, filed Dec. 10, 1999 pending. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to electrically operated power tools and in particular, to portable hand-held power tools which can operate in a cordless mode from a self-contained power source. 
     BACKGROUND OF THE INVENTION 
     Electrically operated devices that function in a cordless mode typically include a housing which has a chamber for receiving and retaining a removable battery module. The battery module completely encloses one or more cells and provides the necessary DC power for operation of the device. Historically, cordless electrically powered devices have included relatively low power devices such as shavers and hand-held calculators. Recently, improvements in battery technology have led to the development of batteries that store more energy and are capable of driving higher power devices. These devices include for example, portable hand-held power tools and appliances operating at power levels from 50 watts up to hundreds of watts. A hand-held power tool is typically powered by a battery module that comprises a number of batteries connected in series. To provide the higher power levels required by high power devices an increased number of batteries are connected in series resulting in higher input voltages and battery module volumetric requirements. 
     Cordless power devices permit work operations to be performed in areas where a conventional AC power source is not available or inconvenient to use. However, the additional power interface that is required to allow battery modules to be removed for replacement and charging leads to a decrease in the reliability of the power tool. Cordless power devices universally employ electrical contacts that are incorporated into terminal blocks and connectors as the interface to electrically couple the battery module to the power device. Over the lifetime of a power device, the electrical contacts are subjected to numerous events that may lead to the eventual wear-out or premature failure of the power interface. The wear-out mechanisms include wearing due to contamination, damage, or misalignment of the terminals, as well as high currents and contact bounce caused by high vibration environments. In addition, material discrepancies and high cross-sectional currents may contribute to wearing of the contacts. As the power interface degrades, the impedance of the connection increases leading to higher power losses in the interface. The higher power losses cause increased localized heating of the contacts that is further exacerbated by the thermally isolated nature of most power interfaces, resulting in a further increase in temperature. The high temperatures contribute to the degradation of the contacts and might eventually lead to thermal runaway, resulting in melting of the connector case. A cordless power device that is not repaired before thermal runaway occurs might be irreparable. Since, a cordless power device receives electrical energy from a limited source, the battery, the device is less likely to suffer thermal runaway than a power device that operates from an unlimited power source such as 115 Vac line power. The limited nature of battery power restricts the quantity of power that might be dissipated in the power interface, thereby limiting the amount of damage to which the interface will be subjected. 
     There is another class of power devices, dual-mode power devices, that have recently been introduced that have a power interface and an unlimited source of electrical energy. Dual-mode power devices include an optional corded converter module that connects to an AC power source and is designed to be interchangeable with the battery module. The corded converter module converts power from the AC source to a regulated low-voltage DC level that is usable by the motor of the power device. The converter module allows a power device operator to use the device in either the cordless battery mode or the corded AC mode as needed. Thus, the availability of a converter module enables the operator to complete a project when the battery module has been discharged. However, when the dual-mode device is operated with the converter module, the power interface has the potential for receiving much greater damage under failure conditions due to the unlimited power source. Therefore, it is desirable to disable a degraded power device before extensive damage occurs to the power interface. 
     While the prior art can be used to provide cordless and dual-mode power devices, it has not proven capable of minimizing the potential failure of the associated power interface for the power devices. 
     SUMMARY OF THE INVENTION 
     The present invention provides a low-voltage DC power tool that includes a removable power module for supplying electrical energy. The power module is coupled to the power tool through a power interface that includes at least two terminals. A thermal control circuit senses the operating temperature of a terminal. In response to detecting an operating temperature that exceeds an overtemperature level, the thermal control circuit permanently interupts the flow of electricity from the power module to the motor. 
     For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a three-dimensional view partially showing the manner of connecting a battery module to the power device; 
     FIG. 2 is a three-dimensional view partially showing the manner of connecting a converter module to the power device; 
     FIG. 3A is a three-dimensional exploded view of the battery module; 
     FIG. 3B is a three-dimensional exploded view of the converter module; 
     FIG. 4 is an end view of the battery module illustrating an attached terminal block; 
     FIG. 5 is a three-dimensional view of the power tool terminal block that mates to the battery module terminal block; 
     FIG. 6 is a two-dimensional view of the interface between the battery module terminal block and the power tool terminal block; 
     FIG. 7 is a two-dimensional view of the interface between the converter module terminal block and the power tool terminal block; 
     FIG. 8 is a two-dimensional view of a cover for the converter module terminal block; 
     FIG. 9 is a block diagram of a power converter assembled and contained within the converter module; and 
     FIG. 10 is a schematic diagram of a switch assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a dual-mode portable power tool  12  according to the present invention is shown. While the present invention is shown and described with a reciprocating saw  12 , it will be appreciated that the particular tool is merely exemplary and could be a circular saw, a drill, a sander, or any other similar cordless or dual-mode power tool constructed in accordance with the teachings of the present invention. 
     The power tool  12  includes a tool interface (not shown) which is driven by a DC motor  11 . The motor  11  is mounted within a double-insulated housing  91  that includes a handle  92  extending therefrom. A trigger switch  93  is mounted in the handle  92  behind the motor  11 . The DC motor  11  is adapted in the preferred embodiment to be powered by a 24 volt DC source, although other DC voltage systems, such as 18 volts or 100 volts, could be used. In a first operating mode shown in FIG. 1, the power tool  12  is powered by a removable battery module  14 . Alternatively, as shown in FIG. 2, the power tool  12  may be powered from common 115 volt AC line power via a converter module  16  which is adapted to be plugged into the power tool in place of the battery module  14 . Additionally, the power tool  12  may be powered from a AC/DC generator (not shown) via the converter module  16 . 
     With particular reference to FIGS. 3A and 4, the rechargeable battery module  14  of the present invention is illustrated to generally include a housing  18 , a battery  20  which in the exemplary embodiment illustrated is a 24 volt nickel-cadmium battery, and a battery module terminal block  22 . To facilitate releasable attachment of the battery module  14  to the tool  12 , the upper portion  25  of the housing  18  is formed to include a pair of guide rails  24 . The guide rails  24  are adapted to be slidably received into cooperating channels  13  (FIG. 1) formed in the power tool handle  92 . To further facilitate removable attachment of the battery module  14  to the tool  12 , the upper portion  25  of the battery module housing  18  further defines a recess  26 . The recess  26  is adapted to receive a latch (not shown) carried by the handle  92  of the tool  12 . The latch is conventional in construction and operation and is spring biased to a downward position so as to engage the recess  26  upon insertion of the rechargeable battery module  14 . Removal of the battery module  14  is thereby prevented until the spring bias of the latch is overcome in a conventional manner insofar as the present invention is concerned. 
     With continued reference to FIGS. 3A and 4, the battery module terminal block  22  comprises a main body portion  28  constructed of rigid plastic or other suitable material and a plurality of blade-type terminals  30 . In the exemplary embodiment illustrated, the battery module terminal block  22  includes four blade terminals  30 . Two of the blade terminals  30  comprise the positive and negative terminals for the battery  20 . A third terminal  30  may be used to monitor the temperature of the battery  20  and a fourth terminal may be used to identify the battery type (e.g., 24 volt NiCad). As best shown in FIG. 4, a pair of holes  32  are formed in the two guide rails  24  in the upper portion  25  of the battery module housing  18  on either side of the row of blade terminals  30 . The function of these holes is described below. 
     Turning now to FIG. 5, the terminal block  34  of the power tool  12  is shown. The main body of the tool terminal block  34  is also constructed of a rigid plastic material and is formed with a row of four U-shaped guideways  36  guiding the four corresponding blade terminals  30  of the battery module  14  when the battery module  14  is inserted into the tool  12 . Located within the guideways  36  are female connectors  38  that are adapted to engage and make electrical contact with the blade terminals  30  of the battery module  14 . Although the tool terminal block  34  shown is designed to accommodate four female connectors for each of the four battery module blade terminals  30 , only two female connectors  38  adapted to engage the positive and negative blade terminals  30  of the battery module  14  are used in the tool terminal block  34 , as the remaining two battery pack blade terminals  30  are only used when recharging the battery module  14 . 
     Also connected to the positive and negative female terminals  38  in the tool terminal block  34  are positive and negative male terminals  40  that project through openings  42  in the terminal block on either side of the row of guideways  36 . As will subsequently be discussed below, the male positive and negative terminals  40  are used to electrically connect the tool  12  to the converter module  16 . 
     With additional reference to FIG. 6, the interface between the battery terminal block  22  and the tool terminal block  34  is illustrated. As the guide rails  24  of the battery module  14  are slid into the channels  13  in the tool housing, the battery module terminal block  22  is guided into alignment with the tool terminal block  34  as shown. To further facilitate proper alignment between the two terminal blocks  22  and  34 , the main body portion of the tool terminal block  34  includes a pair of laterally spaced rails  44  that are adapted to be received within the grooves  46  provided in the battery module housing  18  immediately below the guide rails  24 . Further insertion of the battery module  14  onto the tool  12  results in the positive and negative blade terminals  30  of the battery module  14  passing through the openings in the U-shaped guideways  36  and engaging the female connectors  38  in the tool terminal block  34 . Note that the male positive and negative terminals  40  from the tool terminal block  34  simultaneously project into the openings  32  formed in the rails  24  on the upper portion  25  of the battery pack housing  18 , but do not make electrical contact with any terminals in the battery module  14 . Similarly, the remaining two blade terminals  30  from the battery terminal block  22  project into empty guideways  36  in the tool terminal block  34 . 
     Returning to FIG. 2 with reference to FIG. 3B, the converter module  16  according to the present invention is adapted to convert 115 volts AC house current to 24 volts DC. The housing  48  of the converter module  16  in the preferred embodiment is configured to be substantially similar to the housing  18  of the battery module  14 . In this regard, the housing  48  includes first and second clam shell halves joined at a longitudinally extending parting line. An upper portion  50  of the housing  48  includes a pair of guide rails  52  similar to those of the battery module  14  for engaging the channels  13  in the tool housing. The upper portion  50  also defines a recess (not shown) which includes a latch (not shown) for preventing the inadvertent removal of the converter module  16 . The housing  48  also defines a recess  51  in which a fan  45  is adapted for providing cooling airflow to the converter module  16 . Attached to the fan  45  is a fan cover  47  for preventing foreign objects from impeding the operation of the fan  45 . Within the housing  48  several heatsinks  43  provide heat spreading and cooling for selected power converter components. 
     With additional reference to FIG. 7, the interface between the converter module  16  and tool terminal block  34  is shown. The converter module  16  includes a pair of female terminals  54  that are adapted to receive the male terminals  40  of the tool terminal block  22 . Due to the non-isolated nature of the converter module  16 , the female terminals  54  are recessed within the upper portion  50  of the housing  48  of the converter module  16  by at least 8 mm to meet safety requirements. In a manner similar to that described above in connection with the installation of the battery module  14  on the tool  12 , the guide rails  52  on the upper portion  50  of the converter housing  48  are adapted to engage the laterally spaced rails  44  on the tool terminal block  34  as the converter module  16  is installed on the tool  12  to ensure proper alignment between the female connectors  54  of the converter module  16  and the male connectors  40  of the tool  12 . A pair of temperature cut-offs (TCOs)  53  are co-located near the female terminals  54 . The TCOs change state from a short to an open when the operating temperature of the female terminals  54  exceeds 102° C. The scope of the invention includes using any type of thermostatic device that changes state or resistance when the device is subjected to an operating temperature that is greater than a predetermined temperature. Additionally, the scope of the invention encompasses using a single TCO that is thermally coupled to one or more terminals. 
     Referring to FIG. 8, a cover  57  for enclosing the converter terminal block is illustrated. A heat pipe  55  affixed to the cover  57  thermally couples the TCOs  53  to the female terminals  54 . In the presently preferred embodiment an electrically insulating thermal pad is used as the heat pipe, however the scope of the invention encompasses also using electrically conductive thermal conductors. For ease of assembly, a self-adhesive backing of the heat pipe  55  is used to affix the heat pipe  55  to the converter terminal block cover  57 . 
     As illustrated in block diagram form in FIG. 9, the converter module  16  of the presently preferred embodiment includes a non-isolated buck converter that operates at a frequency of about 40 kHz. 115 volt AC power is converted to 24 volt DC power by the converter module  16  and delivered to the tool  12  through the female terminals  54 . When the converter module  16  is operatively installed on the tool  12 , the female terminals  38  of the tool terminal block  34  are electrically inoperative. Although the presently preferred embodiment of the converter module  16  is a fixed-frequency, non-isolated, buck-derived topology; the principles of the invention encompass using variable-frequency converters, transformer-isolated converters, and topologies other than buck-derived, such as Cük and flyback converters. A control circuit  102  regulates the output voltage, Vtool, of the converter  100  by varying the duty cycle of a power MOSFET  104  that chops the filtered input voltage. The converter output voltage is coupled through the power interface to the motor  11  in the power tool  12 . A driver  105  within the converter module control circuit  102  provides a buffered drive signal for controlling the MOSFET  104 . The control circuit  102  includes a voltage regulator  106  to generate an internal voltage, Vcc, for powering the control circuit  102 . A thermal control circuit  49  that includes the TCOs  53  and heat pipe  55  connects to the control circuit  102 . The thermal control circuit  49  disables the converter output when the temperature of the power interface exceeds a predetermined temperature. The pair of TCOs  53  electrically couple to a control circuit output transistor  108  that supplies the drive signal to the driver  105 . Each of the TCOs  53  normally presents a low impedance. When the operating temperature of one of the TCOs  53  exceeds a predetermined threshold temperature, the TCO changes state permanently to a high impedance. As a result, the drive signal that flows through the output transistor  108  and TCOs  53  is permanently disabled until the TCO is replaced. When the drive signal is disabled, the MOSFET  104  changes to an open impedance, thereby interrupting the flow of current through the power interface to the motor  11 . The scope of the invention encompasses using other control inputs to disable the control circuit  102 . Control inputs such as shutdown inputs, current sense inputs, overcurrent inputs, reference voltage shutdown, and voltage feedback inputs are envisioned. The scope of the invention also includes a similar connection of a TCO to the switch assembly  110  (see FIG. 10) to interrupt the flow of power through the power interface. 
     The power tool  12  of the present invention uses TCOs in a unique manner. Conventional usage of TCOs entails connecting the devices in series with a device that is to be protected from operating when ambient temperatures exceed a predetermined level. When the ambient temperature exceeds the predetermined level, the TCO changes state to an open, thereby preventing current from flowing into the protected device. To reset the TCO to a short, the TCO must be replaced. Since the current that flows through the protected device also flows through the TCO, the physical size of the required TCO increases with increasing current. To protect a power interface that passes currents that are several amps or more, a relatively large TCO is required for each terminal. The power tool  12  of the present invention uses a TCO that has a current rating that is substantially less than the current that flows through the protected device (power interface). In addition, the TCO changes state based upon the operating temperature of the power interface, not the ambient temperature of the surrounding air. Thermally coupling the power interface to a TCO prevents the power interface from being damaged by a runaway thermal failure. Instead, once the temperature of the power interface reaches a predetermined temperature, the flow of current through the interface is disabled. In addition, not placing the TCO in the current path of the power interface terminal, enables the use of a significantly smaller TCO to protect the power interface. 
     Referring to FIG. 10, an alternative method of using a TCO  109  to protect the power interface is illustrated. Here, the TCO  109  electrically connects to the switch assembly  110 . The switch assembly  110  controls the application of power to the motor  11 . The trigger switch  93  is connected in series with the power to the tool  12  and the motor  11 . The trigger switch  93  provides on/off control of the application of power to the motor  11  in a manner known to those skilled in the art. A variable resistance output of the trigger switch  93  connects to a tool control  112  to provide variable control. In response to resistance changes, the tool control  112  provides variable control of the application of power to the motor  11  ranging from approximately 0% to 100% power. An output  114  of the tool control  112  provides a variable duty cycle that is related to the position of the trigger switch  93 . The output  114  controls the switching of a switch  116  that is in series with the motor  11 , thereby providing variable motor speed. A thermal control circuit  107  comprising the TCO  109  and heat pipe  111  connects to the tool control  112 . The thermal control circuit  107  disables the tool control output when the temperature of the power interface exceeds a predetermined temperature. The TCO  109  is thermally coupled to both pairs of tool terminals  38  and  40  by means of a single electrically insulated heat pipe  113 . The TCO  109  electrically couples to the tool control  112  to disable the tool control output when the operating temperature of the terminals  38  and  40  exceeds a predetermined temperature. Disabling the output  114 , permanently interrupts the current flowing through the power interface, thereby eliminating the temperature increase caused by electrical power loss in the interface. To re-enable the output  114 , the TCO must be replaced. 
     The thermal control circuit of the present invention protects the power interface of a dual-mode power tool from thermal runaway. A power interface operating temperature that exceeds a predetermined threshold temperature causes the thermal control circuit to latch-off the flow of power through the interface, thereby preventing self-heating of the interface. Latching-off power prevents additional heating that would be caused by continuously cycling power into a failure condition. 
     In addition, the thermal control circuit is designed with a comparatively small number of components while protecting the power interface from relatively large currents. This enhances the lightweight, compact features of the power tool  12 . The size of the thermal control circuit further permits the use of a power converter in power-operated devices, such as the reciprocating saw  12 , which heretofore were too small to support and contain conversion units providing power in a range of at least 50 watts and higher. 
     Further, while the preferred embodiment of the thermal control circuit disables the output of the converter module, the thermal control circuit can be used to disable the switch assembly output. This permits the power interface to be protected when the power tool  12  is operated from a battery module having a large quantity of stored energy relative to the output power of the power tool. 
     The reciprocating saw  12  is merely illustrative of one example of many power-operated, cordless-mode and dual-mode devices. Other examples of power-operated cordless devices which are enhanced by the inventive concept include, but are not limited to, drills, screwdrivers, screwdriver-drills, hammer drills, jig saws, circular saws, hedge trimmers, grass shears, as well as battery-operated household products and the like. 
     Thus it will be appreciated from the above that as a result of the present invention, a power interface for power-operated cordless and dual-mode devices is provided by which the principal objectives, among others, are completely fulfilled. It will be equally apparent and is contemplated that modification and/or changes may be made in the illustrated embodiment without departure from the invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawings are illustrative of preferred embodiments only, not limiting, and that the true spirit and scope of the present invention will be determined by reference to the appended claims and their legal equivalent.