Abstract:
A linear portable actuator including a direct current electric motor ( 2 ) rotationally driving a screw with the aid of a reduction element, wherein the motor ( 2 ) is controlled by an electronic module ( 5 ) comprising means ( 12 ) for the acquisition of instantaneous intensity of a supply current for the motor. Further, the electronic module ( 5 ) also comprises means ( 13 ) for calculating the differential coefficient in relation to supply-current intensity time, which are connected to means ( 14 ) for comparing the differential coefficient to a first predetermined value, and a comparing means controlling means ( 15 ) for switching off the supply current it the differential coefficient is greater than the first predetermined value.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a portable linear actuator, a method of controlling an electric motor of such an actuator to limit the maximum force generated thereby, and to the use thereof in a machine-part puller or in a tool for working on tubes. The invention also relates to a portable electromechanical tool for working on tubes and including such an actuator. 
   2. Description of the Related Art 
   Actuators are known that are capable of exerting forces on the basis of a direct current (DC) electric motor. 
   Such actuators are connected to specialized tools. 
   One example of such tools is a machine-part puller for pulling ball bearings. Another example concerns tools for working on tubes for expanding them. 
   Nevertheless, such actuators are often limited in the maximum force they are capable of producing. 
   Such actuators are confronted with two types of working situations. 
   In the first type of situation, the tool is in normal operation. The material against which the tool is applied thus presents resistance that means that the force to be provided by the motor is progressive up to a certain maximum working value which corresponds to the maximum working force on a loaded stroke of the actuator. 
   In the second type of situation, the tool is put into operation while unloaded. “Operating unloaded” is used to designate a situation in which a tool is connected to the actuator but the tool is not applied against any material or part. Since there is no material to oppose movement of the tool, the force exerted by the actuator is practically zero until the tool reaches the end of its stroke (e.g. because the tube expansion sectors of the tool are at the end of their stroke). Since the tool is then blocked, it generates an instantaneous force on the actuator that is extremely large. 
   The Applicant has thus observed that the force produced in the second situation can be more than 50% greater than the maximum working force. 
   This has the drawback of making it necessary to dimension the actuator mechanically in terms of its maximum unloaded force and not of its maximum working force, which in addition to increasing cost, also increases the weight and size of the actuator, and thus makes it less portable. 
   In addition, the reliability of the actuator is decreased by deteriorating the motor and the mechanism by the sudden increase in the magnitude of the force. 
   The object of the invention is to remedy those drawbacks. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention provides a portable linear actuator. 
   In another aspect, the invention provides a method of controlling an electric motor of a portable linear actuator in order to limit the maximum force generated thereby. 
   In another aspect, the invention provides a portable electromechanical tool for working on tubes. 
   Advantageously, such a tool is a multipurpose machine for working on tubes, is portable, and is electromechanical. 
   The invention also provides the use of such an actuator in a machine-part puller for bearings, or in a tool for working on tubes for expanding them. 

   
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood in the light of the following description given purely by way of example, and making reference to the accompanying drawings, in which: 
       FIG. 1  is a diagram of an actuator; 
       FIG. 2  is a block diagram of the electronics module of the  FIG. 1  actuator; 
       FIG. 3  is a graph plotting force as a function of time in a prior art actuator; 
       FIG. 4  is a graph plotting force for the actuator of  FIG. 1 ; 
       FIG. 5  is a control flow chart for the electronics module of the  FIG. 1  actuator; 
       FIG. 6  is a diagrammatic exploded side view of a portable tool of the invention; 
       FIG. 7  is a section view of means for fastening a working head; 
       FIG. 8  is a section view of the means for fastening a working head on section line VIII-VIII of  FIG. 7 ; 
       FIG. 9  is a perspective view of means for fastening a working head; 
       FIG. 10  is a perspective view of end-of-stroke sensors; 
       FIG. 11  is an axial section view of a working head for radially expanding a tube; 
       FIG. 12  is an axial section view of a working head for radially press fitting a tube; 
       FIG. 13  is a longitudinal section view of a working head for in-line axial press fitting; and 
       FIG. 14  is a longitudinal section view of another working head for orthogonal axial press fitting. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A portable linear actuator  1 ,  FIG. 1 , comprises an electric motor  2  rotating a screw  3  via a stepdown gearbox  4  of conventional structure, e.g. of the epicyclic type. The screw  3  is preferably a recirculating ball screw which, as is well known, enables the rotary movement of the motor  2  to be transformed into movement in translation of the screw along its axis. Since the portable linear actuator  1  needs to be capable of delivering a large force, a recirculating ball screw which minimizes mechanical friction is particularly well adapted. 
   The electric motor  2  is controlled by an electronics module  5  which in particular controls the electrical power supplied to the motor from a battery  6 . 
   End-of-stroke sensors  7  and  8  are also connected to the electronics module  5  in order to monitor the amplitude of the displacement of the screw  3 . By way of example, the sensors  7  and  8  may be Hall effect sensors. 
   In conventional manner, the actuator also has a manual switch  9  in the form of a trigger so as to enable the user to control the movements of the actuator. The manual switch  9  is also connected to the electronics module  5 . 
   The electronics module  5 ,  FIG. 2 , conventionally comprises a control module  10  comprising a microcontroller connected to a power module  11  that manages the supply of electricity to the motor  2  and that is thus interposed between the battery  6  and the motor  2 . 
   The manual switch  9  and the end-of-stroke sensors  7  and  8  are connected to the control module  10 . 
   The power module  11  has means  12  for acquiring the instantaneous magnitude of the current supplied to the motor  2 . This acquisition is performed conventionally, e.g. by using a current-to-voltage converter connected to an analog-to-digital converter, or by using a MOSFET. 
   The acquisition means  12  thus provide the control module  10  with a numerical value that is representative of the instantaneous current being supplied to the motor  2 . 
   The control module  10  also has calculation means  13  for calculating the time derivative of the power supply current. 
   It also includes means  14  for comparing the derivative with a first predetermined value S 1  and for comparing the power supply current with a second predetermined value S 2  in order to operate a switch  15  for cutting off the power supply to the motor  2  if the derivative is greater than the first predetermined value S 1  or if the current is greater than the second predetermined value S 2 . 
   The operation of the actuator is explained with reference to  FIG. 3 . 
   Observation of  FIG. 3 , which plots force as a function of time while the actuator is loaded in a curve  20 , and while the actuator is operating unloaded in a curve  21 , shows that when operating under load, the curve increases regularly before reaching a quasi-asymptote. The slope, i.e. the derivative, of the curve  22  is therefore never very high. 
   Conversely, the curve  21  shows that on becoming blocked at  23 , growth is almost vertical so the slope is very steep, curve  24 . 
   It is recalled that with a DC electric motor, there is a linear relationship between the current supplied to the motor and the force delivered thereby, so it will readily be understood that detecting a derivative of this current greater than a certain predetermined value is an indication of the actuator becoming blocked, and therefore requires the motor to be stopped by interrupting its power supply. 
     FIG. 4  shows force curves in the two preceding situations, under load in curve  25 , and with blocking after operating unloaded in curve  26 , as they apply to an actuator including an electronics module as described above. It can be seen that detecting too great a derivative does indeed prevent the actuator from finding itself in a situation where it is delivering a force greater than the maximum useful force. 
   The method of controlling the electric motor  2  by the electronics module  5  is described in detail below with reference to  FIG. 5 . 
   After an initialization stage  31 , the microcontroller waits for a motor start signal at  32  as delivered by the trigger  9 . When the microcontroller receives at  32  the order to start, it causes power to be supplied to the motor. In parallel, it starts an inhibit time-out (not shown) during which no account is taken of acquisition signals in order to ignore transient starting phenomena, and it starts acquisition timing at  33  for acquiring values representative of the current supplied to the electric motor  2 . This timing serves to acquire values at regular intervals. 
   Then, at  34 , the microcontroller acquires a value representative of the current on which it performs signal processing at  35  in order to discriminate in particular between signal and noise. 
   If at  36  the inhibit time-out has not terminated, then new acquisition timing is triggered at  33 . Else, the microcontroller at  37  compares the value of the current with a predetermined maximum threshold S 2 . This second predetermined value S 2  serves to verify that the force does not exceed a maximum value. 
   If the magnitude of the current is greater than this maximum threshold, a power supply interrupt order is issued at  38 . Else, the time derivative of the current is calculated at  39  and compared at  40  with the predetermined maximum value S 1 . If this derivative is greater than the maximum value S 1 , then the power supply is interrupted at  38 , else the value of the current is recorded at  41  prior to starting timing for the following acquisition of current at  33 . 
   It should be observed that if the acquisition timing performed at  33  is constant, i.e. if acquisition takes place at regular intervals, then the derivative can be calculated approximately by calculating the difference between two values acquired with a regular period, and the maximum slope is then replaced by a maximum difference. Thus, the electronics module  5  includes means for sampling at regular intervals, the calculation means storing the sampled values over a moving time window of predetermined duration, and then taking the difference between the values of the most recent current sample and of the oldest current sample, with the comparator means comparing the difference with the first predetermined value. 
   It is thus possible to retain the n most recent successive current values, where n is equal to 10, for example. These values can be stored in a first-in first-out (FIFO) type memory. The difference is thus taken between the present current value and the current value that was obtained n−1 intervals earlier. 
   The value selected for n is a compromise between maximizing the accuracy of the calculation microcontroller and the desired detection speed. 
   Timing means are also included to ensure that the interrupter means cannot be activated until after a predetermined length of time has elapsed after starting the actuator. 
   The end-of-stroke sensors  7  and  8  are connected to the electronics module  5  in such a manner that the power supply switch  15  is activated when these sensors detect that the screw is at the end of its stroke. 
   The operation of the end-of-stroke detectors  7  and  8  is described below with reference to  FIG. 10 . 
   The operating principle of a -recirculating ball screw implies that it is prevented from rotating so that the rotary movement of the nut containing the balls is transformed by the screw into movement in translation. 
   The recirculating ball screw  3  includes a transverse anti-rotation pin  80  with each end thereof sliding in a groove  81  of a stationary part  82  of the portable linear actuator  1 . 
   The anti-rotation pin  80  has a magnet  83  at one of its ends. 
   In addition, the stationary guide part  82  includes, placed on the guide groove, two Hall effect sensors  7  and  8  which act as end-of-stroke detectors. Thus, the distance between the two sensors  7  and  8  defines the maximum stroke of the appliance. 
   When the magnet  83  goes past the Hall effect sensor it modifies the magnetic field of the sensor, thereby triggering a modification in the current flowing through the sensor. Appropriately positioning of these sensors thus makes it possible to inform the control electronic  5  that the recirculating ball screw  3  is reaching the end of its stroke. 
   In a variant embodiment, the guide part  82  comprises an electronics card having a plurality of locations adapted to receive Hall effect sensors. 
   Thus, a single electronics card can be used for one or more models of portable linear actuator having, in particular, different displacement lengths for their recirculating ball screws. During manufacture, the sensors are positioned at locations that are appropriate for the recirculating ball screw in question. 
   By limiting the force of the actuator in this way, it is possible to dimension the motor and the mechanical parts as a function solely of the maximum useful force. This produces an actuator that is less expensive and lighter in weight, and therefore more easily portable. 
   Furthermore, the motor is not subjected to any sudden large forces, thereby increasing its reliability. 
   This makes it possible to use the actuator with tools that require large forces. One example of such a tool is a puller of machine parts, in particular for pulling a bearing. Tools for working on tubes, in particular for expanding tubes are likewise tools that require large forces and that can therefore be used with this actuator. 
   One such electromechanical tool for working on tubes is described below with reference to  FIG. 6 . 
   The portable tool for working on tubes comprises,  FIG. 6 , a portable linear actuator  1  as described above and at least two working heads, with one of those heads  60  being illustrated. 
   In the description below, terms such as “forwards”, “outwards”, “to the left” are synonyms designating movement caused by the recirculating ball screw in the direction of arrow A in  FIG. 6 , i.e. from the inside toward the outside of the portable linear actuator  1 . 
   Similarly, since the orientations of the various drawings are identical, “front” corresponds to the left-hand portion of the items shown, and thus, for the portable linear actuator  1 , to the end where the head  60  is attached. 
   The portable linear actuator  1  also has means  62  for fastening the working head, which means are in the form of a hollow cylindrical endpiece having the same axis as the recirculating ball screw  3 . 
   Each specialized working head  60  has fastener means  63  complementary to the fastener means  62  of the portable linear actuator  1  for the purpose of fastening these heads  60  securely to the portable linear actuator  1  in releasable manner. 
   The main characteristics of the fastener means  62 ,  63  are to enable the head  60  to be fastened quickly on the portable linear actuator  1 , to withstand high levels of stress, in particular along the longitudinal axis of the recirculating ball screw  3 , and to make handling easy and fast when changing working heads. 
   The fastener means  62 ,  63  can be made in numerous ways, such as, for example using a bayonet assembly or an axial snap-fastening mechanism provided with a plurality of radially-movable locking balls. 
   One particular embodiment of the fastener means  62 ,  63  is described below with reference to  FIGS. 7 ,  8 , and  9 . 
   The fastener means  63  of the working head are constituted by a hollow cylinder  90  having an annular groove  91  around its periphery. The inside diameter of the hollow cylinder  90  is suitable for passing the recirculating ball screw  3  without stress. The right or proximal end of the cylinder  90  includes a chamfer  92 . 
   The fastener means  62  of the portable linear actuator  1  comprise an annular ring  93  that screws at  94  onto a hollow cylindrical endpiece  95  of the portable linear actuator  1 . The inside diameter of the cylindrical endpiece  95  is adapted to enable the cylinder  90  of the fastener means  63  of the working head to slide therein. The axes of the recirculating ball screw  3 , of the cylindrical endpiece  95 , and of the annular ring  93  coincide. Thus, the recirculating ball screw  3  is placed in the hollow cylindrical end  95  with a large amount of clearance. 
   In front of the endpiece  95 , the annular ring  93  has an inside groove  96  connected over a fraction of its perimeter to the outside surface of the annular ring by a slot  97 . 
   A normally eccentric annulus  98  is housed in the inside groove  96 . It includes an element  99  that projects radially outwards, and that is adapted to slide in the slot  97 . Opposite from the slot  97 , a spring  100  is placed between the bottom of the groove  96  and the annulus  98  so as to urge the annulus towards the slot  97 , towards an eccentric limit position where it is in abutment against an end step  95 A of the endpiece  95 . 
   The annulus  98  also has on its inside surface a collar or collar portion  101  including a chamfer  102  on its left-hand side. 
   Operation of the fastener means is described below: 
   The working head  60  is positioned by the operator in front of the fastener means  62  of the actuator  1  in such a manner that the end of the cylinder  90  engages in the annular ring  93  and the recirculating ball screw  3  engages in the end of the cylinder  90 . By pushing the working head  90  towards the actuator  1 , the chamfered end  92  comes to bear against the chamfer  102  of the annulus  98  and pushes it radially until it moves onto the general axis (downwards in  FIG. 7 ), thus enabling the cylinder  90  to pass through until the annular groove  91  is in register with the collar  101 . Under drive from the spring  100 , the collar  101  is received in the annular groove  91 , thereby locking the working head  60  in position. A radial shoulder  104  of the head  60  adjacent to the cylinder  90  is then pressed firmly against the radial front face  103  of the ring  93 . 
   To change the head, the operator pushes back the annulus  98  by pressing on its projecting portion  99 , thereby disengaging the collar  101  from the annular groove  91  and enabling the working head  60  to be released. 
   The annulus  98  thus behaves like a sliding catch that becomes inserted in the annular groove  95  to fasten the working head. 
   This mechanism advantageously enables the working head  60  to have a degree of freedom in rotation about the linear actuator. 
   It is thus possible to fit numerous heads for working tubes on the actuator  1 , such as heads for increasing the diameter of the tube, or for press fitting a tube radially or axially, or for cutting through a tube, without this list being limiting. 
   By way of example, a plurality of working heads are described below. 
   A first working head,  FIG. 11 , serves to provide localized expansion of a tube. This radial expansion head operates on a principle that is well known and is therefore described only briefly. 
   It has fastener means  63  as described above, forming part of a cylindrical body  110 . This body is extended forwards by a screwed-on ring  111 . 
   A pusher  112  having a conically-shaped front end slides inside the body and bears via a conical part  113  against the inside faces of sectors  114 , e.g. six sectors, forming a frustoconical nose. These sectors  114  can pivot in relatively limited manner about respective annular grooves  115  provided at their rear ends, having an inwardly-directed radial collar  116  formed at the front end of the ring  111  engaged therein. 
   The nose made up of sectors  114  is positioned so as to press against the inside periphery of a tube, as represented by arrows, and the recirculating ball screw  3  pushes the conical part  112  forwards, thereby pushing the part  113  forwards, which has the effect of moving the elements  114  radially apart, away from their rest position, thereby locally increasing the diameter of the tube. 
   A second working head,  FIG. 12 , is a radial press fitting head. Fundamentally similar to a hand press fitter, it comprises two jaws  120  and  121  hinged about two parallel pins  122 ,  123  carried by a body  124 , the body having the fastener means  63  at its rear end. A pusher  125  slidably mounted inside the body carries two wheels  126 ,  127  at its front end, which wheels are mounted on two rotary pins  128 ,  129  parallel to the pins  122 ,  123 . The two wheels bear against the inside cam-forming surfaces  120 A,  121 A of the rear arms  120 B,  121 B of the jaws. 
   As represented by the arrows, by pushing the sliding pusher  125  forwards, the recirculating ball screw  3  causes the arms  120 B,  121 B of the jaws to move apart under drive from the wheels  126 ,  127 , thus enabling the jaws to clamp together, thereby radially press fitting a tube disposed between them. 
   A third working head,  FIG. 13 , is an in-line axial press fitting head. Axial press fitting, also referred to as “ring-pushing”, consists in positioning a ring B in such a manner that it creates a firm connection between the tube T and a connection part P. After expanding the end of the tube, the part P is engaged in the flared end of the tube, and the ring B slides freely thereover. 
   The head has two main parts: a body  130  which carries a stationary jaw  131  at its front end and the fastener means  63  at its other end, and a moving part  132  provided with a second jaw  133  adapted to slide inside the body  130 . Each jaw forms a semicircular cradle for receiving the part P and the tube, the tube cradles sharing a common axis X-X parallel to the sliding direction Y-Y of the part  132 . 
   The recirculating ball screw pushes the second part  132  thus enabling the two jaws  131  and  133  to move towards each other, enabling the end of the tube to come into abutment against an outside collar  134  on the part P, and enabling the ring B to be forced onto the part P clamping the end of the tube tightly on the part P in order to provide a leaktight connection. 
   Of operation that is very simple, this tool suffers from the drawback of needing to be positioned together with the portable linear actuator  1  on the axis of the tube. This can be a drawback if the operation is performed in a confined space having very little room on the axis of the tube on which work is to be carried out. 
   In order to solve this problem, the head of  FIG. 14  enables a tube to be press fitted axially while the tool is held perpendicularly to the axis of the tube. 
   This head,  FIG. 14 , operates in a general plane corresponding to the plane of the figure. 
   It comprises a stationary body  140  that is generally T-shaped with the foot of the T-shape including the fastener means  63 . 
   The body  140  is hollow so as to allow a part  141  to slide therealong on the displacement axis of the recirculating ball screw  3 . Two opposite sides of the body define two plane and parallel surfaces  142 ,  143 . 
   The solid front portion  140 A of the stationary body  140  has two rotary pins  144  and  145  at its ends, extending perpendicularly to the plane of the movement. 
   Inside the body, a return spring  146  connects the body to the sliding part  141 , urging it rearwards. 
   A part  148  that is symmetrical about the displacement axis X-X of the ball screw  3  and that is substantially lozenge-shaped has its short diagonal parallel to the displacement axis of the part  141 . This part is secured to the moving part  141  by a rod  141 A that is guided in two opposite slots  141 B in the body  140 . The part  148  has four guide wheels  149  that run along surfaces  142  and  143  of the body. 
   At two ends corresponding to the long diagonal of the lozenge-shaped part  148  there are two triangular parts  152  and  153  that are symmetrical to each other about the axis X-X, and that are hinged about two rotary pins  150 ,  151 . 
   These two parts  152  and  153  are substantially in the form of isosceles triangles having their bases substantially parallel to the body of the stationary part  140  when the head is in the open position and having their apexes lying between their bases and the stationary body  140 . 
   The first ends of these bases are hinged on the pins  150 ,  151  of the lozenge-shaped part, while their second ends carry respective rotary pins  154 ,  155  having another pair of mutually symmetrical triangular parts  156 ,  157  fitted thereto. 
   The two parts  156 ,  157  are likewise substantially in the shape of isosceles triangles. 
   Their apexes are hinged about the pins  154 ,  155 . The first ends of their bases are hinged about the pins  144 ,  145  of the stationary body  140 , and the second ends of their bases carry rotary pins  158 ,  159  on which two arms  160  and  161  are hinged. In front, these arms carry jaws  162 ,  163  for receiving the tube and the part to be engaged in the tube. 
   These jaws  162 ,  163  are identical to the jaws  131 ,  133  of  FIG. 13 , their cradles of axis Z-Z being perpendicular to the axis X-X of the body  140 . They are mounted differently so as to enable press fitting to be performed close to a wall. 
   The guide arms  160 ,  161  themselves have two inner cam surfaces  164 ,  165  at their right-hand ends over which there slide guide studs  166 ,  167  that are positioned at the apexes of the first pair of triangles  152 ,  153 . The surfaces  164 ,  165  are inclined and converge rearwards. 
   This working head operates as follows. 
   At rest, the sliding part  141  is at the rear end of the stationary part  140  and the jaws  162 ,  163  are open, being spaced apart from each other ( FIG. 14 ). 
   By pushing against the sliding part  141 , the recirculating ball screw  3  moves it forwards together with the lozenge-shaped part  148  to which it is secured, and thus pushes the lozenge-shaped part towards the front of the stationary body  140 . 
   This forward displacement also pushes the first pair of triangles  152 ,  153  which in turn push the ends of the second pair of triangles  156 ,  157  that are connected thereto towards the left. 
   These parts  156 ,  157  with their apexes secured to the stationary body  140  serve to transform the movement in translation into movement in rotation that brings the pins  158 ,  159  towards each other. 
   However, it is clear that for mutual engagement to be effective, it is necessary for the two jaws  162 ,  163  to remain parallel to each other throughout this movement in rotation. 
   That is the purpose of the guide arms  160 ,  161 . The opposing force exerted by the tube and its connection part exerts a rotary couple in the direction opposite to the rotation of the parts  156 ,  157 . Thus, the guide arms  160 ,  161  are held pressed against the studs  166 ,  167  along their surfaces  164 ,  165 . 
   The shape of these surfaces  164 ,  165  is specially adapted to ensure that at all times during the movement, the jaws  162 ,  163  remain parallel to each other. 
   Advantageously, this working head thus makes it possible to make a joint while keeping the tool perpendicular to the tubes to be joined together, making it possible to operate even when the tube is already fixed to a wall or situated in an environment that is difficult of access. 
   This working head preferably operates together with the portable linear actuator described above. Nevertheless, the person skilled in the art knows how to adapt this head without difficulty to other actuators capable of generating linear movement, such as electro-hydraulic actuators. 
   It will be understood that in a variant, other heads for working tubes could be used together with the portable linear actuator  1 , e.g. a guillotine type of head for cutting a tube. 
   The portable tool described thus makes it possible by means of its recirculating ball screw to deliver a large force by electromechanical means of small weight while using a motor powered by a battery.