Patent Publication Number: US-6662560-B2

Title: Motion transducer efficient for small amplitude movements

Description:
The present application is a continuation-in-part of U.S. patent application Ser. No. 09/642,756 filed Aug. 22, 2000 which is a continuation of U.S. patent application Ser. No. 09/034,266 filed Mar. 4, 1998, now U.S. Pat. No. 6,139,324. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hydraulic motion transducer, and more particularly to tactile and movement sensation generators, as used in motion simulators, for example a home theater simulator. 
     2. Description of the Prior Art 
     A traditional movie involves the use of two viewer senses: the sight and the sound. For more than fifty years this was the way of transmitting sensations to viewers. However, recently, new movie technologies have allowed the use of other senses in order to provide an even more realistic experience for the viewers. The tactile and equilibrium senses were added to sight and hearing in order to allow viewers to even “feel” action during a movie. 
     In many places where the affluence of clients permits, expensive technologies have been implemented, especially in amusements parks. For example, U.S. Pat. No. 5,678,889 to Purcell discloses a modular assembly of theater seats moved by a hydraulic cylinder while U.S. Pat. No. 3,865,430 to Tanus presents a theater chair movable by remote control. However, the hydraulic motors used for moving these chairs in amusement parks have a built-in moving system and are consequently large and expensive. They may only be employed in a large scale application, such as in public theaters. These technologies may not be employed for the large majority of movie screens, which are the home theaters. 
     Nevertheless, in recent years, new devices for providing tactile sensations to movie viewers emerged in the field of home use. The U.S. Pat. No. 4,750,208 to Yamada et al. disclosed an audio-band electromechanical vibration converter that provides small amplitude vibrations according to a soundtrack. Like this patent, most devices in the field usually comprise a magnetic vibration generator that reproduces the low frequency vibrations of movie soundtrack on contact with the viewers skin. Other devices transmit vibrations to the body of the viewer&#39;s chair. However, only very small amplitude vibrations may thus be transmitted to the viewers using those devices and this kind of vibrations do not always reflects the action occurring in the movie. Moreover, such small amplitude vibrations are attenuated by the chair cushion in the case of devices that transmit vibrations to the viewer through the chair, so the viewer may barely feel those tactile sensations. Current devices do not provide enough amplitude of the movements for creating real movement sensations for movie viewers. Since they only perform very simple signal processing such as filtering, they do not provide controlled movements. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a multi-sense theater chair transducer system for use with a chair or a sofa, that provides both tactile and movement sensations to a home movie viewer by transmitting these sensations to the viewer via the chair or the sofa. 
     It is another object of the present invention to provide a multi-sense chair transducer system that receives the sound output from a sound system having a plurality of outputs (currently available sound outputs for most sound formats are front, back, left, right, center and subwoofer sound outputs) and computes these sound signals in order to provide directional tactile and movement sensations to the viewer according to the sound signals. In this manner, the movie viewer will feel sensations which are synchronized with the current action from the movie being played. 
     The present invention comprises three main modules: the first one is the main encoder module which receives the sound signals from a video system such as a digital video disk (DVD or LD), a VCR system, a computer or a simple TV set and performs a digital sound signal analysis in order to output movement codes. Any source of video signal that also comprises a form of sound signals, such as analog or digital encoded sound signal may be employed as well, such as a satellite decoder (SAT) or a PC running a video game. The second module is the motors controller which receives the movement codes from the main encoder module and outputs a power drive signal for the motors in such a way that the motors may run in correlation with the initial sound signals and provide the tactile and movement sensations to the viewer. The mechanical movement module is the last module and it comprises the electrical motors as well as a shaft encoder and a protection circuit for the motors. The present application will only cover the second and the third module of the present system, i.e. the motors controller module and the mechanical movement module. 
     The movement generator module preferably comprises four devices, each intended to be placed under a corner of a platform, or a leg of a chair or of a sofa in order to transmit the tactile and movement sensations to the viewer. Each device comprises an electrical motor working in association with a cam system or a hydraulic system in order to generate reciprocate movements. The term movement will be used throughout the present text for covering the range of movements starting with vibrations having very small amplitudes and ending with movements having higher amplitudes of the range of one inch. Thus, in the present text, the term movement sensations will cover both sensations involving the tactile sense (created by small amplitude vibrations) and the sense of equilibrium (sensations created by higher amplitude movements) of a movie viewer. 
     According to the present invention, when a signal is processed by the main encoder module, the movement codes are sent to the motors controller which then activates the motors as commanded by the main encoder, in accordance with the sound signals processed. For example, in a movie having a plurality of output sound signals such as Dolby Surround (TM) or Dolby Digital (TM), one to six different sound signals may currently be available. These signals are processed in real-time by the main encoder which outputs movement action codes for the motor controller and this last one drives the motors for providing directional movement sensations to the movie viewer. For example, with such technology, if a helicopter passes from the left to the right of the screen, the sound will first be output by the left speaker and then gradually pass to the right speaker, so the viewer hears the real passage of the helicopter. The corresponding movement sensations will be at first movements of the device(s) placed under the left leg(s) of the viewer&#39;s chair and this movements will gradually pass to the right leg(s) of the chair for providing the same sensation of movement as the sound does. 
     By using the tactile and equilibrium senses in combination with the sight and the hearing of the movie viewer, the present invention provides tremendous sensation of realism that has never been reached before in a home theater. 
     According to a preferred embodiment of the invention, there is provided a home entertainment transducer system for generating movement in a chair having at least three legs is provided, the system comprising: 
     chair leg movement generator means for generating reciprocate movements at each one of said leg of a chair; and 
     motor controlling means for processing a series of movement input codes and generate in response to each input code a sequence of power drive signals for powering said movement generator means to produce small amplitude reciprocate movements in said chair according to said movement codes. 
     According to another preferred embodiment of the invention, there is provided a movement generator for producing mechanical movements in a chair having at least three legs, the movement generator comprising: 
     a chair leg receiving surface for supporting a leg of said chair and for transmitting said mechanical movements to said chair; 
     rotative means for lifting and lowering at small amplitude and low frequency said chair leg receiving surface; and 
     an electric motor coupled to said rotative means. 
     According to the invention, there is also provided a movement generator for producing small amplitude movements, comprising a supporting surface, a first piston slidable within a chamber for lifting and lowering at small amplitude the supporting surface, a second piston slidable within the chamber for driving up and down the first piston under hydraulic fluid pressure, the first and second piston providing a piston and cylinder arrangement, and an electric motor for linearly reciprocating the second piston in accordance with power drive signals communicated to the electric motor. 
     Preferably, the movement generator includes a ball screw connected to the electric motor and the second piston, the second piston carrying out axial movement in response to a torque transmitted by the motor to the ball screw. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described and will be better understood with reference to the following drawings, in which: 
     FIG. 1 illustrates the main components of the present invention; 
     FIG. 2 shows the high level block diagram of a preferred embodiment of the present invention; 
     FIG. 3 shows the hardware block diagram of a preferred embodiment of the invention; 
     FIG. 4 illustrates a detailed view of the movement generator means as contemplated in a preferred embodiment of the invention; 
     FIG. 5 shows a detailed view of the roller bearing and the actuator that are comprised in the rotative means of a preferred embodiment of the present invention; 
     FIG. 6 illustrates a movement generator unit for a pair of chair lateral legs according to a preferred embodiment of the present invention; 
     FIG. 7 shows a detailed view of the movement generator for a pair of chair lateral legs; 
     FIG. 8 shows a preferred embodiment of the invention related to the data transmission between the main encoder and the motor controller using the direct access mode; 
     FIG. 9 shows a preferred embodiment of the invention related to the data transmission between the main encoder and the motor controller using the mapping mode; and 
     FIG. 10 is a perspective view of a movement generator unit in accordance with a further preferred embodiment of the present invention; and 
     FIG. 11 is a cross-sectional side view of the movement generator unit of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In a preferred embodiment of the present invention, the multi-sense chair transducer system  10  (also called in the present application a home entertainment transducer system) comprises a main encoder module  12 , a motor controlling means  14 , a chair leg movement generator means  16  associated with each leg  11  of a chair  13  and a remote controller  18 , as shown in FIG.  1 . The intended location of each module may also be seen from that figure: the main encoder  12  is preferably a digital processing system for processing the sound output of a video system. Such a video system may be a VCR system, a simple TV set, a computer running a video game or any other presentation, a satellite decoder or any other means for playing a video presentation, i.e. a presentation that comprises both sound and image. The main encoder  12  may be placed anywhere in the room, preferably in the neighborhood of the video system. It may communicate via cables or infra-red beams with the motor controlling means  14  which is preferably placed near the chair leg movement generator means  16  which are responsible for the mechanical movement and have to be placed under the legs of the chair in order to transmit the movement to the viewer. It is to be understood that even if the present text only refers to a chair, the term “chair” also cover a sofa or any other supporting means (e.g. a platform) that a movie viewer may use to sit on for watching. 
     FIG. 8 shows a preferred embodiment of the invention related to the data transmission between the main encoder and the motor controller using the direct access mode a video presentation on a video system. 
     FIG. 2 shows the basic structure of the multi-sense system  10 . A form of sound signals is used as input for the multi-sense system  10  and are processed in order to produce movement sensations being synchronized with the video presentation sound for the viewer. In the preferred embodiment of the invention, the source of the sound signals may be a video system that preferably outputs a digital or analog form of sound signals, such an AC-3, a DTS or a Pro-Logic format. These sound formats currently comprise up to six different sound signals such as the front-left, the front-right, the rear-left, the rear-right, the center and the subwoofer sound signals. All these signals are received by the main encoder  12  as shown in FIG.  2 . The main encoder  12  is preferably a digital signal processing unit that processes the incoming form of sound signal and outputs toward the motor controlling means  14  a series of movement action codes designating the movement sensations the movement generator means  16  are to produce. The movement action codes are preferably strings of digital data that the motor controller uses for outputting a power drive signal for powering the motors in order to produce the movements in accordance with the current sound signal read from the video presentation soundtrack. 
     FIG. 2 shows a home entertainment transducer system  10  comprising a main encoder  12 , a motor controller means  14  and a movement generator means comprising four movement generator units  16 , each of them supporting a chair leg  11 . However, in FIG. 2, the motor controller  14  comprises only two independent motor controllers sub-modules  15  and therefore using this configuration, the left movement generator units  16  (front and rear) are independent from the right movement generator units  16  (front and rear). Nevertheless, in the preferred embodiment of the present invention, four independent movement generator units are provided, each of them requiring independent control from the motor controller  14 . Such a situation is shown in FIG. 3, wherein each of the dotted line blocks is a motor controller submodule  15  independently controlling each of the four movement generator units  16 . The movement generator units are preferably operated in pairs. 
     The Motor Controller 
     In a preferred embodiment of the present invention, the multi-sense system comprises a motor controller means  14  for driving the movement generator motors according to a series of incoming movement codes representing mechanical movement to be generated according to a video presentation soundtrack, such as a movie soundtrack. 
     The motor controller  14  is preferably a digital processing unit that receives the movement action codes from the main encoder  12  and, according to these action codes, it generates a power drive signal for powering the electrical motors in order to produce mechanical movements. These action codes are in a digital form and represent the action the motors are to perform. For example, if the rear-left movement generator  16  has to generate a movement following a given path for producing a reciprocate movement, a series of consecutive positions composing this path would be transmitted through the movement action codes, from the main encoder  12  to the motor controller  14 . Upon receipt of each action code, the motor controller  14  outputs a power drive signal dedicated to the movement generator  16  to be activated. The electric motor  56  of that movement generator  16  modifies its angular position according to the new coordinates that it is suppose to reach. 
     In a preferred embodiment of the present invention better seen in FIG. 3, the motor controller  14  comprises a high speed CPU  24 , which is the core of the module. The CPU  24  performs the required calculation based on the input movement action codes for generating the power drive signals. It also performs calculation for verification purposes and controls the receiving of the data. The action codes may be received from the main encoder  12  through an RS-232 port  26 , allowing data to come in preferably at a rate of 115 Kb/s. However, other communication ports having higher communication speed may be used as well, if required. From this point, incoming data may enter a Serial Communication Interface (SCI) module  27  which is an interface for formatting the incoming data from a serial digital string into a parallel form saving work time to the CPU  24 . A secondary microprocessor may also be used for this data formatting purpose rather than the SCI module. A program PROM module  30  may be used for storing the basic program modules required for the CPU operation. These modules may contain the line codes for the unchanging operations of the system, such as the power-up program code. A flash memory  28  may further be used for storing the movement parameters, the movement data banks and the movement status, and for periodic updates of the program. Periodic updates of these parameters may be performed via the serial port  26 . These movement parameters will be described in greater detail further in this text. 
     A fault indicator module  29  may receive feedback from the movement generators  16  in order to provide safety-related information to the user, such as when an overload situation or a mechanical malfunction occur. A data transfer indicator  31  may also provide information to the user concerning the data transfer state. The movement action codes received from the main encoder  12  are processed by the CPU  24  and a digital signal is output toward the sub-module  15  to be used and which appears in dotted line on FIG. 3. A digital to analog (DIA) converter  32  receives the digital signal and transforms it into an analog voltage that may further be filtered and/or processed by a servo drive  34  such as a Pulse-Width Modulator PWM. This signal then enters a power amplifier  36  that outputs a power drive signal having a higher voltage for driving the motors inside the movement generator modules  16 . The motor controller  14  may also comprise an operation LED indicator  38  that may stay ON as long the module is turned ON, and a proximity circuitry  40 , which safety purpose will be discussed further in this text. As the motor rotates in order to produce movements to be transmitted to the movie viewer, a line receiver module  42  and a quadrature counter  44  are employed in conjunction with a shaft encoder means  45  for keeping track of the motor angular position. This may be performed either by counting the motor&#39;s angular rotation or by measuring its angular speed. Preferably, an optical encoder may be used for this purpose but other mechanical or electric means may be employed as well. 
     In another preferred embodiment of the present invention, the motor controller module  14  may perform an initialization routine each time it is turned ON. Different parameters, such as the weight of the people sifting on the chair  13 , may be recorded so that the speed of the movement, the acceleration and the force needed to move the chair  13  may be accurately calculated. The same initialization routine may further comprise adjustment of the motor angular position and a series of verifications regarding the mechanical and electronic components of the system. 
     In another preferred embodiment of the present invention, the system comprises a remote controller  18  that allows a movie viewer to turn ON and OFF the home entertainment transducer system  10  or even to adjust the characteristics of the movements. For example, switching between movement modes, such as an automatic mode, a music mode or the movie mode would be possible. Choosing the type of movements to be generated for a given movie would be also possible, such as choosing science-fiction movements when watching a science-fiction movie. Each of these modes may have predetermined kinds of movements recorded in databases and when the movie viewer selects a mode, these characteristics of the movements are read and applied to the movement. In another variant of the invention, this feature involves the presence of an encoded mode for reading codes specific to a particular movie. For example, the main encoder  12  may comprise encoded movements for particular movies and when these movies are played these codes representing encoded movements may provided even more accurate movements to be produced. 
     In another preferred embodiment of the invention, the movement action codes may be inserted among the image and sound bit stream recorded on a video support, such as a DVD. These movie DVDs may be especially designed and manufactured for use with the multi-sense systems and may comprise well-defined codes that the system may recognize at the lecture time, thus producing even more accurate movements in particular situations. 
     The Movement Generators 
     In a preferred embodiment of the present invention, the multi-sense system comprises a movement generator means composed of four movement generator units intended to be installed under the legs of the viewer&#39;s chair  13  for transmitting movements to the viewer through the chair  13 . The movement generator units receive a power drive signal from the motor controller  14  and are responsible for converting this signal into mechanical movement which produces the movement sensations to the viewer. However, greater amplitudes may be used as well for providing even greater equilibrium-related sensations to movie viewers, when needed. As better seen in FIG. 4, a movement generator module is mainly composed of an electric motor  56 , a movement transmitting system such as a gear system  52  and an actuator or rotative means  54  that transforms the rotative movement into a translation movement and transmits it from the movement generator unit  16  to the chair leg  11 . 
     The electric motor  56  is preferably a DC motor having its rotating axis in a vertical position, as shown in FIG.  4 . The motor shaft gear  58  transmits the rotative movement to the intermediate gear  60  which is in contact with the actuator gear  62 . The actuator gear  62  sits onto a thrust bearing means  64  that allows the gear  62  to rotate. Such a thrust bearing means may be a flat bearing having a small clearance, rolling either on small cylinders or on small balls. Its purpose is to allow rotation of the cam onto the base  69 . A steel cam  63  is contained in the interior of the actuator gear  62 , that steel cam  63  having a plurality of consecutive circular slopes for allowing a roller bearing means  64  (better shown in FIG. 5) to roll on it. Since the steel cam  63  is firmly attached to the actuator gear  62 , when the actuator gear  62  rotates the steel cam  63  rotates with it and the roller bearing means  64  is pushed upwardly. The steel cam  63  may also form a single piece with the actuator gear  62  so no more attaching is needed. Since the roller bearing supports the top actuator assembly  66 , also herein referred to as the chair leg receiving surface  66 , that part is also pushed upwardly according to the movement of the roller bearing and thus transmits the translation movement to the chair leg  11 . A linear bearing means  67  may be employed for allowing the up-and-down translation movement to be linear. In the preferred embodiment, the linear bearing means is fixed and has a hexagonal section in which the chair leg receiving surface  66  is firmly attached. In this manner, the chair leg receiving surface  66  can not rotate but only translate following the cam  63  axial direction, i.e. the vertical direction. 
     The roller bearing means  64  is preferably a three or six roller bearing having each of its rolls rolling on three slopes of the cam  63 . However, other configurations of cam may also be used provided that the cam in combination with the roller bearing have a low clearance. If the amplitude of the movement is small, the roller means may only be a frictionless surface sliding on the cam slopes. Concentric track roller bearing means may also be used in association with one or more cams forming concentric slopes, depending on the characteristics of the system. A compression spring  65  may be placed at the center of the roller means a shown in FIG. 5, between the chair leg receiving surface  66  and the base  69 . Its purpose is to help the electric motor  56  lifting the chair leg receiving surface  66 . 
     The frequency of the movement may be controlled by controlling the rotative speed of the motor  56 . The upward translation is performed when the steel cam  63  rotates in a first direction, so the roller bearing  64 , which does not rotate, is lifted when it rolls on the raising slopes of the steel cam  63 . When the steel cam  63  is then rotated in the opposite direction, the roller bearing  64  is lowered because it rolls down on the slopes of the cam  63 . By rotating the steel cam  63  very quickly forward and backward, an up-and-down movement may be induced to the roller bearing  64  which then transmits this vertical translation to the chair leg receiving surface  66 . For avoiding that the chair leg  11  loose contact with the chair leg receiving surface  66  during the downward translation, the system may be set such that the amplitude of the movement decreases with the frequency. 
     In another preferred embodiment of the present invention, instead of having four movement generator modules  16 , the system only comprises two movement generators, as shown in FIGS. 6 and 7. In that case, such a module supports two lateral legs of the viewer&#39;s chair, as shown in FIG. 6, and may comprise only one motor  56  that drives a transmission shaft  58 . In that preferred embodiment, the motor  56  lays in a horizontal position and transmits the rotation through the transmission shaft  58  toward the rotative means  66 . In that preferred embodiment of the invention, a cable transmission means  67  replaces the gear system  52  for transmitting the rotative movement to the rotative means for lifting and lowering the chair leg receiving surface  66 . A cable  68  has its two ends wrapped around the transmission shaft  58  in such a way that when the shaft rotates, one end of the cable is rolled around it while the other end of the cable is unrolled from the shaft, as better shown in FIG.  7 . The approximate middle part of the cable  68  may be wrapped around the perimeter of the gear  62  which may have now the form of a pulley, and drives the steel cam, causing it to rotate. The cable may wrapped around the gear&#39;s perimeter for more than one complete revolution and may be attached onto a point or a fraction of the gear&#39;s perimeter for preventing it from sliding onto the gear&#39;s side, or onto the entire gear&#39;s perimeter if it makes more than one complete tour of the gear  62 . Compression spring means  70  may also be used for tightening up the cable  68 . A torsion spring  72  may be added between the transmission shaft  58  and a fixed axis for combating the rotative means moment of inertia, especially when the electric motor begins to rotate or when passing from one angular direction to another. The torsion spring is attached in such a way, that it also helps more the motor  56  to rotate when the motor has to lift the charge than when the motor  56  lowers its charge. In fact, the torsion spring may pester the motor to rotate when lowering the charge for equilibrating the power the motor has to provide between lowering and lifting charges. When the motor  56  rotates, the rotative movement is transmitted to the transmission shaft  58  that pulls the cable  68  by rolling it around the shaft and the cable  68 , which is attached to the perimeter of the circular steel cam  63  or of the gear  62 , transmits the rotation to the cam  63 . From that point, the rotative means transforms the rotative movement into a translation movement, as previously explained in detail. 
     In another preferred embodiment of the present invention, a proximity sensor  74  may be used and installed around the movement generator units  16  for preventing that people insert a part of their body under the chair when the chair is lifted at a maximum height from the floor. In that case, the proximity detector  74  may send an emergency signal to the motor controller  14 , and the motor controller may immediately either cut off the power drive signal or may lift to the highest position and keep there the roller bearing  63  from the cam  62 , for avoiding any possible injury to the person. Such proximity sensor may use infra-red beams between the movement generator units  16  or physical movement detectors, such as cables between the units. 
     In another preferred embodiment of the present invention, the chair transducer system may receive the sound signals from a sound system that does not provide also video images. In that case, the user will only listen to the music and feel tactile and motion sensations related to the music. 
     In another preferred embodiment of the invention, the motor controller  14  may be incorporated into the chair leg movement generator units  16 . Each of these units  16  may comprise their own motor controller module that may receive its own movement codes from the main encoder  12 . This way, the movement controller unit  14  would be eliminated since all its parts would be incorporated into the movement generator units  16 . 
     FIG. 10 illustrates a further construction of a movement generator unit  110  which is adapted to be placed under a leg of a chair to generate reciprocate movements thereat. Typically, such a movement generator unit is placed under each leg of a chair or at each corner of a platform on which the chair is mounted. The movement generator units are typically operated in pairs to generate a variety of movements. 
     As shown in FIG. 11, the movement generator unit  110  generally comprises a base  112  defining a chamber  114  in which a first piston  116  is slidably mounted for raising and lowering a chair adaptor  118  having a leg supporting surface  120  defining an axially extending threaded bore  122  adapted to receive a fastener (not shown) for releasably securing one leg of a chair thereto. As will be seen hereinafter, a motor-driven piston  124  is also slidably mounted within the chamber  114  to displace the first piston  116  under fluid pressure. 
     The base  112  includes a main rectangular base member  112   a  and a secondary circular base member  112   b  connected to the main base member  112   a  by an intermediate tube  126 . The main base member  112   a  includes a horizontal hollow cylinder  128  mounted between a pair of opposed end plates  130  and  132  attached to each other by means of axially extending threaded rods (not shown). A bearing  133  is mounted at one end of the cylinder  128  adjacent the end plate  130  for rotatably supporting a coupling  134  securely mounted on an output shaft  136  of a DSP-controlled brushless AC motor  138 . The motor  138  could also be linear motor acting directly on the cylinder  190 , and thus eliminating the need for the ball screw. 
     The cylinder assembly and the electric motor  138  are supported on a bottom plate  140  upon which a cover  142  can be releasably attached to form an enclosure. A bore  144  is defined in the end plate  132  for receiving the tube  126 . The tube  126  is thereby connected in fluid flow communication with the interior of the hollow cylinder  128 . The tube  126  is connected at its opposed end to a cylindrical body  146  forming part of the secondary base member  112   b.  A recess is defined in the top surface of the cylindrical body  146  for receiving a cover  148 . Circumferentially space-apart holes  150  (FIG. 10) are defined in the cover  148  for receiving corresponding fasteners (not shown) in order to secure the cover  148  to the cylindrical body  146 . The cover  148  and the cylindrical body  146  cooperate to define a cavity  150  which is in fluid flow communication with the tube  126  and, thus, the hollow cylinder  128 . An inlet  152  is defined in the cylindrical body  146  for allowing the chamber  114 , which is formed by the internal space of the hollow cylinder  128  and the tube  126 , and the cavity  150 , to be filled up with a liquid, such as oil. A closure (not shown) is provided for closing the inlet  152  once the chamber  114  has been filled up. 
     The chair adaptor  118  has a bottom ball formation  154  freely received in an axially extending socket  156  defined in an axially extending stem portion  158  of the first piston  116 . The stem portion  158  extends upwardly from a downwardly facing surface  160  of the first piston  116 . A central opening  162  is defined in the cover  148  for allowing the stem portion  158  to project outwardly of the chamber  114 . An annular peripheral wall  164  extends upwardly from the periphery of the downwardly facing surface  160  about the stem portion  158 . The annular peripheral wall  164  is received in an annular recess  166  formed by a central cylindrical projection  168  depending downwardly from the inner surface of the cover  148 . In operation, the first piston  116  moves vertically about the central cylindrical projection  168 . 
     As seen in FIG. 11, a diaphragm or web member  170  has an integral peripheral ring  172  captively received in an annular seat  174  formed by an annular recess  176  defined in the inner surface of the cover  142  and an annular inner shoulder  178  defined in the cylindrical body  146 . The web member  170  extends across a vertical section of the chamber  114  to act as a piston seal. The web member  170  has a central portion  180  which covers the downwardly facing surface  160  of the first piston  116  and which is secured thereto by means of a fastener extending through a disc  182  and into an axially extending threaded bore  184  defined in the downwardly facing surface  160  of the piston  116 . The portion of the web member  170  extending between the peripheral ring  172  and the central portion  180  acts as a foldable skirt  186  which can telescope upon itself for allowing the first piston  116  to slide vertically within the chamber  114  while preserving the integrity of the piston seal. The downward movement of the first piston  116  will cause the skirt  186  to unfold, whereas the upward movement of the piston  116  will cause the skirt  186  to fold upon itself. The web member  170  is made of a flexible, pressure resistance, fluid impermeable material (i.e. rubberized nitrile). 
     A second web member  188  similar to the first web member  170  can be provided to act as a piston seal for the motor-driven piston  124 . The duplicate description of the characteristics thereof will be omitted for brevity. 
     The motor-driven piston  124  generally comprises a cylindrical piston member  190  securely mounted on an internally threaded member  192 , which is in turn threadably mounted on a motor-driven shaft  194  for longitudinal movements therealong in response to rotative movements of the motor-driven shaft  194 . The member  194  is a ball screw spindle, and the member  192  is a ball screw nut. According to a preferred embodiment of the present invention, the cylindrical piston member  190  and the internally threaded member  192  are jointly displaced by a distance of ½ inch for four complete turns of the motor-driven shaft  194 . By reversing the direction of rotation of the motor-driven shaft  194 , the direction of travel of the cylindrical piston member  192  and the internally threaded member  194  is reversed, thereby allowing the motor-driven piston  124  to be reciprocated within a horizontal section of the chamber  114 . 
     A longitudinal slot  196  is defined in the horizontal cylinder  128  for receiving a pin  198  supporting a bearing extending radially outwardly from the cylindrical piston member  190 . The pin  198  is constrained to move within the slot  196 , thereby preventing the cylindrical piston member  190  and the internally threaded member  192  from rotating with the motor-driven shaft  194 . The motor-driven shaft  194  is drivingly connected to the coupling  134  which is, in turn, drivingly connected to the output shaft  136  of the electric motor  138 . 
     In operation, the electric motor  138  receives a power drive signal from a motor controller and rotates the motor-driven shaft  194  in accordance with the received signal. For instance, when it is desired to raise the chair adaptor  118 , the motor  138  rotates the motor-driven shaft  194  in an appropriate direction so as to axially displace the cylindrical piston member  190  against the fluid contained in the chamber  114 . This will cause the first piston  116  to rise under the fluid pressure by a distance corresponding to the displacement of the cylindrical piston member  190 . As will be appreciated, the cross-section ratio of the horizontal and vertical cylinder sections defines the actuator&#39;s linear transmission ratio. Knowing the pitch of the motor-driven shaft  194  and of the associated internally threaded member  192 , it is possible to precisely control the mechanical movement transmitted to the chair adaptor  118 . 
     When it is desired to lower the first piston  116 , the motor-drive shaft  194  is rotated in the opposite direction so as to cause the retraction of the cylindrical piston member  190  into the horizontal cylinder  128 , thereby allowing the first piston  116  to move downwardly under the load supported thereby. The retraction of the cylindrical piston member  190  also creates a “suction effect” which contributes to lower the first piston  116 . 
     The position limits are detected by sensing the current drawn by the motor as a function of rotational position. Rotational position is detected using the optical encoder  12 . The upper limit is determined by forcing the actuator upwardly until the upper edge of  164  contacts the upper edge of  166 . At the point of contact, the current drawn will be great and the rotation zero, as measured by the encoder  12 . The lower position can be detected by counting the rotation of the motor, or by detecting an increase in current near the bottom of the travel as screw  184  contacts the bottom plate of housing  146 . The DSP is programmed to set “soft” limits for the upper and lower limits which are safely within the actual physical limits (e.g. within 10 to 15 mils above and about 60 mils below of physical limits). This prevents any knocking and any damage to the web member. 
     The hydraulic actuator according to the preferred embodiment has the ability to lift up to 400 hundred pounds and to move such a heavy mass in an oscillatory motion with about a 0.25″ (6 mm) amplitude using a brushless DC motor (average power consumption of 100 W) at up to 150 Hz and an acceleration of 2 g. The presently preferred nominal maximum operating frequency of the actuators is 100 Hz. 
     Data Transmission 
     The motor controller module  14  exchanges data in a bi-directional manner with the main encoder  12 . Information regarding the full travel area (fta) or status fault indicators are passed along to the main encoder  12  to organize data accordingly. 
     The Direct Access Mode is the preferred mode of data transmission. In this mode, the input bit stream is composed of digital scheme codes called blocks. Each block may be formed of sequential data words (Ag, Ga, Dp) that the motor controller module  14  may deformat and decode. These codes are: 
     Ag: is the movement generator grouping parameter; each movement generator can be grouped with other movement generator(s) or can be addressed individually. 
     Ga: is the movement generator identification number or the movement generator group identification number; in the individual address mode, the Ga data indicates the movement generator number that will receive the position data Dp, which will give the position to be reached within the Full Travel Area fta. In group mode, the movement generators are grouped together and the Ga data indicates the movement generators group number addressed while the last data words of the bit stream will provide the requested position in respect to the full travel area fta. 
     Dp: position information in respect to the full travel area. 
     After complete reception and data verification of the transmitted data block(s), the motor controller  14  decodes the information and generates the required power drive signal for the motors  56  for performing the requested movements. It also controls the complete motors feedback to make sure the movements are accurate. When performing these tasks, the motor controller  14  may also use data read during the initialization routine, such as: the weight of the chair or sofa Ws, the weight of the users Wu, the zero position Zp, the bottom of travel Bt, the end of travel Et, the overload protection status Op, the amplifier fault status Af, the proximity sensors status Ps and all the motor drive parameters such as the torque, the velocity, the inertia, and the resolution. 
     The mapping mode is a second mode that may be used as a data transmission protocol between the main encoder  12  and the motor controller  14 . In this case, the bit stream is also composed of data blocks but these blocks comprises different data words. These words that the motor controller  14  deformat and decodes are described bellow: 
     Ag: is the movement generator grouping parameter; each movement generator can be grouped with other movement generator(s) or can be addressed individually. 
     Ga: is the movement generator identification number or the movement generator group identification number; in the individual address mode, the Ga data indicates the movement generator number that will receive the position data Dp which will give the position to be reached within the full travel area fta. In group mode, the movement generators are grouped together and the Ga data indicates the movement generators group number addressed while the last data words of the bit stream will provide the requested position in respect to the full travel area fta. 
     Mb: are the movement data banks. These data banks include the necessary data information to complete a motion scheme and serve as a base for the motion sequence being called upon by the main encoder  12 . For example, one data bank may be composed of data representing a starting car engine. Other movement data banks may represent in digital format a complete sequence of motion, such as a starting engine, a car acceleration and cruise movements. 
     A: is the amplitude parameter of the power drive signal. The data transmits to the system the magnitude of the motion to be performed and modulates the movement data bank accordingly. 
     D: is the movement direction parameter that indicates the movement transfer from one movement generator to another. 
     T: is the transfer rate used in conjunction with D. They indicate the percentage of motion transfer to be done from one movement generators to others and ranges from 0 to 100%. 
     P: is the phase control signal of a movement generator with respect to another movement generator; it may range from 0 to 180 degrees. 
     St: is the sweep time, used in conjunction with D and T in order to complete a sweep sequence. 
     Sc: represent a synchronization mode. The movements of the movement generators may be either synchronized or non-synchronized. 
     In the mapping mode too, after complete reception and data verification of the transmitted data block(s), the motor controller  14  decodes the information and generates the required power drive signal for the motors  56  for performing the requested movements. It also controls the complete motors feedback to make sure the movements are accurate. When performing these tasks, the motor controller  14  may also use data read during the initialization routine, such as: the weight of the chair or sofa Ws, the weight of the users Wu, the zero position Zp, the bottom of travel Bt, the end of travel Et, the overload protection status Op, the amplifier fault status Af, the proximity sensors status Ps and all the motor drive parameters such as the torque, the velocity, the inertia, and the resolution.