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This application is a continuation of co-pending application Ser. No. 10/467,414 filed on Aug. 8, 2003, which is the 35 U.S.C. §371 national stage of International PCT/IB02/00786 filed on Mar. 13, 2002, which claims priority to Swiss Application No. 0473/01 filed on Mar. 15, 2001. The entire contents of each of the above-identified applications are hereby incorporated by reference. Any disclaimer that may have occurred during prosecution of the above referenced applications is hereby expressly disclaimed. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to automatic mechanical car parks. Such car parks generally consist of a containment building, with a reinforced-concrete or steel structure, installed in which are the necessary handling systems and machinery, with automatic collection from the entrance bay, where the user leaves the motor vehicle, and automatic return to the user at the exit bay, of motor vehicle which are contained within the said building throughout the parking period. To be specific, the present invention relates to one of the systems normally used in handling motor vehicles in this field, namely a carriage for the horizontal transfer of the motor vehicles from the parking bay (or from the entrance bay) to a handling platform, the function of which is to transport the carriage, with or without a motor vehicle, between the parking bay and the entrance and exit bays, or from a handling platform to the parking bay (or to the exit bay). During transfer of the vehicle, the handling platform, on which the carriage is normally parked, and the parking bay (or the entrance or exit bay) involved in the transfer, lie in the same plane and their respective longitudinal axes, in the line of the transfer movement, are aligned. 
     As regards known carriages and accessory systems, the following may be cited as the more significant of the prior art: EP 430892, EP 236278, EP 875644, EP 933493, WO 96/05390, WO 88/04350, DE 3820891, DE 19741638, U.S. Pat. No. 5,148,752, U.S. Pat. No. 3,159,293, U.S. Pat. No. 2,890,802. 
     None of these satisfactorily solves all of the problems connected with reliable transfer of the motor vehicle, speed of transfer, minimization of the space necessary for transferring and parking the motor vehicle, and minimization of the combined cost of the carriage and associated systems for transferring and parking the motor vehicle. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to solve all of these problems in such a way as to provide a carriage that is innovative in the sum of the distinguishing characteristics which make it optimal for carrying out its functions and for overcoming the limits of the prior art. 
     These distinguishing characteristics are as follows:
         Reliability of transfer of the motor vehicle:   Critical is the method of locking on to the motor vehicle which, according to the present invention, is lifted only via its wheels from beneath, so as to reproduce as far as possible its normal operating condition.   The wheels of the motor vehicle are locked by the carriage during the transfer in such a way that it does not matter whether or not the handbrake and any gear are or are not engaged, no problems of any kind arising from this during transfer of the motor vehicle.   The mass to be transferred is automatically limited by the carriage in order to avoid damage or malfunction caused by vehicles that may be too heavy.   When the carriage is transferring the motor vehicle from the entrance bay, as it positions itself underneath the motor vehicle there is a risk that, if the motor vehicle has been left by the user with its longitudinal axis very far from the longitudinal axis of the carriage and if the vehicle has not first been centered by means independent of the carriage, it may interfere, in its movement, with a wheel of the vehicle and get stuck against the tyre; for which reason the width of the carriage of the present invention or at any rate the width of that part of the carriage which rises above the height of the bottoms of the wheels of the vehicle, is made very small so as to allow a generous tolerance of displacement of the longitudinal axis of the motor vehicle from that of the carriage. Systems are also provided to help the user to position the motor vehicle in the entrance bay so that its longitudinal axis is as closely as possible aligned with the longitudinal axis of the carriage.   Speed of transfer of the motor vehicle:   Given the same acceleration and speed of translational movement of the carriage—which however, within certain limits, can be greater the more securely the vehicle is clamped to the carriage, —the overall speed of transfer is greater if the method of lifting the motor vehicle is such as to minimize this time, and therefore, according to the present invention, this lifting action is carried out simultaneously on all four wheels of the vehicle, following positioning of the carriage underneath the vehicle in one step, rather than first on the two wheels of one axle and then on the two wheels of the other.   The shorter the vertical lifting stroke permitted by the design of the carriage, the less time is needed to carry out this function, and therefore, in the carriage of the present invention, the vertical stroke is minimized.   The time required to centre the motor vehicle in the carriage of the present invention is superimposed on the time used for another function of the cycle of the carriage. This reduces the total cycle time and increases the speed of transfer of the motor vehicle.   Minimization of the amount of space required for transferring and parking the motor vehicle:   For the same maximum dimensions of motor vehicles to be stored, the carriage of the present invention allows the parking bays to be very small.   On this subject:   The width is minimized by the longitudinal alignment of the motor vehicle in the line of the transfer movement (centering).   To minimize the length, the carriage is designed to allow the vehicle to be released in the parking bay in a variable position depending on the length of the vehicle itself.   To minimize the height, the carriage is the lowest it can be for the vehicle to be able to park, when the carriage is inserted beneath it, with its four wheels resting on a surface only slightly higher than the surface supporting the carriage or supporting the systems that will be carrying it; also, the design of the carriage is such as to allow a very small vertical travel when lifting the four wheels of the motor vehicle off the surface on which they are standing.   Minimization of the combined cost of the carriage and associated systems for transferring and parking the motor vehicle:   There are functions, such as for example that of centering or that of limiting the load, which at present are performed by dedicated systems, separate from the carriage, but obviously with the increased costs of providing housings and additional supports. It is therefore advantageous that the carriage according to the present invention is able as far as possible autonomously to perform not only its own functions, i.e. transferring the vehicle, but the accessory functions as well.   In addition, the biggest cost is represented by the systems that park the vehicles in the parking bays, which, although being “other” than the carriage, must be regarded as a consequence of the design of the carriage itself; and the latter is therefore designed, in the present invention, in such a way as also to minimize the cost of these systems.       

     The present invention possesses all the optimal characteristics indicated above and is advantageous when compared with all the known inventions cited. 
     In particular, compared with EP 430892, it has the advantage of greater reliability in the transfer of the motor vehicle because it limits the mass of this motor vehicle and includes the systems for helping the user to position the motor vehicle more accurately in the entrance bay; the advantage, too, of a faster transfer because, without modifying any of the other conditions influencing the length of time required for the transfer cycle to be carried out, it allows a shorter vertical lifting stroke; then, too, the advantage of making it possible for the motor vehicle parking bays to be shorter because it includes systems for sensing the size of the conveyed motor vehicle and for sensing the translational position of the carriage in order that the vehicle can be released in a variable position depending on the length of the vehicle, as well as for the height of the motor vehicle parking bays to be lower due to the design of the carriage which allows lowering of the height, relative to the floor of the parking bay, to which the wheels of the motor vehicle are lifted during the transfer; and lastly, the advantage of a lower combined cost of the carriage and associated systems for transferring and parking the motor vehicle, in that the systems for parking vehicles in the parking bays, which in EP 430892 are highly complicated and expensive, being unable simply to be stood on the floor of the parking bay but having to work cantilever-fashion, can be much simpler and less expensive because they simply stand on the floor of the parking bay. 
     These and other advantages will be evident from the description of the preferred form of construction and from the characteristics listed in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The preferred, but not limiting, form of construction of the invention is described below with reference to the accompanying drawings, in which: 
         FIG. 1  is a plan view of the transfer carriage according to the invention, positioned on a handling platform  52  with the means  58  and  59  for supporting the wheels of the motor vehicle in the retracted position inside the carriage itself. 
         FIG. 2  is a plan view of an entrance bay. 
         FIG. 3  is a plan view of the carriage positioned in the entrance bay with the means for supporting the wheels of the motor vehicle in the extended position. 
         FIG. 4  is a plan view of an enlargement of one part of the carriage. 
         FIG. 5  is a cross section through the carriage positioned in the entrance bay with the means for supporting the wheels of the motor vehicle in the retracted position inside the carriage itself. 
         FIG. 6  is a cross section through the carriage positioned in the entrance bay with the means for supporting the wheels of the motor vehicle in a partially extended position and not raised. 
         FIG. 7  is a cross section through the carriage positioned in the entrance bay with the means for supporting the wheels of the motor vehicle in the extended position, that is after the vehicle has been centered, and not raised. 
         FIG. 8  is a cross section through the carriage positioned in the entrance bay with the means for supporting the wheels of the motor vehicle in the extended and raised position. 
         FIG. 9  is a side view, partially in longitudinal section, of one part of the carriage. 
         FIG. 10  is a cross section through a system, fitted to the carriage, for limiting the mass that is to be transferred, according to the invention. 
         FIG. 11  is a plan view of the transfer carriage according to the invention, positioned in a parking bay, in the course of depositing or collecting the motor vehicle. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, the carriage consists of an articulated frame  1  with hinges and hinge pins  2  to permit relative rotation between the front and rear parts into which the said frame  1  is divided. The front part has four wheels  3 , two of which are driving wheels driven by a motor/speed reducer assembly  4  via a shaft  5  and a chain and toothed wheel system  6 , while the rear part has two driving wheels  3  driven as described above. 
     The said frame  1  is guided by four rollers  7  acting on the two sides of the trench  53  which is sunk into the entrance and exit bays, the handling platform, and the parking bays. 
     Electrical power and the signals are supplied to the carriage via a suitable electrical cable  48  which is wound onto a cable reel  50  installed on the handling platform  52 , and is attached to the carriage by a shaped support  51  and guided by pulleys  49 . 
     The instantaneous position of the carriage, in the direction of longitudinal translation, is known by means of a rotating electronic system  47  mounted on the handling platform  52  to decode the linear displacement of a rope  45  attached to the frame  1  and guided by the pulleys  46 . 
     This system may conveniently be replaced with other electronic systems suitable for the purpose, e.g. one or more laser signal emitters installed on the carriage and aimed at a fixed reflective surface so as to measure the instantaneous distance between the emitting surface and the reflecting surface. 
     On each of the front and rear parts of the frame  1  is a frame  8  that can be moved vertically with respect to the frame  1 . Acting via the toothed wheel  13 , the deflecting toothed wheels  13 ′ and the chain  14 , the motor/speed reducer assembly  12  simultaneously turns the toothed wheels  15  and hence the axial cams  16 , of which there are three and which are positioned on the longitudinal axis of the carriage and are capable of lifting both frames  8 , which are borne by the cams, via pairs of arms  9  and steel wheels on roller bearings  17 . 
     The said cams  16  each have two identical straight helical surfaces rotated through 180°, on which, as the cams rotate, the three pairs of wheels  17  connected to the arms  9  roll and are raised or lowered. The arms  9 , being positioned symmetrically at a certain distance from the longitudinal axis of the carriage, provide stability to the frames  8 . 
     In order partly to further stabilize the frames  8  against potential loads that may be eccentric with respect to the longitudinal axis of the carriage, and partly to connect the frames  8  one-to-one with the frame  1 , two pins  11  are fixed to the frame  1 , and two bronze bushes  11 ′ integral with the front frame  8  run vertically on these. 
     A pair of opposing frames  58  are positioned on the two frames  8  to support the wheels  22  and  22 ′ of the front axle of the motor vehicle, and similarly a pair of opposing frames  59  to support the wheels  23  and  23 ′ of the rear axle of the motor vehicle. 
     Each frame of these pairs of frames  58  and  59  is made up of a centering bar  18  or  18 ′ designed to act horizontally against the inner side wall of the wheels  22 ,  23  or  22 ′,  23 ′. 
     Each bar  18 ,  18 ′ has metal supports  19 ,  19 ′ situated beneath and perpendicular to the said bar so as to raise the wheels  22 ,  23  and  22 ′,  23 ′, respectively, by engaging them from beneath. 
     Each frame of these pairs  58  and  59  is moved horizontally, symmetrically with respect to the longitudinal axis of the carriage with the opposite frame, by means of the racks  20 ,  20 ′, the toothed wheels  30 , the motor/speed reducer assembly  31  and the chain  32 . 
     The said frames are also equipped with balancing bars  24  and  24 ′ which engage with the guide  10  of the frame  8  via the rollers  25 ,  26  and  25 ′,  26 ′ to give stability and guided movement to the frames. 
     Referring to  FIG. 2 , when the dividing door  35  between the entrance bay and the multistorey car park is closed, the user drives the vehicle onto the entrance bay. 
     The photocells  33 , together with the reflective mirrors  34 , are positioned symmetrically and at a predefined distance relative to the longitudinal axis of the carriage in such a way that, when one of the wheels  22 ,  22 ′,  23  and  23 ′ of the motor vehicle comes too close to the said axis, the signal of one of the photocells is interrupted. 
     While a motor vehicle is moving in, whenever a photocell  35  is cut off the system control activates a light signal indicating to the user that he must modify the direction in which the motor vehicle is moving. 
     When the wheels,  22 ,  22 ′ are positioned on the rest  55 , the sensor  54  enables the stop signal. 
     As the vehicle is entering, the direction signalling made possible by the photocells  33  helps the user, while after the vehicle has come to a stop, the blocking of the rays of the photocells by one or more wheels is used as a safety lock for the carriage which, if it tried to position itself underneath the motor vehicle to transfer it, would hit one of the wheels of the vehicle. 
     Referring to  FIGS. 3 to 8 , after the motor vehicle has been correctly positioned in the entrance bay and the user has left the vehicle and initiated the parking operation, the doors  35  are opened and the carriage starts the cycle of transferring the motor vehicle by travelling from the handling platform to the entrance bay in the guide trench  53 . 
     The position at which the carriage stops in the entrance bay is determined in such a way that the axis of the pair of supporting means  58  of the front wheels  22  and  22 ′ of the motor vehicle coincides with the axis of the rests  54  on which the said wheels  22  and  22 ′ have been positioned by the user. 
     Because the rear wheels  23  and  23 ′ of the motor vehicle may be nearer to or further from the front wheels  22  and  22 ′ depending on the wheelbase of the motor vehicle, the pair of supporting means  59  of the rear wheels  23  and  23 ′ is made elongate in the direction of the longitudinal axis of the vehicle so that it can support the wheels  23  and  23 ′ of the rear axle within the range of variations of the wheelbases of motor vehicles on the market. 
     Despite the use of signals to minimize the misalignment of the motor vehicle with respect to the longitudinal axis of the carriage as the user is driving in, the front left wheel  22 , referring to the direction of movement of the vehicle, will undoubtedly be at a different distance from the longitudinal axis of the carriage than the front right wheel  22 ′ and the same will go for the rear left wheel  23  as compared with the rear right wheel  23 ′. 
     The two pairs of wheel supporting means  58  and  59  begin the horizontal outward symmetrical movement. 
     The metal supports  19  and  19 ′ fit underneath the wheels  22 ,  22 ′,  23  and  23 ′ into the free spaces between the fixed supports  28  of the wheels. 
     Continuing the horizontal outward movement, one of the centering bars  18  or  18 ′—bar  18  in FIG.  6 —meets the side wall of the tyre of the corresponding wheel and pushes it out. 
     This operation is facilitated by the presence of the rollers  29  inserted in the fixed supports  28 , which minimize the resistance to displacement of the wheel. 
     Continuing the horizontal outward movement, as shown in  FIG. 7 , the centering bar  18 ′ which has hitherto not made contact, now meets the side wall of the tyre of the wheel  22 ′,  23 ′ of the motor vehicle. 
     At this point the longitudinal axis of the motor vehicle coincides with that of the carriage: the car has been “centered” by the same movement as enabled the wheel supporting means  58  and  59  to position the metal supports  19  and  19 ′ beneath the four wheels  22 ,  22 ′,  23  and  23 ′ of the motor vehicle. 
     The horizontal outward movement of the motor vehicle wheel supporting means  58  and  59  stops when the vehicle is central, that is aligned with the longitudinal axis of the carriage, and this condition occurs when the pressure-sensitive tapes of variable-resistance conductive rubber  21  and  21 ′ applied to the surface of each centering bar  18  and  18 ′ that comes into contact with the motor vehicle tyre are simultaneously compressed, making them electrically conductive, thus enabling the said movement to be stopped. 
     The amount of horizontal outward movement of the motor vehicle wheel supporting means  58  and  59  is variable as a function of the inside tracks of the two wheels of each axle of the motor vehicle and, when this movement stops, the distance between the pressure-sensitive tapes  21  and  21 ′ applied to the centering bars  18  and  18 ′ is equal to the inside track of the wheels of the corresponding axle of the vehicle. 
     At this point the motor vehicle wheel supporting means  58  and  59 , extended as described above and pressed against the tyres so as to clamp the wheels  22 ,  22 ′,  23  and  23 ′ and prevent them moving on their resting surface, are lifted by the rising of the frames  8 . 
     To avoid transferring a vehicle whose mass is too great, unacceptable for example on the handling platform, the system shown in  FIG. 10  is used to limit the amount of mass that can be lifted by limiting the force transmissible through the chain  14 . 
     On the output shaft  39  of the lifting speed reducer  12 , the toothed wheel  13 , which passes the torque of the speed reducer  12  to the chain  24 , is housed, by the friction rings  40 , between a hub and an axially movable anchor  41  pressed via the spring  42  by a disc  43  which compresses the spring  42  by a controlled amount adjusted by means of the screw  44 . 
     Depending on the force exerted by the spring  42  and the coefficient of friction of the friction rings  40 , the toothed wheel  13  can transmit a variable force to the chain  14  to limit how much mass can be raised. 
     Once the vehicle is raised, the carriage can transfer it to the handling platform. 
     The job of the sensor  38  is to detect the presence or absence of the motor vehicle on the carriage. 
     Since the position of the wheels  22  and  22 ′ of the front axle of the motor vehicle is stationary on the carriage, the sensors  36  positioned on the carriage on the longitudinal axis at predefined distances from the axle of the front wheels make it possible to determine whether the distance between the front axle and the front end of the motor vehicle is greater or less than certain preset values. 
     In the same way, the sensors  37  make it possible to determine whether the distance between the front axle and the back end of the motor vehicle is greater or less than certain preset values. 
     On the basis of the state of the sensors  36  and  37  and the data from the rotating electronic system  47 , the system control can deposit the motor vehicle, as shown in  FIG. 11 , in a variety of positions, and in particular, if the distance between the front axle of the car and the front end is short, it can deposit the motor vehicle in such a way that its front axle coincides with the axis  56 , whereas if this distance is large, it can deposit the motor vehicle in such a way that its front axle coincides with the axis  57 . 
     The vehicle is deposited by lowering the wheel supporting means  58  and  59 , by lowering the frame  8 , in such a way that the metal supports  19  and  19 ′ fit into the spaces between the fixed supports  28  of the frames  60  installed in the parking bays, allowing the wheels  22 ,  22 ′,  23  and  23 ′ to rest on the said fixed supports  28 , after which the supporting means  58  and  59  can be withdrawn horizontally into the carriage in the rest position. 
     On the basis of the state of the sensor  38 , the data of the rotating electronic system  47  and the lifting or lowering function of the motor vehicle wheel supporting means  58  and  59 , the system control writes to memory whether the parking bay is empty or full. 
     In other words when the motor vehicle is present on the carriage and the carriage is inside a parking bay, in the right position for releasing or gripping the motor vehicle, the operation of lifting the frames  8  is always interpreted by the control as a collecting operation and hence the parking bay is stored in memory as empty, while the operation of lowering the frames  8  is always interpreted by the control as a depositing operation and hence the parking bay is stored in memory as full.

Summary:
Self-propelled carriage on wheels includes:
       one or two pairs of supporting elements for the wheels of either or both of the axles of the motor vehicle, these elements being movable symmetrically and perpendicularly with respect to the longitudinal axis of the carriage and designed to center, immobilize and lift from beneath the wheels;   elements for limiting the mass to be transferred;   elements for sensing, continuously during the transfer, the translational position of the carriage;   elements for sensing the presence of the motor vehicle on the carriage and measuring the front and rear lengths of the motor vehicle relative to its front axle; and
 
elements for sensing excessive displacement of the longitudinal axis of the motor vehicle relative to the longitudinal axis of the carriage when the motor vehicle is being positioned by the user in the entrance bay.