Abstract:
An anti-theft system for a shopping cart includes a mounting structure for supporting anti-theft apparatus on the cart, the mounting structure being secured to the base frame of the cart. The anti-theft apparatus is removably carried by the mounting structure, with the anti-theft apparatus including a collapsible front suspension assembly having a housing; a first set of casters mounted to the housing, retractable pin elements carried by the housing and engagable with the mounting structure for maintaining the front suspension assembly in operative mode in which the first set of casters engage a supporting surface, a sensor assembly for detecting an activation signal and actuating the retractable pin elements to cause retraction of the pin elements and render the front suspension assembly inoperative, and a second set of casters fixed to the shopping cart base frame for preventing the cart from moving in a linear direction when the front suspension assembly is rendered inoperative.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to theft-thwarting devices for preventing the loss of shopping carts from shopping stores. The invention disables operation of the front wheels of the cart, and keeps the cart from being pushed off the market premises or out of the parking lot once the front wheels are disabled. Other new theft-defeating features are incorporated in this invention.  
           [0003]    2. Description of related Art  
           [0004]    Millions of dollars are lost each year by grocery stores and shopping markets as a result of shopping cart theft. The expenses associated with shopping cart theft include costs for cart replacement, retrieval of carts taken off the premises, and rehabilitation of the carts once they are returned.  
           [0005]    The prior art is replete with devices intended to prevent shopping cart theft, but none before applicants&#39; invention have achieved such a result in the manner suggested by applicants. For example, U.S. Pat. No. 3,717,225 to Rashbaum (1973) discloses a piston rod and wheel lock device which includes a frame mounted actuator that locks one of the wheels of the cart. It does not eliminate the potential for the use of one bank of cart wheels rotating by lifting the disabled wheel from the contact with the ground. U.S. Pat. No. 5,315,290 to Moreno (1994) discloses an electronic wheel lock device, which also fails to eliminate the possibility of leaning the cart to the side of the still active wheels, and thus prevent cart theft. U.S. Pat. No. 4,2422,668 to Herzog (1980) discloses a collapsible sub frame causing the wheels to swing out of operative position. This device immobilizes the front wheels of the cart, but does not eliminate the possibility the cart can be leaned back, and rolled away using the rear wheels of the cart. U.S. Pat. No. 5,357,182 to Wolfe et al. (1994) discloses a braked wheel device. This device also fails to eliminate the possibility of leaning the cart to the side of the still active wheels once the cart has left the boundary of the parking lot. U.S. Pat. No. 4,524,985 to Drake (1985) discloses an arrest device for a wheeled cart, which relies on a hooking device in the parking lot, but does not appear to eliminate the possibility that the cart could be leaned to one side to avoid the hooking device. Similarly, U.S. Pat. No. 5,576,691 to Coaklet et al. (1996) discloses another form of a wheel locking device that also fails to prevent the removal of the cart by a user who can simply tip the cart onto its back wheels and roll the cart away. U.S. Pat. No. 4,577,880 to Bianco (1986) discloses still another form of a wheel locking device that fails to eliminate the removal of the cart by simply tipping the cart on to the back wheels and rolling the cart away. U.S. Pat. No. 4,772,880 to Goldstein (1988) discloses another form of the wheel locking device, disabling only one of the front wheels. U.S. Pat. No. 5,194,844 to Zelda (1993) discloses a proximity wheel locking mechanism. This device also fails to eliminate the removal of the cart by simply tipping the cart on to the back wheels and rolling the cart away. U.S. Pat. No. 4,591,175 to Upton (1986) discloses a magnetic wheel locking mechanism. This device also fails to eliminate the removal of the cart by simply tipping the cart on to the back wheels and rolling the cart away.  
           [0006]    Many of the devices disclosed by these patents demonstrate that disabling only one wheel of a shopping cart is insufficient to completely immobilize the cart, for even if one wheel is disabled, the person removing the cart can continue to push the cart with sufficient force to override the traction of the locked wheel. Moreover, a shopping cart of the Herzog design, in which the two wheels of the front wheel assembly were disabled by allowing the sub frame of the cart to collapse around the front wheel assembly, failed to prevent the leaning of the cart backwards over, and rolling it away on, its rear wheels.  
           [0007]    Not one of these previously known devices totally prevents forward motion of the cart. If one wheel is locked, a person intent on removing the cart from the premises can exert enough force to overcome the friction of the locked wheel. If two wheels are disabled, as is taught by the Herzog patent, the cart can still be maneuvered by tilting the cart so that it can still be rolled away.  
           [0008]    In U.S. Pat. No. 6,271,755 to Prather et al., there is disclosed a theft-thwarting mechanism for use with a shopping cart for preventing removal of the cart from the premises of a market. Also known is U.S. Pat. No. 6,054,923 to Prather et al. which discloses a shopping cart that includes a disabling mechanism that renders the cart&#39;s swivel wheels inoperative upon actuation of the disabling signal.  
           [0009]    The object of these inventions was to make it nearly impossible for a person having the intent to remove the cart from the store&#39;s premises by rolling it away. In fact, tests performed by applicants have shown that cart losses due to theft, during a six-month test period, were virtually non-existent. However, various drawbacks and difficulties have since been noted, which the present invention seeks to cure. The present invention is drawn to improvements over the previously presented subject matter, and incorporates additional shopping cart theft thwarting features.  
           [0010]    For example, in the past, the cover for the front wheel suspension assembly was easily removable just by removing the fasteners that secured the cover to the suspension assembly. The present invention proposes a solution. In the past, often after repeatedly resetting the cart disabling mechanism, the reset wand failed to properly engage the reset screw associated with the disabled mechanism and resetting was prevented. The present invention provides a novel solution to this difficulty. In the past, various attempts have been made to remove the wheels attached to the front wheel suspension assembly. The present invention offers a solution to this problem. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings, in which:  
         [0012]    [0012]FIG. 1 a  shows an anti-theft shopping cart  10  according to the present invention including a base frame  12  and a basket  14 , and in which a front wheel suspension assembly of the cart is in a functional, operative, position;  
         [0013]    [0013]FIG. 1 b  shows a variation of basket according to the present invention in which the vertically extending wires at the front of the basket have been doubled in number in the front and side vertical panels of the basket;  
         [0014]    [0014]FIG. 2 shows the shopping cart base frame  12  (without the basket) with the front wheel suspension assembly of the cart in a disabled, non-functional, “triggered” state and the fixed directional casters in an enabled, functional, state;  
         [0015]    [0015]FIG. 3 a  shows details of a mounting box member which is utilized to mount the collapsible front suspension assembly to the base frame  12  of the shopping cart;  
         [0016]    [0016]FIG. 3 b  shows the mounting box member affixed to the underside of the base frame  12 ;  
         [0017]    [0017]FIG. 3 c  depicts the components of which the front wheel suspension assembly of the cart is comprised;  
         [0018]    [0018]FIG. 3 d  is a view of the underside of the front of a shopping cart modified in accordance with the invention showing the mounting box member  200  and pair of fixed directional casters;  
         [0019]    [0019]FIG. 4 is a schematic drawing showing the collapsible front suspension assembly  24  in operative and disabled states, and its functional relationship relative to the fixed directional casters;  
         [0020]    [0020]FIG. 5 a  is a perspective, partial sectional, view of a tool designed for resetting the disabled collapsible front suspension of the shopping cart  10  shown in the figures above;  
         [0021]    [0021]FIG. 5 b  is a cross-sectional view of the tool shown in FIG. 5 a;    
         [0022]    [0022]FIG. 6 schematically shows a parking lot with markings to define boundaries beyond which the front suspension assemblies of the carts will be triggered, as well as buried electrical conduits (shown in phantom) disposed in a primary loop and in secondary loops, the conduits being used to generate triggering signals according to the invention. And  
         [0023]    [0023]FIGS. 7 a  and  7   b  show details of pivoting members  144  and  150 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Referring now to the drawings in which like reference numerals refer to like elements throughout the figures, a shopping cart  10  according to the present invention has a basket  73  and a U-shaped base frame  12  comprising a front member  14  and two side members  16  and  18  extending rearwardly therefrom. The side members  16  and  18  have back ends  20  and  22  respectively.  
         [0025]    [0025]FIG. 1 a  depicts a basket in accordance with the present invention in which the vertical wire count in the forward portions of the side panels and the front panel of the basket has been doubled up. This makes it impossible for a person with intent to steal a shopping cart to insert his fingers between the wires. In the conventional basket shown in FIG. 1 a in order to lift the front of the cart off the ground and then walk the cart away from store premises in an attempt to defeat the effect of the otherwise disabled cart.  
         [0026]    A collapsible front suspension assembly  24 , shown in an untriggered functional state in FIG. 1, and a triggered disabled state in FIG. 2, extends between the two side members  16  and  18  at a location adjacent the front member  14  of the base frame  12 . Details of the suspension assembly  24  are shown in FIG. 3 c , and are described below. The front wheels  26  of the cart are supported on the underside of the suspension assembly  24 . A protective guard or shield  30  is secured to and over the suspension assembly  24  once it is properly seated in the box member  200 . A channel-shaped cover  25  having inwardly facing legs  28 ,  28 ′ is configured to be laterally slid over the suspension assembly  24  as well as the protective guard or shield  130  (see description below). Both the cover  25  and the protective shield are provided with an openings (see openings  125  in the cover  25  and opening  132  in the shield  130 ) for external access to the “reset” mechanism (described below) of the suspension assembly. The protective guard or shield  130  is mounted and secured to the suspension assembly of the cart to prevent undesired access to the suspension assembly.  
         [0027]    Also supported by the side members  16  and  18  at a more forward position (see FIGS. 2, 3 b  and  3   d  in particular) is a fixed direction caster asssembly  60  comprising a pair of wheels or casters  62 ,  64  that are arranged at an acute angle to the longitudinal axis of the base frame  12  and respective support members  66 ,  68  (see FIG. 3 d ) from which are supported the casters  62 ,  64  to the base frame  12  of the shopping cart  10 . Preferably, the support members  66 ,  68  are welded or bolted to the base frame of the cart (for example to the underside of the base frame) such that they each are positioned adjacent the intersection of one side member and the front member  14  of the base frame and diagonally spanning a respective pair of adjacent side and front members of the base frame. The wheels  62 ,  64  are preferably of the same or smaller diameter as the wheels of casters  26  attached to the suspension assembly  24 , and are arranged parallel to one another and at an acute angle to the longitudinal axis A-A of the base frame.  
         [0028]    A mounting box assembly  200  (see more specifically in FIGS. 3 a ,  3   b  and  3   d ), in which the suspension assembly  24  is disposed and by which it is mounted to the base frame of the cart, is seen to comprise outer longitudinal side walls  202 ,  204  and lateral walls  206 ,  208  spanning and interconnecting at right angles the longitudinal side walls. Disposed parallel to the outer side walls and spaced inwardly therefrom are a second set of lateral walls  210 ,  212 . An L-shaped supporting flange  214  is attached to the rearmost lateral wall  208  at the lower portion thereof and functions as a seat for supporting the suspension assembly  24  when the latter is secured in the box assembly  200 . Each of the inner side walls has a pair of spaced apart openings  232 ,  232 ′,  234 ,  234 ′ with the openings  232  and  232 ′ being laterally aligned with each other, and the openings  234  and  234 ′ being laterally aligned with one another. Bushings  236 ,  236 ′ are positioned between the inner and outer side walls of the box assembly for receiving the laterally outer ends of the pins  158  of the suspension assembly. These paired, laterally aligned sets of openings, combined with the bushings  236 ,  236 ′ are for the purpose of mounting the suspension assembly  24  in the box assembly  200  (in a manner described below). Once the suspension assembly has been properly placed in the box assembly  200 , the cover  25  is secured to the box assembly  200  via rivets or other fasteners which engage with openings  238 , and the shield  130  is then mounted atop the cover  25 . The cover  25  of the box assembly  200  essentially renders the suspension assembly tamper proof.  
         [0029]    [0029]FIG. 2 shows the collapsible front suspension  24  in a collapsed state.  
         [0030]    [0030]FIG. 3 c  depicts a U-shaped tray-like support  102  for supporting the triggering mechanism for the front suspension. Support  102  has a forward end  102   a  and a rearward end opposite the forward end. As shown, the longitudinally extending sides of the support  102  each has an upstanding laterally extending edge portion  104  and  106 . Each one of a pair of swivel wheels  26  is attached to the support  102 . Each of the wheels includes a substantially cylindrical threaded stud or bolt which is inserted through a polygonal opening  149  (see FIG. 3 d ) in the floor of the support  102 , and on which a lock nut is threaded and turned to secure the wheel to the underside of the support  102 . Immediately below the stud is a portion that includes ears or other similar radially projecting protrusions. These ears lodge in the openings in the floor of the support to lock the wheel studs against rotation relative to the support  102 , thereby making it impossible to remove the wheels  26  once they have been assembled to the support  102  and the cover  25  has been secured on the mounting box assembly  200 .  
         [0031]    The support  102  has two release pins (only pin  110  is shown in FIG. 3 c ) located at lateral sides of the support  102  and extending from a centrally located pivot area to and through the opposing edge portions of the support. The two release pins are movably mounted relative to the support and to one another (described below in greater detail) such that the outer ends of the pins are extendable beyond the exterior side surfaces  106 . This is to permit the outer ends of the pins to extend into the bushings  234 ,  234 ′ located between the respective side members  202 , 210  and  204 ,  212  of the rectangular mounting box member  200 . Preferably, the outer end surfaces of the release pins are beveled to reduce the amount of force is required to insert and extract the pins from the bushings. The support  102  also has a pair of spring-biased, retractable, hinge pins  114 ,  116  extending along the forward edge of the support. Each hinge pin is biased laterally outwardly of the support  102  and has an outer end extending beyond the exterior sides  104 ,  106  of the support. The pin outer ends are linearly aligned and engage in correspondingly situated openings  232 ,  232 ′ in respectively adjacent side members  212 ,  210  of the mounting box member  200 . Other embodiments of the invention may have a single hinge rod having opposing spring-biased end elements that perform a similar function as the two hinge pins. Another acceptable arrangement would encompass a solid, single-piece, hinge rod with ends that function as the pins.  
         [0032]    [0032]FIG. 4 is a partially cross-sectional view showing the suspension assembly in a non-collapsed, functional, orientation designated by the reference numeral  400 ′, and a collapsed, non-functional, orientation designated by the reference numeral  400 ′. When the suspension assembly becomes disabled and collapses, the fixed casters (only caster  64  is shown since FIG. 4 is a view of the forward region of one longitudinal half of the base frame) drop downwardly toward the surface  28 , placing the wheels  62 ,  64  of the fixed directional caster assembly  60  in contact with the surface  28 , and thereby inhibiting the shopping cart  10  from being pushed forward for any substantial distance. In fact, once the wheels  62 ,  64  make contact with the surface  28 , motion of the shopping cart is constrained to circular motion only, in the direction in which the two wheels  62 ,  64  are directed, as long as the collapsible suspension assembly remains disabled or collapsed. It is to be understood that when the collapsible suspension becomes disabled, the weight of the shopping cart is absorbed by the wheels  62 ,  64 , and the momentum of the cart is re-directed circular motion only. Moreover, the combination of the collapsible suspension assembly with the fixed directional caster assembly prevents destruction of cart caster components when the collapsible suspension assembly becomes disabled, as is the case with various devices taught by the prior art, and also prevents injury to customers by absorbing the weight of the cart and smoothly redirecting motion instead of abruptly ceasing motion.  
         [0033]    The suspension assembly of the cart is disabled by a triggering signal, such as an audio, magnetic or radio frequency signal, which is detected when the cart is pushed past a signal emitting member, such as an underground cable or a line-of-sight emitter. A sensor assembly  140 , mounted on the support  102 , detects the triggering signal The sensor assembly transmits a disabling signal via a trigger conduit  142  (eg, a wire) when a triggering signal is detected. Designs for sensors of the type needed to detect a triggering signal are well know in the art. Preferably, the sensor assembly  140  and the disabling signal produced thereby is powered by batteries B mounted on and carried by the suspension assembly. The sensitivity of the sensor assembly&#39;s trigger signal detecting capability can be adjusted by moving the sensor assembly toward or away from the pivoting member  144 . Such adjustability is enabled by slots  141  formed in the sensor assembly substrate. When a triggering signal is sensed, the sensor assembly  130  transmits a disabling signal to activate the mechanism for retracting the pin ends. The disabling signal preferably is an electrical current from the batteries to a titanium nickel wire  142 . The alloy of wire  142  has the property of contracting when heated. Thus, the current from the batteries heats the wire and causes it to contract. The invention also contemplates the use of other resistive materials that contract upon heating. Wire  142  is mounted to the sensor assembly substrate and extends to and from mounting pins on the pivoting member  144 .  
         [0034]    Referring again to FIG. 3 c , as well as FIGS. 7 a  and  7   b , it will be noted that the contracting of the wire  142  moves a member  144  about a pivot point  146  against the tension of one or more spring elements  143  (FIG. 3 c  shows two such spring elements). As the member  144  pivots about the pivot point  146  toward the sensor assembly  130 , a notch  148  on the member  144  disengages from holding a tail portion  145  (see FIGS. 7 a  and  7   b ) on the pivoting member  150 . As a result, the member  150  pivots, thereby moving a notch  152  on the rear side of pivoting member  150  from engagement with the side of a pivoting member  154 , which in turn is rotatably mounted to the support  102 . Most preferably, the pivoting members  144  and  150  are identical, thereby saving manufacturing costs and reducing the number of parts that are required. The the pivoting members  144  and  150 , as well as the pivoting members  154  and  155 , are formed with sets of dimples  242 ,  244  at various regions thereof. Preferably, the dimples are formed via a stamping process, and act as bearings to reduce friction as well as to prevent the build-up and collection of debris which will hamper, or even prevent (after enough time), movement of the pivoting members  144  and  150 . Through-bores  246  and  248  in pivot members  144  and  150  enable interchangability of the pivoting members  144  and  150  by permitting either through-bore to act as the pivot point, with the preferred arrangement of pivoting members being depicted in FIG. 3 c.    
         [0035]    The release pins  110  and  112  are connected to the pivoting member  154  via rigid linking members  156  such that when the pivoting member  154  pivots in one direction, the pins  110  are driven to extend beyond the side  102 ,  106  of the support  102  into the bushings  234 ,  234 ′ in the support box member  200 , and when the pivoting member  154  pivots in an opposite direction, the pins  110  are retracted from the bushings. Springs  158  are disposed about the release pins, and between the respective linking member  156  and the support sides  104 , 106  such that the springs urge the release pins to retract. This arrangement permits the release pins to retract when the notch  152  is removed from the side of the second pivoting member  154 .  
         [0036]    After the suspension assembly  24  has been triggered into the inoperative position, whereupon forward or rearward motion of the cart is controlled by the fixed directional casters  62 ,  64 , the batteries continue to discharge and generate the triggering current. This leads to premature draining of the battery power, even if rechargable batteries are used. To prevent this condition when the suspension assembly is not reset within a predetermined time, a switch, placed in the circuitry between the batteries and the trigger wire, is opened to disconnect the batteries (or other power source when used) from the trigger wire (i.e., the load) to prevent continuous drain of power following triggering of the suspension assembly into its inoperative state.  
         [0037]    When the suspension assembly is reset (i.e., the release pins  110  are reset to their extended positions so that the suspension assembly  24  once again becomes operative, as seen in FIG. 1), the switch is closed so that power to the triggering mechanism can again be tapped whenever the cart sensor assembly detects a triggering signal.  
         [0038]    To reset the release pins  110  into their extended position, the pivoting member  154  is rotated or pivoted such that the release pins are caused to extend beyond the support sides  104 ,  106  and the notch  152  is re-engaged with the side of the pivoting member  154 . This is accomplished by engaging a reset tool  800  (shown in FIGS. 5 a  and  5   b , and described in greater detail below) with a tamper proof screw  900  and rotating the pivoting member  155  by rotating the screw  900 .  
         [0039]    Preferably, the screw  900  has a polygonal exterior shape which can only be engaged by a tool possessing a complementarily configured recess. The purpose behind this mutuality of configuration is to insure a positive interlock between the screw and the tool used to turn the screw, without causing damage to either. Different sizes of the screw head can be used, and many different shapes of the screw head can be envisioned, including triangular (as shown in the drawings), star, diamond, cross-shaped, rectangular, square, etc. In this way, there can be an array of screw head configurations that work with different tool engagement zones, where the one constant is the depth of penetration into the tool engagement zone for a positive interaction between screw and tool with concomitant reduction of slippage of the screw head in the tool engagement zone A huge advantage of the variation in configurational engagement is that the tools and screws can be customized for each market chain. It is to be understood that the engagement zone of the tool is that end region which is intended and configured for engagement with the screw, and further that the screw and the tool engagement zones can be either male or female, with a preferred arrangement being a male screw head engagement zone and a female tool engagement zone for receipt of the male screw head engagement zone.  
         [0040]    The tool  800  is designed to limit the amount of torque delivered to the screw  900 . The reset tool  800  comprises an elongated body shaft portion  802  having a handle portion  804  at the upper end thereof. The handle portion includes a horizontally extending housing  806  and a short vertically extending shaft housing  808 . The handle portion is rotatably mounted to the shaft portion  802  of the tool The shaft portion can be fitted at its reset screw-engaging end  810  with a removable tool socket  812  having an internal, polygonal, receptacle configured to engage the polygonal exterior of the reset screw  900 . At the upper end of the reset tool shaft portion  802  is a spring  814  which sits concentrically about the shaft portion of the tool. The lower end of the spring  814  sits against a shoulder  816  inside the shaft housing. Urged upwardly, or away from the reset screw-engaging end  810  of the shaft portion, is a lower slip gear  818  positioned above the spring  814 , an upper slip gear  820  seated on the lower slip gear, a pivot  822  having lateral facets, and a spring plug  824  that can be rotated with a tool (not shown), such as a flat-head screw driver or other implement that can engage a slot or other similar engagement surface on the upper side of the spring plug  824  The spring plug  824  is provided with external threads and is adjustably housed inside a threaded bore  826  for adjusting the pressure on the upper and lower slip gears. It should be noted that the lower and upper slip gears bear facing, engaging, beveled gear teeth  828 ,  830  permitting rotation of one slip gear relative to the other in one direction only. The tool handle portion includes radially extending, diametrically opposed housings, each having a threaded bore  834 ,  834 ′ extending partially inward from the outer end of the respective housing. A spring alignment seat  836  is positioned in abutment with the pivot of the reset tool in each of the handle portion housings. Situated radially outwardly from the spring alignment seat  836  in each tool handle portion housing is a coil spring  838 ,  838 ′, and then even further radially outwardly Is an externally threaded spring plug  840 ,  840 ′ which makes threaded engagement with the threading in the bore  834 ,  834 ′ in each of the tool handle portion housings. By adjusting the radial portion of the spring plugs relative to the pivot against the spring action of the springs  838 ,  838 ′, a predetermined pressure can be applied to the pivot element so that slippage of the handle portion as it rotates on the shaft portion of the tool can be controlled, while ensuring that a limited torque will be applied through the shaft portion of the tool to the reset screw  900 . The slip gears also ensure that the end of the tool shaft portion can be maintained in contact with the reset screw  900  throughout the entire reset process, thereby eliminating the possibility of slippage of the tool shaft end portion off the reset screw.  
         [0041]    The shopping cart  10  preferably may also have theft-deterring wheels or casters at the rear end of the cart. These casters can be of two types—a pivotable caster  70  mounted for pivoting movement about its attachment to the base frame of the cart, or a fixed caster  80  fixedly mounted to, and at an attachment location on, the base frame of the cart (see FIGS. 1 and 2). Insofar as a pivotable caster typically can pivot freely about the attachment spindle with which it is attached to the base frame of the cart, preferably each caster  70  is provided with a forward facing stop dog that will face rearwardly when the cart is tilted backward onto its rear casters and the caster  70  pivots about it spindle  1800 .  
         [0042]    Preferably, each pivotable caster  70  (a cart may have two pivotable casters at the rear end) on the cart includes a U-shaped strap  72  which is positioned in a forward-facing manner, while each fixed caster  80  on the cart includes a U-shaped strap  82  facing rearwardly. The straps  72 ,  82  are provided as a means of preventing the cart from being moved in the direction in which the strap faces. Thus, the strap functions as a “stop dog” to further inhibit the shopping cart  10  from being pushed forwardly or rearwardly after the front suspension assembly has been disabled. Non-rotatable, fixed, wheels are prevented from rotating in a plane that is parallel to the surface  28 .  
         [0043]    If the front member of the cart is tilted upwardly to facilitate the wheeling away of the cart on its rear wheels, the stop dogs on the rear wheels will assist in preventing this by the engagement of the bottom edge  72 ,  82  of the stop dogs with the surface  28 . As the bottom edge  38  is in contact with the surface  28 , the wheel  34  is inhibited from rolling. It is to be noted that the pivotable caster  70 , as shown in FIGS. 1 and 2, is reversed, with the strap or stop dog facing forwardly. The purpose of this is to insure that motion of the cart would be thwarted if, when the front member  14  of the cart is tilted upwardly, a would-be thief instead tried to wheel the cart away in a direction. It is to be further noted that both rear casters on the cart  10  may be fixed casters or rotatable, and further that the rear casters may be one each of fixed and rotatable.  
         [0044]    Referring now to FIG. 6, the invention contemplates surrounding the perimeter of the area of protection (eg, a parking lot) with a main loop of cable or wire  400  that emits a “triggering” signal. Secondary loops  410  and  420  may also be provided to encircle cart corrals or other areas where it is desired to store or otherwise hold carts for later use. Once the shopping cart  10  is pushed past the signal-emitting boundary, the front suspension  24  collapses, thereby disabling the shopping cart.  
         [0045]    The present invention may be embodied in other specific forms without departing from the spirit or essential attributes. For example, the collapsible front suspension may be retrofitted onto existing shopping carts according to an embodiment of the invention, wherein the collapsible front suspension assembly is premounted in the mounting box assembly  200  and then the entire combined assembly is secured to a conventional shopping cart. Furthermore, once a conventional shopping cart has been so modified, the suspension assembly  24  may easily and quickly be inserted or removed (eg, for servicing) from the mounting box assembly  200  without having to dismantle any components on the so-modified shopping cart. Moreover, the rear casters on the shopping carts can be interchanged so that the carts can have different types of rear casters or the same type of rear casters. Accordingly, reference should be made to the appended claims, rather than to the foregoing specification.