referring in detail to the drawings where similar parts are identified by like reference numerals , and , more particularly to fig1 , the mobile practice dummy 20 comprises generally an upper body 22 and a remotely controllable , self - propelled base unit 24 . the upper body 22 comprises a pliant mass intended to simulate the body of an opposing player . the upper body preferably comprises a compressible material such as a plastic foam covered with an abrasion resistant outer shell such as a vinyl covering and may have a mass intended to simulate the mass of an opposing player . the upper body may be any shape including the shape of a human torso but commonly available practice dummies have a generally frustoconical shape as illustrated in fig1 . referring also to fig2 , 4 and 5 , the base unit 24 is self - propelled and remotely controllable and comprises generally a propulsion unit 23 and a cover 25 . the propulsion unit 23 comprises generally a chassis 43 supported by a plurality of ground engaging elements , for examples , wheels 40 , 42 or caterpillar treads . the chassis comprises generally a plurality of longitudinal plates 44 spaced apart by and secured to each other by a plurality of lateral members 46 . the wheels 40 , 42 are mounted on axles 52 , 54 which rotate in bearings 56 that are secured to longitudinal chassis plates 44 . batteries 60 , secured in the chassis , provide the energy to operate the self - propelled base unit . the exemplary base unit 24 is propelled and steered by two powered wheels 40 , each driven by a respective motor 62 , 64 . power is transferred to the respective wheel from its motor by a transmission comprising a gear set including a pinion 66 attached to the motor shaft and a driven gear 68 which rotates a jackshaft 70 rotatably supported in the chassis . a first pulley 72 attached to the jackshaft 70 is coupled by a drive belt 76 to a second pulley 74 which is drivingly connected to the respective powered wheel . the speed and direction of rotation of each motor 62 , 64 are separately controllable by a respective electronic speed controller 78 , 80 . the electronic speed controllers 78 , 80 respond to signals from at least one receiver 82 of electromagnetic signals by applying variable width electrical pulses to the respective motors . the motor responds to the pulses by rotating at substantially the same speed as it would if it was excited by a continuous voltage equal to the time weighted average voltage of the series of pulses . although signals in another portion of the electromagnetic spectrum , such as infrared light , might be used to control the practice dummy , the receiver 82 is typically a radio frequency receiver receiving signals from a remote radio frequency transmitter 84 which is typically operated by a member of the coaching staff . the operator is able to remotely control the speed and direction of rotation of the individual motors and thereby the speed and direction of the mobile practice dummy to simulate the actions of a human player . a resilient bumper assembly 47 substantially encircles the chassis 43 of the propulsion unit . the bumper assembly preferably comprises a resilient outer bumper band 48 that substantially encircles the chassis and resilient supporting bumper bands 49 , 50 , 51 which are affixed to respective longitudinal and lateral members 44 , 46 defining the perimeter of the chassis . the bumper bands which preferably comprise a resilient plastic , such as an ultra - high molecular weight ( uhmw ) plastic , resiliently deform when the propulsion unit comes in contact with another object , such as a player , to protect the propulsion unit and the other object . the cover 25 is generally cylindrical in shape and preferably comprises resilient plastic foam covered with an abrasion resistant outer shell , such as vinyl . the cover includes a portion , in the underside of the cover , defining a cavity 26 that is arranged to fit over the propulsion unit and an upper interface surface 33 to engage and support the upper body 22 when it is installed on the base unit 24 . the cover is supported by the propulsion unit and the cavity 26 includes a base surface 28 and a side wall 30 which is arranged to encircle and engage the outer bumper band 48 to support the cover and aid in restraining lateral displacement of the cover and the upper body when the speed and direction of the base unit changes . the cover 25 is detachably restrained to the propulsion unit 24 by a first element 94 and a complementary second element 96 of a hook and loop fastener affixed respectively to the outer bumper band 48 and to the sidewall 30 of the cavity 26 in the cover . the restraint provided by the hook and loop fastener reduces the likelihood of separation of the cover when the speed or direction of the dummy changes or when the dummy is impacted or tipped by a blocker or tackler but allows removal the cover for access to the propulsion unit . the side wall 30 is the inner surface of a skirt portion 32 of the cover 25 which encircles the base unit . when a tackler or blocker engages the upper body 22 it is possible that the player will make contact with the base unit as well . the resilient skirt 32 prevents direct contact with the bumper of the propulsion unit and reduces the likelihood that the blocker or tackler will make contact with more rigid elements of the propulsion unit . a significant portion of the practice dummy &# 39 ; s weight is represented by the base unit 24 and the low center of mass of the practice dummy enables rapid direction and speed changes without tipping , simulating the speed and mobility of a human player . the inventor realized that the upper body could tip or slide and become detached from the mobile base unit during rapid maneuvering . in addition , when a tackler or blocker makes contact with the upper body of the practice dummy , the force of the contact will tend to overturn the practice dummy and , if the base unit remained attached to the upper body and tipped , with the upper body the tackler or blocker could fall on the upturned base unit . to reduce the likelihood of contact between a player and the base unit and to retain the upper body during rapid maneuvering , the inventor arranged the upper body 22 to be supported by and restrained to the base unit , but separable from the base unit 24 when the upper body is contacted by another object such as a player tackling or blocking the dummy . although other mechanisms for increasing the separation resistance of the engaged surfaces of the upper body and the base unit , such as magnets , might be used , the inventor concluded that separability with an appropriate resistance to separation could be achieved by affixing portions of first element 90 and a complementary second element 92 of a hook and loop fastener to respective portions of surfaces , the interface surface 33 of the cover and the supporting surface 35 of the upper body , which engage each other when the upper body is installed on the base unit . the self - propelled , remotely controlled practice dummy can simulate the speed and mobility of a human athlete enabling potential tacklers and blockers to practice appropriate techniques for engaging other players without engaging in live drills with other players . releasably securing the upper body to the mobile base unit allows the practice dummy to change speed and direction rapidly without separating the upper body from the base unit but allows separation of the upper body and the base unit when a player tackles or blocks the dummy . the detailed description , above , sets forth numerous specific details to provide a thorough understanding of the present invention . however , those skilled in the art will appreciate that the present invention may be practiced without these specific details . in other instances , well known methods , procedures , components , and circuitry have not been described in detail to avoid obscuring the present invention . the terms and expressions that have been employed in the foregoing specification are used as terms of description and not of limitation , and there is no intention , in the use of such terms and expressions , of excluding equivalents of the features shown and described or portions thereof , it being recognized that the scope of the invention is defined and limited only by the claims that follow .
a practice dummy comprising a remotely controlled , self - propelled base unit supporting a detachable upper body enables athletes to practice football skills against a dummy that can simulate the speed and mobility of a human player .