Abstract:
The speed of a cable being unwound and wound onto a cable drum disposed on a movable vehicle is controlled by developing a control signal from an error signal which is the difference of an actual value representing the drum speed and a reference value which contains two components, one proportional to the vehicle speed and another proportional to the position of a tensioning means associated with the cable drum.

Description:
SUMMARY OF THE INVENTION 
     This invention relates to an apparatus for controlling the speed at which a cable is unwound from and would onto a cable drum disposed on a movable vehicle in general and more particularly to an improved apparatus of this nature permitting operation over larger ranges of distance and velocity. 
     Various vehicles which are electrically driven such as mobile cranes, loading machines in mines and the like have a cable connection to a feeding source and thus include a cable drum onto which cable must be unwound or rewound as the vehicle moves away from or towards the feeding source. When vehicles of this nature are driven at great speed or when they accelerate or decelerate rapidly, great demands are placed on the control system for the cable drum. This is particularly true when the vehicle must have a large range of operating capability. With large distance capabilities great amounts of cables must be wound on the drum. The greater the amount of cable which is wound on the drum, the greater the moment of inertia. 
     Because of these problems, prior art control devices have not been completely satisfactory for controlling cable drum drives in vehicles of this nature. Thus, the need for an improved cable drum drive system for use in electrically driven vehicles becomes evident. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for controlling the drive of the cable drum and thus controlling the speed at which the cable is wound and unwound reliably under the types of conditions noted above. 
     In accordance with the present invention this is accomplished by measuring the actual cable drum speed, the speed of the vehicle and the position of a tensioning spring associated with the cable drum drive. The difference between the actual drum speed and the sum of the vehicle speed and tensioning device position is used to develop an error signal which is the input to a proportional integral controller. The controller develops a control output which is used to control means for driving a DC motor having its shaft coupled to the cable drum. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic elevation view illustrating cable drum equipment disposed on a movable vehicle. 
     FIG. 2 is a block diagram of the control apparatus of the present invention for controlling the cable speed in the system of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a cable drum system of the type which is controlled by the apparatus of the present invention. Shown is a vehicle 10 movable on wheels 11 which ride on a base 12. Mounted on the vehicle 10 is a cable drum 13 supported for rotation on a shaft 14. The shaft is coupled to a drive unit, not shown on FIG. 1 but to be described below in connection with FIG. 2. A cable 15 is wound onto the drum 13. The cable runs from the drum 14 over traversing wheels 16 and 17, respectively. The shaft of the traversing wheel 16 is mechanically coupled to a spring assembly 18 attached to the vehicle. The spring assembly 18 applies a bias force in an upward direction to the traversing wheel 16. Winding and unwinding of the cable 15 applies a force to the wheel which opposes the spring force. The traversing wheel 17 is fixed to the vehicle 10. Regulation of the cable speed will result in a regulation of the height of the traversing wheel 16 as will be obvious to those skilled in the art. 
     FIG. 2 is a block diagram illustrating the improved apparatus of the present invention for controlling the cable drive of FIG. 1. As illustrated, the cable drum 13 has its shaft 14 coupled to motor 19 which may be, for example, a separately excited DC motor. The motor is driven from a motor driver 30 which may be a double converter. Control of DC motors using converters is well known in the art. See, for example, Thyristor Phase-Controlled Converters and Cyclo-converters by B. P. Pelly, [John Wiley &amp; Sons, 1971], particularly page 13 et seq. Such control is also disclosed in Harwood&#39;s Control of Electric Motors by Ralph A. Miller--master [John Wiley and Sons, 1970]. In the illustrated embodiment there is shown a power input 31 which will be single or 3-phase AC power which in the converter of the motor driver is rectified in a controlled manner in accordance with the control input on line 33, as more fully described in the above-mentioned references. 
     The control signal on line 33 designated δ n  is obtained from a motor controller 20. The motor controller 20 is a proportional integral controller obtaining an input error ε from a summing junction 25. The summing junction 25 obtains as a first, actual value, input a quantity n a  from a direction sensitive tachometer generator 22 coupled by means of a shaft 23 to motor 19. The desired value inputs with which the actual value input n a  is summed at the summing junction 25 includes a quantity n b  proportional to vehicle speed and obtained from a second direction sensitive tachometer generator drive system and a quantity Δ n b  representing the position of the traversing wheel 16 obtained from a position transducer 24. Position transducer 24 may, for example, be a potentiometer coupled to the traversing wheel 16. The position transducer 24 is preferably adjusted so that the quantity Δ n b  is 0 at a preset level of the traversing wheel shaft 16 which is an intermediate position between the uppermost level of that shaft and a lower level which can be reached through compression of the spring assembly 18. What this means is that the quantity Δ n b  provides a control input to the motor controller 20 which causes it to regulate the drive of motor 19 and thus the cable drive to cause the traversing wheel 16 to remain in an intermediate position. This in turn allows the spring assembly 18 to place a suitable mechanical tension on the cable 15 while at the same time being able to move in both directions to absorb temporary deviations. Thus, the output of the summing junction 25, ε, will contain a contribution representing the difference between actual speed n a  and desired speed n b  [a quantity proportional to vehicle speed] and a contribution representing the deviation of the traversing wheel 16 from its desired position. As noted above, this input signal ε is provided to the motor controller 20 which is a proportional integral controller. For example, it may comprise an integrating amplifier which changes its output in response to input changes in order to develop an output δ n  which will result in a motor speed at which the error signal ε is zero.