The present invention relates to a cable reeling system, and more particularly, to a development of the cable reeling system disclosed in patent application WO 93/02957.

The disclosure of WO 93/02957 is incorporated herein. In brief, the disclosure of WO 93/02957 relates to a cable reeling system which seeks to control the tension in the cable as it is reeled onto, and is payed out, from the reel.

It is an aim of the present invention to provide a cable reeling system which is responsive in real time to maintain cable tension within acceptable limits irrespective of the speed at which the cable is reeled onto, or payed out, from the reel; the stretch of the cable; and/or its effective diameter on the reel.

It is also an aim of the invention to provide a reeling system which achieves the above aim at high reeling speeds.

A definition of the present invention is given in the appended claims.

Throughout this specification, the term "reel" should be construed widely to include drum/spool-shaped reels as well as, for example, a mono-spiral reel.

Exemplary embodiments of the invention are herein described with reference to the accompanying drawings, in which:

Figure 1 shows a schematic of the hardware lay-out of the reel system of

the present invention;

Figure 2 shows an overall control scheme for the present invention;

Figure 3 shows a block diagram of the second control means;

Figures 4a and 4b show two schematic views of the second sensing means;

Figure 5 shows a view of the second sensing means in greater detail; and

Figure 6 shows a reel employing the 3/2 layering method.

Figure 1 shows a mobile installation in the form of a mobile crane 10 which is supplied power from a base station 14 via a cable 16. The cable 16 is mounted on the crane 10 via reel 12. The reel 12 is drivingly connected to an AC motor 18 via a gear box 20. The motor 18 has a first sensing means in the form of an incremental shaft encoder 36 (as shown in Figure 2) mounted on its output shaft to provide a motor shaft position signal. The slip rings are denoted 22.

Referring to Figures 4a and 4b a second sensing means 21, which is also mounted on the crane 10, provides a signal s _L representative of the speed of the cable 16 reeling onto, or paging out from the reel 12. Referring to Figure 5, the second means 21 comprises a pair of rollers 80,82 which are biased in contact with the cable 16 by a pair of links 84 and a spring 86, which connects the links 84. The links 84 share a common pivot point 88 which is disposed on the centreline of the cable 16. The roller 80 is a pinch roller and the roller 82 is a measurement roller, which is linked to an encoder which produces the signal s _L which is indicative of the velocity of travel of the cable 16. This arrangement permits the rollers 80, 82 to

follow the cable 16 without in any way diverting it.

Referring now to Figure 2, the torque output of the motor 18 is controlled by the first control means 25 substantially as described in WO 93/02957. For convenience, the portion of the present invention which corresponds generally to that disclosed in WO 93/02957 is enclosed by dashed box 29. Again, reference numeral 43 represents a conventional power output stage and the signals on lines 27 correspond to the windings of the motor 18. The significant difference between the system in box 29 and that disclosed in WO 93/02957 is that the torque which the motor 18 is to generate, Tor _d , is controlled by a second control means 55 (instead of being a constant determined by the system operator). By way of brief summary, the first control means 25, in response to the motor shaft position signal x and the desired torque signal Tor _d , effects continual instantaneous modification, via lines 27, of the stator current of the motor 18 to provide the desired torque output at the motor shaft.

In accordance with the invention, the second control means 55 is responsive to:- (i) a signal Ten,, which is selected by the system operator and may be considered to represent the desired level of tension in the cable 16 during reeling in and paying out;

(ii) the signal s _L generated by the second sensing means 21, which is representative of the linear speed of the cable 16; and (iii) the motor shaft position signal x from the shaft encoder 16.

Figure 3 shows the structure of the second control means 55 in greater detail. It will be noted that the motor shaft position signal x is differentiated to produce the motor shaft speed signal x which appears in Figure 3. The basic principle behind the algorithm implemented by the

second control means is that of combining the linear speed s _L of the cable 16 with the motor shaft speed x to produce a factor F indicative of the amount of cable 16 on the reel 12. Using the factor F, the reference signal TeiLj can be scaled to produce the desired torque signal Tor _d which then controls the first control means 25, in the way discussed previously. In this manner, the present invention varies the desired torque signal Tor _d so as to take into account the amount of cable 16 on the reel 12 and, thereby, better controls the tension in the cable 16.

However, in order to implement a practical algorithm a number of filter stages and systems constants may need to be incorporated into the algorithm outlined above.

Referring to Figure 3:

The motor speed signal x is scaled by the gear box ratio G to determine the angular speed of the reel 12. This operation is represented by box 57.

The angular speed of the reel 12 and the linear speed s _L of the cable 16 are then combined to establish the factor F in any operation represented by box 59. It will be appreciated that the factor F provides a measure of the amount of cable which is on the reel 12 (ie. its effective diameter).

The factor F is then fed through a first filter stage 61 which serves to limit the slew rate of F and prevents its rate of change from exceeding a given magnitude; this filter stage 61 may be required in order to de-sensitize the system to, for example, spurious noise on the cable speed signal s _L .

The factor F is then combined with the reference signal Tenj in an operation represented by box 64. The reference signal Ter^ is a

theoretical reference value which corresponds to the torque which should be applied to the reel 12 when the cable 16 is fully wound thereon and may be selected by the operator. Thus, in the operation represented by 64, the factor F, which is indicative of the actual amount of cable on the reel 12, scales Tenj to generate a required torque signal Tor _r .

In a practical system, it may be necessary to suppress the calculation of the required torque signal Tor _r when the cable speed s _L is below a low threshold value in for reasons of low-speed dynamic stability. This is achieved by a second filter stage 63 which senses the cable speed s _L and freezes the value of F being fed to operation represented by box 64, should the value of s _L fall below the threshold value.

A third filter stage 65 may be necessary to scale the required torque signal Tor _r to provide static compensation for

(i) direction dependant losses down stream of the motor 18, such as backwinding losses in the gear box; or

(ii) a cold start-up.

The output of this stage is denoted Tor _ri .

A fourth filter stage 67 may be necessary to scale the required modified torque signal Tor _ri to provide inertial compensation. Such a filter stage may derive the linear acceleration s _L of the cable 16 using its linear speed s _L , and use this parameter s _L to modify the signal Tor _ri to produce the signal Tor^.

The size of the motor 18 is selected such that for a given application the average output torque of the motor is around 90% of the maximum rated output torque for the motor 18. As is known in the art, motors 18 can be

operated at greater than maximum rates output torque for limited periods; it may be a function of the fourth filter stage 67 to ensure that the maximum rated output torque of the motor 18 is not exceeded for excessive periods.

A fifth filter stage 69 may be necessary to compensate for no-load losses at low levels of torque and power output. These losses are due to bearing and seal friction, and oil churning, and have been found to be approximately proportional to the motor speed x. Accordingly, filter stage 69, using a no-load torque loss constant K scaled by the motor speed x, modifies (either by addition or substraction) the signal Tor^ to generate the signal Tor _d and so compensate for the above losses.

It will be appreciated that although the above algorithm includes five filter stages and requires various system constants, the design of an algorithm in a particular instance depends very much on the proposed use of a system and the hardware which is available. For example, although in the described arrangement a static compensation filter stage 65 is described, if a high quality gearbox is employed, it may be possible to dispense with this stage.

Although the first control means 25 and the second control means 55 have been presented as two discrete units, this is a notional distinction for purpose of explanation. In practice, the first control means 25 and the second control means 55 can be implemented as a two processor system, but they can also be implemented by a single processor. With this in mind, it will be appreciated that no significance should be attached to, for example, the described order of the steps within the algorithm implemented by the second control means 55.

It is considered to be a significant advantage of the present invention that when initialising the system it is not necessary to measure the amount of cable 16 which is on the reel 12. This is because by measuring the motor shaft position x and the linear speed of the cable reeling onto, or paying ) out from, the reel 12, it is possible to calculate a measure of this quantity as previously explained. This is particularly advantageous when using a 3/2 layered drum/spool-shaped reel, as shown in Figure 6, where the amount of cable 16 on the reel is extremely difficult to estimate accurately.

The 3/2 layered reel is preferred in many applications as the combined reel/cable inertia is lower than the monospiral reel (as seen in Figures 4a and 4b) and so enables a smaller and lower cost drive to be used.

It has been found that the system of the present invention using a 45mm diameter cable and a mono-spiral reel is able to maintain the tension within a range of ± 10%, in some cases within a range of 5%, for crane speeds of up to 200 m/min.