DRIVE SYSTEM

Abstract: A drive system (10) for driving an inertial mass (40) over a drive path between first and second extreme positions comprises an electrical drive motor (12, 14), linkage means (30) connecting the drive motor (12, 14) to the mass (40) for driving the mass, a storage capacitance (20) for storing electrical power, and a control system (16, 18) for applying electrical power to the drive motor (12, 14) to accelerate the mass (40) over a first portion of its drive path from one of the first and second extreme positions. The control means (16, 18) connects the drive motor (12, 14) to the storage capacitance (20) such that when the drive motor acts as an electromagnetic brake during deceleration of the mass (40) over a second portion of its drive path towards the other of the first and second extreme positions electrical power generated by the drive motor (12, 14) is used to charge the storage means (20). A monitoring device (22) monitors the level of energy stored in the storage means (20) and the control means applies power from the storage means (20) to the drive motor (12, 14) during acceleration of the mass (40) over the first portion of the drive path when the electrical power stored in the storage means (20) exceeds a preset level.

Description: TITLE: DRIVE SYSTEM The present invention relates to a drive system for driving an inertial mass. In existing machines such as weaving looms and knitting machines all of the moving parts are invariably driven from common drive motors. Where Jacquards, dobbies, tappet motions, cam motions in weaving looms, yarn carriers and needle actuation carriages in knitting machines (flat bed and circular types) are in use, these are generally powered on each loom or machine from independent single centralised power transmissions. Recent developments to speed up these types of fabric forming machines have been limited due to their respective high inertial masses. In the past, drives have been taken from single standard AC or DC motors working at nominal operating speeds, enabling speed reductions to be incorporated, thereby reducing inertial masses, as referred to their motor shafts, by the square of their reduction ratios. To be able to improve the speed factors in fabric forming it has become necessary to reconsider the respective mass inertias of the power driven units and their drive means in a programmable phased relationship to produce fabrics at more economical production rates. Consequently, energy costs become a dominant factor with respect to production improvement benefits. Heald frames in weaving, for example driven by dobbies, tappet motions and cam motions, are reciprocated vertically through a linkage mechanism by means of rotating cams or crank motions on a common shaft, each selected by use of a clutch or other such power selection means. The shape of the cams and/or the paths traced out by the crank motions when driven by the drive motor, accelerates the heald frame from its lowest shedding position to a velocity which is then maintained over a period before the heald frame begins to decelerate towards its upper shedding position. As the heald frame decelerates towards its upper position under the action of gravity and is then driven down, accelerated and then decelerated, towards its lowermost shedding position with the assistance of gravity, its stored potential and kinetic energy is released back to the drive shaft. This returned energy assists in accelerating other heald frames which are being moved from their lower to their upper shedding positions. A consequence of this conservation of energy is that the drive motor can be a relatively low power motor. The linkage mechanism of the heald frames typically provides a suitable mechanical advantage to allow a relatively low power drive motor to be used. This enables the heald frames to be driven at speeds up to 600 picks per minutes. However, to increase the speed of a heald frame substantially above this limit requires a very considerable increase in the power of the drive motor, rendering the system uneconomical. If the linkage is changed to provide a more direct drive to the heald frame, because of the inertia of the heald frame a considerable amount of electrical energy is generated in the drive motor as a result of the motor being "driven" by the heald frame during deceleration towards the end of the stroke of its downward shedding movement. Figure 1 is an idealised graph of velocity against time for movement of a heald frame from its lower extreme shedding position to its upper position during a shedding movement. The graph for the return movement of the heald frame from its upper shedding position to its lower shedding position is identical to Figure 1. This is an ideal motion diagram and shows that maximum power is required from the drive motor only during the acceleration period r β from time t t to t,. Once the heald frame has been accelerated from velocity v β to the required velocity v„ the velocity is then held substantially constant over a period of time t e from time t, to t, as the heald frame passes through its centre shed position. Over this period of time the energy required by the drive motor falls away, requiring just sufficient energy to overcome any frictional forces present to maintain the speed of the frame at v ; . The velocity then begins to fall away to zero over the time period t d from time t 2 to t } as the heald frame reaches its opposite upper extreme position. Over this period the drive motor in effect acts as an electromagnetic brake. As the heald frame thus drives the motor, electrical power is generated by the motor which acts as a generator and this electrical energy normally goes to waste. Because of the inertial constraints of the system the speed of acceleration and deceleration of each heald frame is limited, giving rise to relatively long acceleration and deceleration time periods t a , t d , as a result of which the time period from the lower shed position to the upper shed position is quite long. This in turn limits the speed of each cycle of movement of the heald frame. The present invention seeks to provide an improved drive system for driving an inertial mass. Accordingly, the present invention provides a drive system for driving an inertial mass over a drive path between first and second extreme positions, comprising: an electrical drive motor; linkage means connecting said drive motor to said mass for driving said mass; storage means for storing electrical power; and control means for applying electrical power to said drive motor thereby to accelerate said mass over a first portion of its drive path from one of said first and second extreme positions; wherein said control means connects said drive motor to said storage means whereby when said drive motor acts as an electromagnetic brake during deceleration of said mass over a second portion of its drive path towards the other of said first and second extreme positions said drive motor serves to charge said storage means. The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings in which: Figure 1 is a graph showing velocity against time for the ideal movement of an oscillating mass driven along a drive path between two extreme positions; Figure 2 is a graph similar to that of Figure 1 with friction and damping forces taken into account; Figure 3 is a graph similar to that of Figure 1 additionally showing velocity against time for the ideal movement of a reciprocating mass such as a heald frame of a weaving machine, the heald frame being driven by a preferred form of drive system according to the present invention; Figure 4 is a diagrammatic representation of a preferred form of drive system according to the present invention for a weaving machine; Figure 5 is a diagrammatic representation of the steps of operation of the drive system according to the present invention using relative positional feedback for the drive control; Figure 6 is a diagrammatic representation similar to that of Figure 5 using analogue absolute positional feedback for the drive control; Figure 7 is a graph similar to that of Figure 3; and Figure 8 is a further graph similar to that of Figure 3. Although the invention can be applied to any inertial mass which is driven in an oscillating manner the example described below relates to reciprocating motion of an inertial mass such as a heald or heald frame of a weaving machine. Referring to the drawings, Figure 4 shows a drive system 10 for a weaving machine 11. In this weaving machine each of the healds, and/or one or more heald frames 40, is driven by a motor or actuator. Linear actuators 12 are shown. Rotary actuators 14 may additionally or alternatively be provided. It will be appreciated that the type of actuator may be chosen to suit particular requirements and is not limited to the actuators shown in Figure 4. In the preferred embodiment each heald frame is driven by its own actuator or actuators 12, 14 (although in the following description reference is made only to heald frames it will be appreciated that the embodiment applies equally to the driving of independent healds or a combination of independent healds and heald frames). This enables very low power motors to be used to drive the heald frames directly and in rum the heald frames can be made relatively lightweight. Because each heald frame requires only a very low power actuator, actuators such as brushless servo motors (which can be accelerated very rapidly up to the required heald frame velocity) can be used. This reduces both the acceleration and deceleration time periods for the heald frame and thus enables an increase in the weaving speed. The control system has a microprocessor 16 which controls the actuation of each actuator 12, 14 individually, thereby controlling the weaving process, according to a preselected weaving programme. The microprocessor 16 determines the selection of heald frames and times the selection and the movement of the weft thread. Control signals from the microprocessor are passed to an actuator controller 18 which monitors the power drawn by each actuator. This indicates whether or not each actuator (and thus each heald frame) is being accelerated from one of its extreme lowermost or highermost shed positions. The actuator controller 18 is also connected to an electrical power storage means, preferably in the form of a capacitance 20, in turn connected both to a storage monitor 22 and an electrical power controller 24. The power controller 24 controls the supply of power from the mains supply. A DC/AC converter 26 is also provided. This is coupled to the storage capacitance 20 and can be controlled by the microprocessor 16 (or the actuator controller 18) to supply power to the mains from the storage capacitance 20, if desired. Each of the actuators 12, 14 is also provided with a sensor 28 which monitors the position of the actuator (linear or rotary) and thus of the heald frame, and supplies the actuator controller 18 with a signal which is representative of the position of the heald frame. The actuator controller 18 may alternatively or additionally use the signal from the sensor 28 to indicate whether or not each actuator and thus the associated heald frame is accelerating or decelerating rather than just monitoring the power drawn by each actuator. Figure 5 is a diagrammatic representation of the operation of the drive system and is described here with reference to the operation of a linear actuator 12. It will be appreciated that this operation applies equally to any other type of actuator. Starting from the position where the microprocessor 16 supplies a desired position signal for the position of the linear actuator 12 (i.e. of the associated heald frame), this signal is applied to an error calculation logic circuit 100 which may be in the form of a simple comparator. This compares the desired position signal with an actual position signal supplied from a store 118 to generate a position error signal which is applied to an output selection logic circuit 102. The illustrated linear actuator 12 in Figure 5 is a two-phase motor having two coils 104, 106 and drive circuits 108, 110 for the respect coils. The output selection logic 102 controls the drive circuits 108, 110 to drive the coils and move the actuator and thus the heald frame to its new position. Each coil on the actuator 12 is associated witii a respective position sensor 28, each of which provides a signal representative of the position of the actuator in relation to its associated drive coil. Although two sensors are shown here, it will be appreciated that only one sensor need be used to provide a single position signal. The change and rate of change of the position with respect to time can also be used to give both velocity and acceleration of the actuator and thus the heald frame. Each signal from the sensor 28 is supplied to a respective Inverted Position Lookup Table 112, 114 where it is compared with a datum or reference signal to provide a signal representative of the absolute position of the actuator 12 and thus of the associated heald frame. Each signal from the comparator 112, 114 is then compared in a position selection logic circuit or comparator 116 with a signal representing the previous position of the actuator 12. The comparator 116 generates an error signal as a result, the error signal being applied to a store 118. Store 118 stores the current position of the actuator 12 and updates this each time the error signal is received. The signal representing the previous position of the actuator 12 is retained in a logic circuit 120 to which a current position signal is applied from the store 118. The logic circuit 120 updates the stored signal representing the previous position of the linear actuator in response to receipt of the error signal to store a new signal representing the current position of the actuator, and thus of the heald frame. In addition, the logic circuit 120 includes a subtractor which subtracts the signals representing the new position of the actuator and the signal representing the previous position and compares this difference with a pre-selected reference value which is set by the microprocessor 16. In effect, by monitoring the change in position of the actuator with respect to time and comparing this with the pre-selected reference value the circuit is monitoring the velocity and acceleration of the actuator and thus the heald frame. It will be appreciated that the position signal which is supplied by the microprocessor 16 to the error calculation logic circuit 100 varies over the cycle of movement of the actuator and its rate of change represents the acceleration and deceleration of the actuator and thus of the heald frame. In effect, the circuit is thus monitoring the velocity of the actuator and comparing it with a desired velocity set by the microprocessor 16 by virtue of the change with time of the position signal from the microprocessor 16 and the comparison of the difference position signal with the pre-selected reference value. The comparison gives the indication of the velocity and acceleration of the linear actuator 12 and, in dependence upon the comparison of the different signal with the reference value, the logic circuit 120 applies a further error signal to a speed calculation circuit 122. The latter also receives a signal from the store 118 indicating the current position of the linear actuator and thus of the heald frame within its cycle. The speed calculation circuit 122 uses the signals to determine whether or not the heald frame should be maintained at a constant speed, accelerated or decelerated and applies a resulting signal to the output selection logic 102 to maintain the linear actuator at the same speed or accelerate or decelerate the actuator accordingly. The position signal from the store 118 is also fed to a polarity and coil selection circuit 124 which determines the direction of movement of the linear actuator 12 and thus the heald frame in dependence on this position signal and accordingly applies a directional signal to the output selection logic circuit 102 which either maintains the actuator 12 moving in the same direction or reverses its direction. Figure 6 shows a simplified control system in which the position sensor 28 on the actuator 12 supplies a position signal to a subtractor 200. This compares the signal with a desired position signal from the microprocessor 16 and applies a resulting error signal to a polarity and coil selection circuit 202, the output of which is applied to the drive circuits 108, 110 for the coils of the actuator 12. In addition, the position signal from the sensor 28 is digitised by an analogue to digital converter 204 and then applied to a lookup table 206. The latter is pre-programmed with digital values representing various absolute positions of the actuator 12 and thus of the associated heald frame and a comparison with the signal from the converter 204 determines the absolute position of the actuator 12. Resulting directional and speed signals are applied to the polarity and coil selection circuit 202 to control the signal supplied through the output drive circuits to the coils. It will be appreciated that the pre-selected reference value which is set by the microprocessor 16 and which the logic circuit 120 compares with the position difference signal may be held constant over a period of time or may be varied in a pre-selected manner in dependence of the type of movement of the mass (i.e. weaving pattern) required. In addition, it will also be appreciated that the desired position signal which is supplied by the microprocessor 16 to the error calculation logic circuit 100 varies continuously with time in order to provide a smooth variation in the position of the linear actuator 12 and thus of the associated heald frame. The error signal generated by the error calculation logic 100 is also fed back to the microprocessor 16 where it is compared with a pre-set reference level. If the error signal exceeds the pre-set reference level this indicates that the speed or acceleration of the actuator 12 is not meeting the desired value and the current applied to the actuator coils is therefore approaching an unacceptable level. This may be as a result of a breakdown in the mechanical equipment or increased friction forces. If this occurs then the microprocessor 16 applies a current limit to the coil drive circuits 108, 110 to prevent the applied current reaching unacceptably high levels. This could also be achieved by monitoring the difference between the reference value set in the logic circuit 120 and the position difference signal. If this difference increases beyond a pre-selectable level then it again indicates that the actuator is failing to reach the required velocity or acceleration levels as a result of a malfunction and that the coil currents are approaching unacceptably high levels. A velocity against time graph is shown in Figure 3 which compares the idealised conventional drive system (curve A of Figure 1) with that of the described embodiment of the present invention (curve B). As can be seen, because the acceleration and deceleration times t , t of the preferred embodiment of drive system of the present invention are much shorter than those of the conventional system, the speed of reciprocation of the heald frames can be increased dramatically without adversely affecting the time t' e over which the shed is open for traverse of the weft thread. Speeds in excess of thousand picks per minute can be achieved compared with the conventional maximum of about 600 picks per minute. The use of individual actuators 12, 14 would normally give rise to a much higher power consumption. However, the prefeπed drive system avoids this by using the kinetic energy (half the product of the mass and the square of its velocity) of the heald frames during the weaving process. If we assume that the actuator 12 is driving a heald frame from its lowermost shed position, upwards in accordance with curve B of Figure 3, the actuator controller 18 monitors the speed of the actuator in the manner described above and compares this with a desired speed v, which is set by the variation in the desired position signal from the microprocessor 16. Since initially the speed of the actuator 12 and therefore the heald frame is less than v ; the controller 18 applies power to the actuator to increase the actuator speed. However, the acceleration of the actuator 12 is maintained within certain limits (such as set by the slope 30 of the curve B) by the reference value which is set by the microprocessor 16 and compared with the difference signed in the logic circuit 120. This reference value is increased by the microprocessor 16 at a pre-selected rate from a first minimum preset value to a second maximum preset value. This ensures power is supplied to the actuator 12 at a controlled rate to ensure a steady acceleration of the actuator 12 at a pre-selected rate. At or shortly before the time t the reference value reaches the second preset value which is a maximum set by the microprocessor 16, and is held at that value. This has the effect of preventing further acceleration of the actuator 12 which then continues at the desired velocity v The reference value is maintained at its maximum value over the time period t' e in order to maintain the velocity of the actuator 12 and thus the heald frame at v ; . At or shortly before the time t the microprocessor 16 begins to reduce the reference value from its maximum value back towards the first preset value. This reduces power to the actuator 12 at a relatively constant rate and thus decelerates the actuator 12 in a controlled manner. During this period of deceleration of the actuator 12, as the heald frame moves towards its uppermost shed position, the actuator 12 is acting an electromagnetic brake on the heald frame and is therefore generating electrical power. The excess power which is generated by the actuator is fed by the controller 18 to the power capacitance 20 for storage. Once the heald frame has reached its uppermost shed position, the actuator 12 is then accelerated in the reverse direction and the cycle is repeated in the same manner as previously described. Again, when the actuator is decelerated towards the lowermost shed position of the heald frame electrical energy is generated by the motor and this is again stored in the power capacitance 20. The actuator controller 18 monitors the power required by each actuator and where this rises above a threshold, supplies power from the power storage means 20. The level of the power capacitance 20 is momtored by the monitor 22 such that if the power capacitance level drops below a certain level the momtor 22 switches mains power from the mains supply through the power controller 24 into the capacitance to top it up. On start up of the weaving machine, the power returned to the system by each actuator will initially be below a threshold which is monitored by the monitor 22. Whilst this power level is below the threshold none is returned to the power capacitance, it being used by the system to bring the machine up to operating speed as quickly as possible. Once the power level returned to the system by the actuators 12, 14 rises above the threshold of the monitor 22 it is channelled to the power capacitance. When power is required by any one of the actuators it is transmitted to the actuator by the actuator controller from the power capacitance. Using the above described system the energy required to be drawn from the mains can be reduced considerably, and under certain energy imbalance conditions energy may be regenerated and returned to the mains to reduce further the overall energy costs. Although the above description relates to the application of the invention to a weaving machine, it will be appreciated that the above described invention can be applied equally to the control of the needles and/or yam carriers of knitting machines. The invention can also be applied to flat bed and circular knitting machines, needle boards of needling machines, needles of tuft machines, yarn winding machines and any other machines in which an inertial mass is driven in an oscillating manner such as in a simple harmonic motion or any compound motion of variable strokes and frequencies. It will be appreciated that several inertial masses, such as the heald frames of a weaving machine or needles of a knitting machine, can be driven independently of one another at variable strokes and frequencies. Referring to Figure 7, although Figure 3 shows the more usual form of movement of a heald frame of a weaving machine, there is no reason why the movement should not be as shown in idealised form in Figure 5. In other words, the heald frame is accelerated from its lowermost position to a maximum speed at its centre shed position and is then decelerated towards its uppermost shed position, and vice versa. Figure 8 shows a further simple form of motion for a heald frame which is sinusoidal. It will be appreciated that each of the above forms of motion can be applied to any inertial mass oscillating between two positions.

Claims: CLAIMS 1. A drive system (10) for driving an inertial mass (40) over a drive path between first and second extreme positions, comprising: an electrical drive motor (12, 14); linkage means (30) connecting said drive motor (12, 14) to said mass (40) for driving said mass; storage means (20) for storing electrical power; and control means (16, 18) for applying electrical power to said drive motor (12, 14) thereby to accelerate said mass (40) over a first portion of its drive path from one of said first and second extreme positions; wherein said control means (16, 18) connects said drive motor (12, 14) to said storage means (20) whereby when said drive motor acts as an electromagnetic brake during deceleration of said mass (40) over a second portion of its drive path towards the other of said first and second extreme positions said drive motor (12, 14) serves to charge said storage means (20). 2. A drive system as claimed in claim 1 further comprising: monitoring means (22) for monitoring the level of energy stored in said storage means (20); and wherein said control means is operable to apply power from said storage means (20) to said drive motor (12, 14) during acceleration of said mass (40) over said first portion of said drive path when said electrical power stored in said storage means (20) exceeds a preset level. 3. A drive system as claimed in Claim 1 or 2 further comprising: monitoring means (28) for monitoring the current position of said mass and providing successive monitored current position signals in dependence thereon; and wherein said control means (16, 18) comprises: a microprocessor means (16) for providing a desired position signal representing the desired position of said mass (40); and controller means (18) for comparing said desired and monitored positions and controlling the power supplied to said drive motor (12, 14) in dependence thereon. 4. A drive system as claimed in Claim 3 wherein said desired position signal is variable in a continuous manner. 5. A drive system as claimed in Claim 3 or 4 wherein said controller means (18) further comprises: first store means (120) for storing a previously monitored cuπent position signal; first comparator means (116) for comparing a current position signal with said previously monitored current position signal and generating a position error signal in dependence thereon; and wherein said controller means (18) is operable to control the power supplied to said drive motor (12, 14) in dependence on said position error signal. 5. A drive system as claimed in Claim 4 wherein said controller means (18) further comprises: second store means (118) for storing each said current position signal. 6. A drive system as claimed in Claim 5 wherein said second store means (118) is coupled to said first comparator means (116) and is operable to update said stored cuπent position in response to receipt of said position error signal. 7. A drive system as claimed in any of Claims 4 to 6 wherein said controller means (18) further comprises: means (120) for comparing the difference between said stored previously monitored and current position signals with a reference value and generating a second error signal in dependence thereon; and wherein said controller means (18) is operable to control the power supplied to said drive motor (12, 14) in dependence on said second error signal. 8. A drive system as claimed in Claim 7 wherein said second eπor signal is a function of said position error signal. 9. A drive system as claimed in Claim 7 or 8 wherein said reference value is generated by said microprocessor means (16). 10. A drive system as claimed in Claim 7, 8 or 9 wherein said reference value is variable. 11. A drive system as claimed in any of Claims 3 to 10 wherein said controller means (18) further comprises: means (124) for controlling the direction of drive of said mass (40) in dependence on said monitored current position signal; and wherein said direction controlling means is operable to reverse the direction of drive of said mass in response to said momtored current position signal indicating that said mass (40) has reached one of said extreme positions. 12. A drive system as claimed in any of Claims 3 to 11 wherein said controller means (18) further comprises: second comparator means (100) for comparing said monitored current position with said desired position signal and generating a third position error signal in dependence thereon; and wherein said controller means (18) is operable to control the power supplied to said drive motor (12, 14) in dependence on said third position error signal. 13. A drive system as claimed in any of Claims 3 to 12 wherein said controller means (18) further comprises: means (102) for controlling the power transferred between said drive motor (12, 14) and said storage means (20) in dependence on the or each said error signal. 14. A drive system as claimed in any of the preceding Claims wherein said mass (40) is a heald or a heald frame of a weaving machine. 15. A drive system as claimed in any of claims 1 to 14 wherein said mass (40) is a needle of a knitting machine or the like.

Inventors: PALMER RAYMOND LESLIE, PALMER, RAYMOND, LESLIE
Applicants: PALMER RAYMOND LESLIE, PALMER, RAYMOND, LESLIE
Application year: 1997
Main classification: D03C1
IPC codes: D03C1, D03C3, D03D51, D04B27