The present invention relates to a dual pinion drive system and an associated method of control, in particular the invention relates to feed forward oscillation damping in a dual pinion variable speed drive system.

Dual pinion variable speed drive systems are commonly used in cement mills where two variable speed drives are connected to the same cement mill. An example of such a system 2 is shown in Figure 1 and comprises a first motor 11 a, a second motor l ib, corresponding drive converters 15a and 15b, and a cement mill 16.

The components of such a system feature a number of physical properties such as inertia, stiffness and damping and during its operation there is interplay of these properties between each component within the system. An illustration of the interplay of mechanical properties between the components is shown in Figure 2. Motor components 1 la, b act upon load component, in this case a cement mill, 16 via coupling in components 12a, b and coupling out components 14a, b. Each of the components 1 lab, 12ab, 14a,b and 16 represents an inertia within the system. Torque 10 a, b is input into drives 1 la, b respectively and between each of the motors 11a and coupling in component 12a, coupling in component 12a and coupling out component 14a, coupling out component 14a and load 16 there is acting a mechanical stiffness 13a and mechanical damping 13b. Output from the load is the load torque 18a as well as mechanical damping 13b.

As a result of the interplay of these mechanical properties within the system, a dual pinion drive system having variable speed drives is prone to problems caused by torsional oscillations. If the system is considered as a three mass system, namely motors 11a, l ib and load 16, the dual pinion drive system exhibits two natural modes of low frequency oscillation. In the first mode, motors 11a and l ib oscillate in counter phase wherein motor 11a speeds up whilst motor l ib slows down and vice versa. This means that motors 11a and l ib alternately pull the load, which maintains a constant speed. The natural frequency of systems, having soft couplings, operating in this mode has often been observed to be around 1 to 5FIz. In the second mode, motors 1 la and 1 lb oscillate in phase with each other but counter to the load 18. The natural frequency of a system operating in this mode has been observed as being around 3 to 8Hz, which, as can be seen, is higher than the natural frequency of the first mode.

In view of the variation in the natural oscillation frequency of systems operating in the first mode or the second mode, the frequency response aspect of their design must take into account the inclusion of controls capable of damping both of the modes of natural oscillation.

The frequency response of such a known dual pinion drive system, when used for example in a Semi Autogenous Grinding Mill, is shown in Figure 3. The graph, line A in the graph illustrates the comparison of the torque reference of the first motor 11a to the rotational speed of the motor 11a and this shows the counter-phase and in- phase oscillations of the frequency response. Also provided for the sake of comparison is the frequency response in a single pinion configuration, shown by line B. As can be seen in from Line A, in addition to the natural modes of oscillation for a three mass system, a high frequency mode can also be seen around 240Hz. This high frequency mode is a result of a flexible coupling in the drive train model. The illustration in the graph primarily shows that a source of oscillations in the torque reference in the drive train has the potential risk of being amplified at three different frequencies. Therefore it is important that the control system is suitably assessed to establish the effectiveness in damping these oscillations.

In a dual pinion variable speed drive system such as that of Figure 1 , the motors 11a, 1 lb are controlled using a master- follower configuration wherein one of the motors, 1 la, is the master drive and is responsible for speed control of the system by virtue of providing input into a common Proportional Integral (PI) speed controller meaning motor 1 lb is the follower drive. The rotational speed of the master drive is input into the PI speed controller where it is compared against a predetermined set-point to calculate a torque reference signal. From this, a torque command is generated by the PI speed controller and is sent to both the master drive converter 15a and the second drive converter 15b. The converters 15a and 15b then, using their existing internal control system (not shown), produce the necessary voltage waveforms to drive their respective motors 11a, l ib according to the generated torque commands. An

illustration of this process is show in Figure 4.

A drawback of this control topology is that regardless of the PI speed controller tuning parameters the counter phase oscillations are not damped and thus pose a potential risk to the system. Figure 5 shows the frequency response of the motor speed of the follower drive as influenced by the speed reference changes in the master drive motor. From this it can be seen that the nominal setpoint tracking performance required from the control system causes significant oscillations in the motor speed of the follower drive and thus the torque load sustained by the shafts interconnecting the components of the system.

Detuning the control settings of the PI speed controller reduces the magnitude of the oscillations, however as a result of the detuning the control performance degrades beyond acceptable limits for the system.

Another approach to overcome these issues has involved carrying out extensive low- pass filtering of the feedback signals. The low pass filtering can damp the

oscillations within the system, however it has the knock on effect of slowing the overall closed-loop dynamics of the system, which is undesirable. In view of the above, an object of the present invention is to obviate or mitigate at least one of the aforementioned problems.

According to a first aspect of the present invention there is provided a control system having first and second electrical machine, the control system comprising a controller component operable to receive a first signal which relates to a measured parameter of a first electrical machine and a second signal which relates to a reference parameter of such a first electrical machine, the controller component being operable to use the first signal and second signal to produce a first control signal; a first filter component which is operable to receive such a first signal from such a first electrical machine and to use a received first signal to produce a second control signal; a first output component which is operable to receive such a first control signal and such a second control signal and to combine said first and second control signals to produce a first output control signal for provision to said first electrical machine, the control system further comprises a second filter component operable to receive a third signal which relates to a measured parameter of a second electrical machine from that second electrical machine and to use the third signal to produce a third control signal; and a second output component operable to receive such a first control signal and such a third control signal and to combine such first and third control signals to produce a second output control signal for provision to a second electrical machine.

Conveniently, the first output control signal is provided to a first converter component operable to drive said first electrical machine and the second output control signal is provided to a second converter component operable to drive said second electrical machine.

The arrangement of the control component and first filter component and second filter component in this control system acts to remove peaks in an aspect of the first measured parameter which forms the first signal when the control system is implemented with a converter component operable to drive a first and second electrical machine and to remove peaks in an aspect of the second measured parameter which forms the third signal when the control system is implemented with a converter component operable to drive the second electrical machine.

Preferably, the control component is a proportional integral controller, each filter component may be a band pass filter and each converter component may be a power electronics converter component.

The first signal may comprise the measured rotational speed of the first electrical machine, the second signal may comprise a reference rotational speed of the first machine and the third signal may comprise a measured rotational speed of the second electrical machine.

Preferably each output control signal is a torque command.

The first and second electrical machines may be mechanically coupled to a load and by way of this the first and second electrical machines can be considered to be mechanically coupled to one another. Each mechanical coupling may be a flexible coupling. It will be understood that the term flexible coupling, also commonly known as elastic coupling, refers to a mechanical coupling arrangement such as one which transmits torque from a motor to a load which it is driving whilst allowing misalignments between shafts of the motor and load, as well as removing resonances and dampening vibrations. Typically such flexible coupling results in a length operational lifetime for the machinery it couples and often also provides the benefit of minimizing maintenance requirements.

The control system may comprise more than two electrical machines and for example may comprise n electrical machines, where n is a whole number of 2 or more.

In such a control system, a (n+l) ^th filter component operable to receive a (n+l) ^th signal which relates to a measured parameter of a n ^th electrical machine from that n ^th electrical machine and to use the (n+l) ^th signal to produce a (n+l) ^th control signal; and a n ^th output component operable to receive such a first control signal and such a (n+l) ^th control signal and to combine such first and (n+l) ^th control signals to produce a n ^th output control signal for provision to a n ^th electrical machine.

According to a second aspect of the invention there is provided a method for controlling a mechanical system having first and second electrical machines, the method comprising: receiving a first signal relating to a first measured parameter of the first electrical machine; receiving second signal relating to a first reference parameter of the first electrical machine;

producing a first control signal from the first and second signals using a controller component;

producing a second control signal from the first signal using a filter component; producing an first output control signal from the first and second control signals using a first output component,

supplying the first output control signal to the first electrical machine; and receiving a third signal relating to a second measured parameter of a second electrical machine; producing a third control signal from the third signal using a filter component; producing a second output control signal from the first and third control signals using a second output component, and supplying the second output control signal to the second electrical machine. By producing a first output control signal from the first control signal and second control signal, and a second output control signal from the first control signal and third control signal, peaks in an aspect of the first measured parameter are removed. and peaks in an aspect of the second measured parameter are removed when the control system is implemented with a converter component operably driving an electrical machine.

In this method, the first and second electrical machine may each be mechanically coupled to a load and by way of this may each be mechanically coupled to one another via the load. Each mechanical coupling may be a flexible coupling. According to a third aspect of the invention there is provided a controller for controlling a mechanical system which includes first and second electric motors that are mechanically coupled to one another, the controller being operable to supply control signals to first and second electric motors using speed feedback and a self-tuned adaptive feedforward filter. Each mechanical coupling may be a flexible coupling. According to a fourth aspect of the invention there is provided a method of controlling a mechanical system which includes first and second electric motors that are mechanically coupled to one another, the method including supplying control signals to first and second electric motors using speed feedback and a self-tuned adaptive feedforward filter technique. Each mechanical coupling may be a flexible coupling.

These and other aspects of the invention will become apparent from the following descriptions when taken in combination with the accompanying drawings in which:

Figure 1 shows a known cement mill dual pinion drive system;

Figure 2 shows a schematic diagram of the interplay of mechanical properties of the components of a system as shown in Figure 1 ;

Figure 3 shows a graphical representation of the frequency response of a known dual pinion drive system compared to the frequency response of a single pinion drive system;

Figure 4 shows a schematic diagram of a known control process for a master - follower type dual pinion drive system;

Figure 5 shows a graphical representation of the frequency response of the rotational speed of the follower motor of the system of Figure 4;

Figure 6 shows a control system for an electrical machine according to a second embodiment of the present invention;

Figure 7 A shows a graphical representation of the frequency response of a master motor of an electrical machine being controlled by the control system of Figure 6, and

Figure 7B shows a graphical representation of the frequency response of a follower motor of an electrical machine being controlled by the control system of Figure 6.

In Figure 6 there is shown a control system 40 for use with a dual pinion drive system such as a dual pinion drive cement mill. For the purposes of the description, a master- follower type dual pinion drive system such as the cement mill of Figure 1 will be referred to. The control system 40 acts upon both the master motor 11a and the follower motor 1 lb of the dual pinion drive system 2

In order to counteract oscillations in the dual pinion drive system 2 in a further improved manner, the control system 40 is provided with a PI controller 42 arranged in parallel to a master band pass filter 44 and a follower band pass filter 45. The control system 40 is further provided with a master output calculator 48 and a follower output calculator 49. At any given moment in the operation of the control system 40, a measurement of rotational speed 52 of the master motor (1 1a of Figure 1) is provided to both the PI controller 42 and the master band pass filter 44. A reference value 54 for the rotational speed is also provided to the PI controller. A measurement of rotational speed 53 of the follower motor (1 lb of Figure 1) is provided to the follower band pass filter 45. The PI controller uses reference value 54 and first measurement 52 to generate a first output signal 56. The master band pass filter uses the first measurement 52 to generate a second output signal 58. The follower band pass filter uses the second measurement 53 to generate a third output signal 59. In this case the function of the master band pass filter 44 is the same as that of the follower band pass filter 45, namely:

where Kd is the filter gain and fo is the frequency of the oscillations to be damped. The simple tuning parameters a and b can be used to further tune the filter properties. The first output signal 56 and second output signal 58 are then provided to the master output calculator 48 which uses the supplied information to generate an output master torque command 60 which in turn would be supplied to the master motor 1 la via the power electronics control converter 12a . The first output signal 56 and the third output signal 59 are provided to the follower output calculator 49 which uses the supplied information to generate an output follower torque command 61 which is then supplied to the follower motor l ib via the power electronics control converter 12b.

As the control system 40 is operating in discrete time this is an iterative process repeated with the sampling time of the control algorithm which, in this case of torsional oscillations, will be around 1ms. The power electronics converter control system (not shown), is responsible for actually driving the cement mill such that the desired torque is produced, has a sampling time of probably 25 μβ.

The operation of the control system 40 results in a swing damping effect upon the frequency response of the master motor speed, which in effect removes the peaks in the master motor, 11a, speed frequency response as well as a swing damping effect upon the frequency response of the follower motor speed which similarly removes the peaks in the follower motor speed frequency response. A graphical representation of this effect is illustrated in Figure 7A and 7B respectively

In the control system 40, the main torque command for the system is still output from the master output calculator 48. However, the additional feedback loop for the follower motor acts to enhance oscillation suppression within the system 2 by enabling the control system to dampen the natural mode of oscillation in the rotation speeds of both the master and follower drive in the dual pinion drive system.

The control system of Figure 6 provides a feed forward oscillation damping control method which acts upon the torque commands provided to each of the motors in a dual pinion drive system thus reducing unwanted torsional oscillations along the shafts of the motors which connects the motors to the load being driven, for example the cement mill of Figure 1. Furthermore the detailed control systems will reduce the torsional oscillations acting on the load.

In the arrangement detailed in Figure 6, the control system can be implemented on any board suitable for use with Medium Voltage (MV) drive systems. Alternatively, a Low Voltage drive control platform in a relevant application may be used.

It will be understood that in the arrangement detailed in Figure 6 with respect to Figure 1 , that master and follower motors 11a and 1 lb are mechanically coupled to one another via the mechanical load which they drive , in this case cement mill 16. In such an arrangement, the mechanical coupling is likely to be flexible coupling, also commonly known as elastic coupling. A flexible coupling transmits the torque from the motor 1 la or 1 lb to the load 16 which it is driving whilst allowing misalignments between shafts of the motor and load, as well as removing resonances and dampening vibrations. Typically such flexible coupling results in a length operational lifetime for the machinery it couples and often also provides the benefit of minimizing maintenance requirements.

It will be further appreciated that the control system may comprise more than two motors and for example may comprise n motors, where n is a whole number of 2 or more. In such a control system comprising n motors, a (n+l) ^th filter component operable to receive a (n+l) ^th signal which relates to a measured parameter of a n ^th motor from that n ^th motor and to use the (n+l) ^th signal to produce a (n+l) ^th control signal; and a n ^th output component operable to receive such a first control signal and such a (n+l) ^th control signal and to combine such first and (n+l) ^th control signals to produce a n ^th output control signal for provision to a n ^th motor.

Various modifications may be made to the embodiments hereinbefore described without departing from the scope of the invention. For example, although the control system embodiments have been described with reference to a dual pinion drive cement mill any equipment having a dual pinion drive would benefit from the advantages of the control system. In addition, it will be appreciated that whilst the embodiments above have been detailed as having flexible mechanical coupling between the motors and the respective loads which they are driving, other mechanical coupling arrangements could be used.