GB2601548A

GB2601548A - Roller controller

Info

Publication number: GB2601548A
Application number: GB2019156.5A
Authority: GB
Inventors: Van Rijswick Rudolfus; Marcellis Renê
Original assignee: Weir Minerals Netherlands BV
Current assignee: Weir Minerals Netherlands BV
Priority date: 2020-12-04
Filing date: 2020-12-04
Publication date: 2022-06-08
Also published as: GB202019156D0

Abstract

A method of controlling a pair of counter-rotating rollers 14, 16, the method comprises providing an initial reference speed to both the rollers; measuring a first torque relating to the first counter-rotating roller and a second torque relating to the second counter-rotating roller; comparing the first and second torque to create a torque difference; calculating a speed adjustment based on the torque difference; creating an updated reference speed based on the initial reference speed and the speed adjustment; and sending the updated reference speed to the second counter-rotating roller. A controller 12 for controlling rollers in a high-pressure grinding roller machine, each roller is independently controlled by a dedicated variable frequency drive 30a, 30b and a controller operable to control variable frequency drives by assigning a first of the drives as a master drive and a second of the drives as a slave drive and re-assigning the drives in response to a defined criterion are also provided. The method may include the step of smoothing the torque difference prior to calculating the torque difference. The initial reference speed provided may be when there is no material to be ground in a nip therebetween.

Description

ROLLER CONTROLLER

FIELD OF INVENTION

The invention relates to improvements in or relating to a roller controller, and particularly, but not exclusively, to a controller for controlling a pair of counter-rotating grinding rollers, such as those used in high pressure grinding roller (HPGR) machines.

BACKGROUND OF THE INVENTION

HPGR machines include counter-rotating grinding rollers, one of which is actuated towards the other at high pressure whilst maintaining a variable distance between the two grinding rollers to form a nip or grinding gap. The material for grinding is delivered to this nip and is subjected to material bed comminution.

The roller surfaces (usually covered by a tyre having tungsten carbide studs) wear out faster if there is too much slip between the outer surfaces (the tyres). In other words, when the peripheral speed difference between the tyres is too high the material being ground will rub more on the tyres, causing excessive wear. This material couples the tyres and causes the faster tyre to exert a force on the slower tyre. The amount of force exerted can be measured indirectly by the torque. When there is no torque difference between the tyres, the wear will be at a minimum, which is the desired condition.

Each of the grinding rollers in an HPGR machine is typically driven by a motor controlled by its own variable frequency drive (VFD). The VFD adjusts the speed of the motor by varying the frequency supplied to the motor. Known VFD apparatus may operate using an AC power input that is transformed into a DC voltage signal, which in turn may be transformed into a sinusoidal output whose frequency is proportional to the motor speed of the motor to be driven.

Each of the two VFDs needs to be controlled, and to some extent synchronised, to minimise the torque difference between the grinding rollers and thereby prevent excessive wear on one or other of the rollers. Some prior art VFD control systems use load sharing, which typically involves using one VFD as the master, which is speed controlled, and using the second VFD as a slave, which is torque controlled to follow the torque of the master VFD.

A disadvantage of this prior art control system is that torque control is not possible when the HPGR is running idle (i.e. without any material in the grinding gap). These prior art systems therefore need complex switching logic to switch between torque and speed control of the VFD slave depending on the load of the HPGR machine. This is expensive, and requires proprietary components from the VFD suppliers. It also requires specific engineering for each VFD, and on-site commissioning and tuning of the HPGR machines.

It is among the objects of embodiments of the present invention to overcome or mitigate one or more of the above disadvantages or other disadvantages of the prior art, or to provide a useful alternative.

SUMMARY OF THE INVENTION

This summary is provided to introduce a selection of concepts that are further described in the detailed description below. This summary is not intended to identify indispensable features of the claimed subject matter, nor is it intended for use as an aid in limiting the scope of the claimed subject matter.

in this application relative terms are used; such as front; rear, pt, down, etc., only for ease of the description and understanding of the embodiments; not by way of limitation Ordinal numbers (first, second; third, etc.) are assigned arbitrarily herein, and are used to differentiate between parts, and do not indicate a particular order or sequence.

According to a first aspect there is provided a method of controlling a pair of counter-rotating rollers, the method comprising: providing an initial reference speed to both the first and second counter-rotating rollers; measuring a first torque relating to the first counter-rotating roller and a second torque relating to the second counter-rotating roller; comparing the first and second torque to create a torque difference; calculating a speed adjustment based on the torque difference; creating an updated reference speed based on the initial reference speed and the speed adjustment; and sending the updated reference speed to the second counter-rotating roller.

The method may include the step of smoothing the torque difference; and calculating the speed adjustment may be based on the smoothed torque difference.

The step of measuring a first torque relating to the first counter-rotating roller and a second torque relating to the second counter-rotating roller may be implemented by sensors measuring the torque directly from the respective rollers, measuring the torque delivered to the respective rollers by a motor dedicated to each roller, or in any other convenient way.

The torque delivered to the respective rollers by a motor may be implemented by the VFD 30a,b calculating the torque delivered based on outputs from the VFD 30a, b.

The initial reference speed provided to both the first and second counter-rotating rollers may be provided when there is no material to be ground in a nip therebetween.

The step of sending the updated reference speed to the second counter-rotating roller may be implemented without changing the initial reference speed provided to the first counter-rotating roller.

The method may further comprise: comparing the calculated speed adjustment with a defined maximum speed adjustment; and limiting the calculated speed adjustment thereto in the event that the defined maximum speed adjustment is reached or exceeded.

The defined maximum speed adjustment may comprise a percentage change to the rotational speed of the rollers (for example, plus or minus 2%, 3%, 4%, 5% or the like), or an absolute rotational speed of the rollers.

The step of smoothing the torque difference may comprise: averaging the torque difference over a defined time period (for example for a period selected between 200 and 2000 milliseconds), removing spikes in the torque difference (for example, using a low pass filter), or any other convenient process.

The method optionally includes the step of ignoring the smoothed torque difference if the smoothed torque difference is less that a defined percentage of the torque (for example, less than 1%, less than 2%, less than 3%, or the like).

The first counter-rotating roller may be an actuated roller. Alternatively, the second counter-rotating roller may be an actuated roller.

It will now be appreciated that in the above aspect both drives (one for each counter-rotating roller) are speed controlled. A controller receives the torque of the individual drives and uses this to slightly adjust the speed setpoint of the second (slave) drive to balance the torque difference between the two counter-rotating rollers. This aspect may be less dynamic than prior art roller controls systems, but it is simpler as it does not require complex switching logic between torque and speed control.

According to a second aspect there is provided a high pressure grinding roller machine comprising: a pair of counter-rotating rollers defining a nip therebetween; a pair of roller motors, each roller motor having a dedicated variable frequency drive; and a controller operable to control the variable frequency drives by: (i) providing an initial reference speed to each of the dedicated variable frequency drives, (ii) measuring a first torque relating to a first one of the counter-rotating rollers controlled by a first of the dedicated variable frequency drives, and a second torque relating to a second one of the counter-rotating rollers controlled by a second of the dedicated variable frequency drives, (iii) comparing the first and second torque to create a torque difference, (iv) calculating a speed adjustment based on the torque difference, (v) creating an updated reference speed based on the initial reference speed and the speed adjustment, and (vi) sending the updated reference speed to only the second variable frequency drive.

The controller may smooth the torque difference prior to calculating a speed adjustment based on the smoothed torque difference.

The controller may be implemented as a programmable logic controller (PLC) that is dedicated to the HPGR. This has the advantage of allowing control of synchronising and load balancing of the HPGR rollers to be performed by a controller that is separate from controllers incorporated into the VFDs.

The controller may implement the further step of comparing the calculated speed adjustment with a defined maximum speed adjustment, and limiting the calculated speed adjustment thereto in the event that the defined maximum speed adjustment is reached or exceeded.

The controller may implement the step of smoothing the torque difference by averaging the torque difference over a defined time period (for example for a period selected between 200 and 2000 milliseconds), removing spikes in the torque difference (for example, using a low pass filter), or any other convenient process.

The controller may ignore the smoothed torque difference if the smoothed torque difference is less that a defined percentage.

Measuring a first torque relating to a first one of the counter-rotating rollers may be implemented by using a torque value delivered by the respective roller motor. Similarly, measuring a second torque relating to a second one of the counter-rotating rollers may be implemented by using a torque value delivered by the respective roller motor.

The pair of counter-rotating rollers may have identical diameters.

This aspect has the advantage that standard variable frequency drives can be used without any customised, built-in, control for switching between speed and torque control, because the controller, which may be external to the variable frequency drives can be used to provide speed control to both variable frequency drives.

According to a third aspect there is provided a controller for controlling counter-rotating rollers in a high pressure grinding roller machine, where each roller is independently controlled by a dedicated variable frequency drive, the controller being operable to control the variable frequency drives by: (i) providing an initial reference speed to each of the dedicated variable frequency drives, (ii) measuring a first torque relating to a first one of the counter-rotating rollers controlled by a first of the dedicated variable frequency drives, and a second torque relating to a second one of the counter-rotating rollers controlled by a second of the dedicated variable frequency drives, (iii) comparing the first and second torque to create a torque difference, (iv) calculating a speed adjustment based on the torque difference, (vi) creating an updated reference speed based on the initial reference speed and the speed adjustment, and (vii) sending the updated reference speed to only the second variable frequency drive.

The controller may smooth the torque difference prior to calculating a speed adjustment based on what is then the smoothed torque difference.

The controller may be implemented as a PLC.

The PLC may include a processor operable to control additional functions of the high pressure grinding roller machine, such as the feed rate of material to be ground, the hydraulic system, the bearings lubrication, communications with the DCS, and the like.

According to a fourth aspect there is provided a controller for controlling counter-rotating rollers in a high pressure grinding roller machine, where each roller is independently controlled by a dedicated variable frequency drive, the controller being operable to control the variable frequency drives by: (i) assigning a first of the variable frequency drives as a master drive and providing a constant reference speed thereto; (ii) assigning a second of the variable frequency drives as a slave drive and providing an updated reference speed thereto, where the updated reference speed is the constant reference speed provided to the first variable frequency drive; and (ii) re-assigning the first variable frequency drive as the slave drive, and the second variable frequency drive as the master drive, in response to a defined criterion.

The updated reference speed may be lower than the constant reference speed. In many configurations, this allows the reference speed to be the highest recommended speed for the rollers.

Re-assigning the first variable frequency drive as the slave drive may be implemented automatically by the controller on detection of the defined criterion, without any human intervention.

The defined criterion may comprise a detected condition where the controller would need to increase the slave speed above the constant reference speed.

Assigning a first of the variable frequency drives as a master drive and providing a constant reference speed thereto may be implemented at commissioning of the high pressure grinding roller machine. The variable frequency drive associated with the roller having the highest torque may be assigned as the master.

According to a fifth aspect there is provided a method of controlling counter-rotating rollers in a high pressure grinding roller machine, where each roller is independently controlled by a dedicated variable frequency drive, the method comprising the steps implemented by the controller in the fourth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a simplified schematic plan diagram of an HPGR including a controller, in accordance with one embodiment of the present invention; Fig. 2 is a simplified schematic diagram showing the controller and other parts of the HPGR of Fig. 1 in more detail; and Fig. 3 is a simplified schematic plan diagram of an HPGR including a controller, in accordance with another embodiment of the present invention.

DL.AILEDDESCRIPTION OF EMBODIMENTS

Reference is now made to Fig. 1, which is a simplified schematic diagram of an HPGR 10 including a controller 12 according to one embodiment of the present invention.

The HPGR 10 comprises a first grinding roller 14, a second grinding roller 16, which is actuated towards the first grinding roller 14, and a nip 18 (or grinding gap) defined between the two rollers 14, 16. Each grinding roller 14, 16 is mounted on a respective shaft 20, 22. Each shaft 20, 22 is mounted on, and supported by, bearings (not shown for clarity). Each grinding roller 14, 16 has the same diameter, in this embodiment the diameter is approximately 2m.

Each grinding roller shaft 20, 22 is driven by an identical motor 24a,b that is controlled by a dedicated variable frequency drive (VFD) 30a,b. In this embodiment, the VFDs are conventional, off-the-shelf VFDs, such as those supplied by Siemens Aktiengesellschaft or ABB Ltd. The controller 12 is coupled to each of the VFDs 30a,b and provides input to these VFDs 30a,b to control the rotation of the grinding rollers 14, 16.

The controller 12 communicates with a Distributed Control System (DCS) (not shown in Figure 1) via a DCS communication line 40.

In this embodiment, the first grinding roller 14 is considered as the master roller, and the second grinding roller 16 is considered as the slave roller; however, this is a matter of convenience at the start of operation. During operation, on detection of a defined criterion by the controller 12, this may be switched so that the second grinding roller 16 is considered as the master and the first grinding roller 14 is considered as the slave. In other embodiments, at the start of operation the second grinding roller 16 may be considered as the master and the first grinding roller 14 may be considered as the slave.

Reference will now be made to Figure 2, which is a simplified schematic diagram showing, in more detail, the controller 12 and the first and second VFDs 30a,b.

The two VFDs 30a,b are identical, but one (the first VFD 30a) is assigned as the master and the other (the second VFD 30b) is assigned as the slave.

The controller 12 receives a reference speed (nref) on the communication line 40 from a computerised DCS that controls the HPGR 10 and other mining equipment.

Initially, on start-up of the HPGR 10, this reference speed (nre() is provided to both the first and second VFDs 30a,b via a reference speed input line 50a,b. The reference speed input line 50a,b is fed into a negative feedback speed adjustment system 52a,b, together with an actual speed of the respective motor 24a,b, (via an actual speed line 54a,b), and an updated (or adjusted) speed is provided on an updated (or adjusted) speed line 56a,b and fed into a speed control system 58a,b in the VFD 30a,b. Initially, the motor 24a,b is not turning so the reference speed is input to the speed control system 58a,b, which creates a reference torque value and outputs this on a reference torque line 60a,b. The reference torque is then fed into a negative feedback torque adjustment system 62a,b, together with an actual torque value provided via a measured torque line 64a,b. The negative feedback torque adjustment system 62a,b creates a torque adjustment value and outputs this on a torque adjustment line 66a,b.

The torque adjustment value on the torque adjustment line 66a,b is then fed into a torque control system 68a,b, which uses the torque adjustment value to create a torque output on a torque output line 70a, b. The torque output line 70a, b feeds a VFD output unit 72a,b which provides an output of the VFD 30a,b that controls the respective motor 24a, b. The actual speed value (on the actual speed line 54a, b) and the actual torque value (on the measured torque line 64a,b) are provided to the VFD 30a,b by the respective motor 24a,b to which the VFD 30a,b is coupled. The actual torque value is the delivered torque of the respective motor 24a,b.

The actual measured torque is also fed into the controller 12 via a measured torque controller input line 74a,b. Various VFD control signals 76a, b are used by the controller 12 to control the operation of the two VFDs 30a,b. It should be appreciated that the negative feedback speed adjustment system 52a,b, the speed control system 58a,b, the negative feedback torque adjustment system 62a, b, and the torque control system 68a, b are provided on conventional VFDs, and can be programmed by a controller, such as controller 12. The controller 12 includes a torque difference adjustment system 80 that receives the actual measured torque from each of the VFDs 30a,b (via the measured torque controller input lines 74a, b). This torque difference adjustment system 80 provides a torque difference value on a difference output line 82 that feeds a smoothing circuit 84. The smoothing circuit 84 removes any spikes from the torque difference value generated by the torque difference adjustment system 80. In this embodiment, the smoothing circuit 84 comprises a low pass filter designed to remove any spiked values that occur for less than approximately three seconds. This number was selected because the grinding rollers 14,16 typically rotate at approximately 20 revolutions per minute (rpm) in this embodiment, and any spike that is of shorter duration than one complete revolution of the grinding roller 14,16 can be safely ignored.

The smoothing circuit 84 outputs the smoothed difference onto a speed control line 86, which feeds the smoothed difference into a speed control system 88 similar to the speed control systems 58a,b. The speed control system 88 uses the smoothed difference to generate a speed adjustment value that is output on a speed adjustment line 90 and fed into a speed adjustment system 92.

The speed adjustment system 92 creates a secondary reference speed (nseeeed) based on the reference speed (Ref) on the communication line 40 and the speed adjustment value on the speed adjustment line 90. This secondary reference speed is output onto the second reference speed input line 50b only (not the first reference speed input line 50a). The second VFD 30b uses this secondary reference speed as input to the negative feedback speed adjustment system 52b, which propagates through the VFD 30b components (e.g. the speed control system 58b, the negative feedback torque adjustment system 62b, and the torque control system 68b) to generate a new torque output on the torque output line 70b controlling the second motor 24b.

This process operates continually during operation of the HPGR 10 so that the speed control (via the speed control system 88) is used (instead of torque control) to minimise the measured torque difference between the first and second grinding rollers 14,16. This embodiment only requires two reference inputs, one for each VFD 30a,b. The reference inputs are both reference speeds. The first reference speed is the required speed provided by the DOS (me) and supplied to the first VFD 30a (either directly from the DCS or via the controller 12). The nief is not usually adjusted during operation of the HPGR 10 (at least, it is not usually adjusted by the controller 12). The second reference speed is the secondary reference speed (neeeeed) that is supplied by the controller 12 to the second VFD 30b.

This embodiment has the advantage of simplicity and access to the controller 12 without requiring knowledge of proprietary internal operations of the VFDs 30a,b, which would be required if switching between speed control and torque control.

However, the speed control system 88 only introduces relatively small changes in speed, typically plus or minus 5% of the reference speed (nref). It is possible that these minor speed adjustments will not be possible to reduce the torque difference between the grinding rollers 14,16. In such circumstances, the controller 12 may transmit an alert signal to the DOS or to some other monitoring system. If the torque difference exceeds a threshold, and the speed adjustment is at its maximum, then the controller 12 may activate an alarm, transmit a request for intervention (e.g. to examine the material being fed into the HPGR 10), or in extreme cases shut down the VFDs 30a,b and thereby stop the grinding rollers 14,16.

In another embodiment, a threshold detector may be introduced between the smoothing circuit 84 and the speed control system 88. The threshold detector would ensure that any minor torque differences (for example a torque difference of less than 1% of the higher measured torque from the motors 24a, b) are ignored, so that the speed control system 88 does not take any action if there is only a minor torque difference.

Reference is now made to Figure 3, which is a simplified schematic plan diagram of an HPGR 100 including a controller 112, in accordance with another embodiment of the present invention.

The main difference between the embodiments of Figures 1 (HPGR 10) and 3 (HPGR 100) is that in the Figure 1 embodiment, the controller 12 receives inputs directly from, and outputs the reference speeds and other signals directly to, the two VFDs 30a,b; whereas, in the Figure 3 embodiment, the controller 112 communicates only with a DOS 102 and receives signals from and outputs signals to the VFDs 30a,b via the DOS 102. To implement this, the controller 112 has a DCS communication line 104 via which signals and information is passed. The DOS 102 is coupled to the VFDs 30a,b by a pair of VFD communication lines 106a,b respectively. In all other respects, the controller 12 and controller 112 are identical. Typical control signals for operating the VFDs 30a,b (for either HPGR 10 or HPGR 100) include those shown in Table 1 below.

Signal Type Digital or Analogue VFD Input or Output VFD Start Digital Input VFD Reverse Digital Input VFD Stand-alone Digital Input VFD Reset Alarm Digital Input VFD Speed setpoint Analogue Input VFD Failure/Alarm Digital Output VFD Warning Digital Output VFD Ready to Run Digital Output VFD Running Digital Output VFD Actual Speed Analogue Output VFD Torque Analogue Output VFD Power Analogue Output Table 1 -VFD input and output signals The choice of which architecture (HPGR 10 or 100) may depend or be influenced by the way that other HPGR machines are controlled at that location. For example, if the other HPGR machines are controlled directly by a DOS at that location, then the HPGR 100 architecture may be the most suitable for that site.

Various modifications may be made to the above embodiments within the scope of the present invention. For example, in other embodiments, a different definition of minor torque difference may be used, such as less than 0.5% of the higher measured torque.

In other embodiments, the controller 12, 112 may be coupled to various sensors that record measurements from different parts of the HPGR 10. These sensors may include: a first torque sensor for measuring the torque at the first grinding roller 14, a first speed sensor for measuring the rotational speed of the first grinding roller 14, a second torque sensor for measuring the torque at the second grinding roller 16, and a second speed sensor for measuring the rotational speed of the second grinding roller 16. The sensors may also include temperature and vibration sensors mounted in the motors 24a,b, bearings (not shown), and other parts of the HPGR 10.

In the foregoing description of certain embodiments, specific terminology has been used for the sake of clarity. However, the disclosure is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes other technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as "upper" and "lower", "above" and "below" and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms, nor to imply a required orientation of the seal assembly.

In this specification, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of'. A corresponding meaning is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear.

The preceding description is provided in relation to several embodiments which may share common characteristics and features. It is to be understood that one or more features of any one embodiment may be combined with one or more features of the other embodiments. In addition, any single feature or combination of features in any of the embodiments may constitute additional embodiments.

In addition, the foregoing describes only some embodiments of the inventions, and alterations, modifications, additions and/or changes can be made thereto without departing from the scope of the disclosed embodiments, the embodiments being illustrative and not restrictive.

List of reference numerals: HPGR 10, 100 Controller 12, 112 First grinding roller 14 Second grinding roller 16 Nip (or grinding gap) 18 First grinding roller shaft 20 Second grinding roller shaft 22 First and second grinding roller motors 24a,b First and second variable frequency drives (VFDs) 30a,b DOS communication line 40 Reference speed input lines 50a,b Negative feedback speed adjustment system 52a,b Actual speed line 54a,b Adjusted speed line 56a,b Speed control system 58a,b Reference torque line 60a,b Negative feedback torque adjustment system 62a,b Measured torque line 64a,b Torque adjustment line 66a,b.

Torque control system 68a,b Torque output line 70a,b Measured torque controller input line 74a,b.

VFD control signals 76a,b Torque difference adjustment system 80 Torque difference output line 82 Smoothing circuit 84 Speed control line 86 Speed control system 88 Speed adjustment line 90 Speed adjustment system 92.

DOS 102 DCS communication line 104 VFD communication lines 106a,b

Claims

CLAIMS1. A method of controlling a pair of counter-rotating rollers, the method comprising: (0 providing an initial reference speed to both the first and second counter-rotating rollers; (ii) measuring a first torque relating to the first counter-rotating roller and a second torque relating to the second counter-rotating roller; (iii) comparing the first and second torque to create a torque difference; (iv) calculating a speed adjustment based on the torque difference; (v) creating an updated reference speed based on the initial reference speed and the speed adjustment; and (vi) sending the updated reference speed to the second counter-rotating roller.
2. A method according to claim 1, wherein the method further comprises the step of smoothing the torque difference prior to calculating the torque difference; and calculating the speed adjustment is based on the smoothed torque difference.
3. A method according to claim 1 or 2, wherein the initial reference speed provided to both the first and second counter-rotating rollers is provided when there is no material to be ground in a nip therebetween.
4. A method according to any of claims 1 to 3, wherein the step of sending the updated reference speed to the second counter-rotating roller is implemented without changing the initial reference speed provided to the first counter-rotating roller.
5. A method according to any preceding claim, wherein the method further comprises: comparing the calculated speed adjustment with a defined maximum speed adjustment; and limiting the calculated speed adjustment thereto in the event that the defined maximum speed adjustment is reached or exceeded.
6. A method according to claim 5, wherein the defined maximum speed adjustment comprises a percentage change to the rotational speed of the rollers of no more than 10% of the initial reference speed.
7. A method according to any of claims 2 to 6, wherein the step of smoothing the torque difference comprises averaging the torque difference over a defined time period.
8. A method according to any of claims 2 to 6, wherein the step of smoothing the torque difference comprises removing spikes in the torque difference using a low pass filter.
9. A method according to any of claims 2 to 8, wherein the method includes the further step of ignoring the smoothed torque difference if the smoothed torque difference is less that a defined percentage of the torque.
10. A method according to any preceding claim, wherein the step of measuring a first torque relating to the first counter-rotating roller and a second torque relating to the second counter-rotating roller is implemented by sensors measuring the torque directly from the respective rollers.
11. A method according to any of claims 1 to 9, wherein the step of measuring a first torque relating to the first counter-rotating roller and a second torque relating to the second counter-rotating roller is implemented by receiving the torque delivered to the respective rollers by a motor dedicated to each roller.
12. A high pressure grinding roller machine comprising: (i) a pair of counter-rotating rollers defining a nip therebetween; (ii) a pair of roller motors, each roller motor having a dedicated variable frequency drive; and a controller operable to control the variable frequency drives by implementing a method according to any preceding claim.
13. A high pressure grinding roller machine according to claim 12, wherein the controller is implemented as a programmable logic controller dedicated to the machine.
14. A high pressure grinding roller machine according to claim 12 or 13, wherein the second counter-rotating roller is an actuated roller.
15. A controller for controlling counter-rotating rollers in a high pressure grinding roller machine, where each roller is independently controlled by a dedicated variable frequency drive, the controller being operable to control the variable frequency drives by: providing an initial reference speed to each of the dedicated variable frequency drives, (ii) measuring a first torque relating to a first one of the counter-rotating rollers controlled by a first of the dedicated variable frequency drives, and a second torque relating to a second one of the counter-rotating rollers controlled by a second of the dedicated variable frequency drives, (iii) comparing the first and second torque to create a torque difference, (iv) calculating a speed adjustment based on the torque difference, (v) creating an updated reference speed based on the initial reference speed and the speed adjustment, and (vi) sending the updated reference speed to only the second variable frequency drive.
16. A controller according to claim 15, wherein the controller is further operable to smooth the torque difference, and calculate the speed adjustment based on the smoothed torque difference.,
17. A controller according to claim 15 or 16, wherein the controller is incorporated into a programmable logic controller that includes a processor operable to control additional functions of the high pressure grinding roller machine.
18. A controller for controlling counter-rotating rollers in a high pressure grinding roller machine, where each roller is independently controlled by a dedicated variable frequency drive, the controller being operable to control the variable frequency drives by: assigning a first of the variable frequency drives as a master drive and providing a constant reference speed thereto; (ii) assigning a second of the variable frequency drives as a slave drive and providing an updated reference speed thereto, where the updated reference speed is different to the constant reference speed provided to the first variable frequency drive; and (iii) re-assigning the first variable frequency drive as the slave drive, and the second variable frequency drive as the master drive, in response to a defined criterion.
19. A controller according to claim 18, wherein the updated reference speed is lower than the constant reference speed, thereby allowing the reference speed to be the highest recommended speed for the rollers
20. A controller according to claim 18 or 19, wherein re-assigning the variable frequency drives is implemented automatically by the controller on detection of the defined criterion, without any human intervention.
21. A controller according to any of claims 18 to 20, wherein the defined criterion comprises a detected condition where the controller needs to increase the slave speed above the constant reference speed.
22. A controller according to any of claims 18 to 21, wherein assigning a first of the variable frequency drives as a master drive and providing a constant reference speed thereto is implemented at commissioning of the high pressure grinding roller machine, where the variable frequency drive associated with the roller having the highest torque is assigned as the master.