APPARATUS AND METHOD FOR CONTINUOUSLY PROCESSING MATERIAL UNDER CONTROLLABLE POWER

Abstract: Disclosed is an apparatus and method for controlling a continuous mixer 10 having a discharge formed as a gear pump 30 or an extruder screw. The continuous mixer 10, which includes a charging orifice 20 spaced apart from a discharge orifice 16, feeds material fed through the charging orifice 20 through at least one chamber 14 by one or more bladed otors 18 which are controlled by a motor 26. The discharge of this material through the mixer 10 is controlled by the gear pump 30, and the RPM of the pump gears 36 is controlled in relation to the torque produced by the motor 26 which drives the bladed rotors 18. A change in torque produced by this motor 26 will result in a change in the RPM of the pump gears 36, thereby maintaining the rotor torque and consequently the mixing energy exerted on the material being mixed in the mixer 10.

Description: This application relates in general to an apparatus and method for continuously processing material under controllable power, and relates more specifically to said processing being performed in a continuous mixer having a moving surface discharge device. Continuous mixers, for mixing plastics materials, which include a mixing enclosure with spaced apart entrance and exit openings and a mixing means, are well known. In general, the mixing time of the material in the enclosure is dependent on the volume of material in the enclosure, which in turn is dependent in part on the rate at which material in the enclosure is moved out through the exit opening. By varying the size of the exit opening, the pressure required to push the material through the exit opening, and the pressure on the material being mixed internally in the enclosure, may be controlled. In some situations, the restraint exercised by the exit opening is not satisfactory to exert the desired pressure on the material because of material characteristics of the mixing. However, attempts to obtain a smoother action by use of walls having polished or lubricated surfaces have not been completely satisfactory. A smoother action is obtainable where the restraint to the flow of material discharged from the continuous mixer is provided by a moving surface discharge device in which one or more surfaces contact the material and move in the direction of the flow of material at controllable speeds. Movement at a speed initially slower than that of the flow when unimpeded restrains the flow and effects a back pressure on the flow. Then, by adjusting the speed so that the rate of feed into and from the mixer is substantially the same, this back pressure remains and is reflected back on the material being mixed in the mixer while moving through the mixer. Adjustment of the moving speed of the surface or surfaces adjusts the internal mixer pressure to give and maintain the pressure desired for the mixing operation involved. The speed can be made automatically responsive to the temperature of the mixing or discharging material, temperature being a function of the pressure on the material being mixed. A screw-type extruder or a gear-type pump may be used to provide such a moving surface, provided that the rotative speed of the extruder screw, or pump gears, can be controlled, the use of continuous mixers with a screw-type extruder and a gear-type pump being taught by U.S. patent 4,310,251. In controlling the operation of a continuous mixer with a discharge device such as an extruder screw or pump gears, it is important that the operation of the starve fed continuous mixing device be synchronised with its closely coupled discharge device in order to achieve stable operation, and it is also desirable to provide for the start up and shut down of both pieces of equipment. In U.S. patent 4,452,750, an in-line melter/mixer-gear pump system is provided for the processing of synthetic, thermoplastics materials. The system employs the pressure between the melter/mixer and the gear pump as the controlling parameter which affects, in a proportional relationship, the speed of the gear pump, the energy transmitted to and the consequent temperature of the materials passing through the melter/mixer. Such a control method has not proven to be of practical use due to the difficulty of developing relationships between the gear pump inlet pressure and the mixer energy input level. In fact, the relationship of pump speed to mixer power taught by US-A-4452750 may be impossible. With the gear pump being a volumetric displacement device, it has a relatively constant output per revolution. It is, therefore, not possible to maintain an output rate of 900 lbs/hr (408 kg/hr) over a gear pump speed range of 75 to 93 RPM. Either the material will back up in the mixer at the low end of the speed range, or the gear pump is running starved at the high end of the speed range, meaning that control of the mixer is now partially due to drag in the space between the mixer and the gear pump. It also has the drawback that the pressure existing at mixer discharge must be derived empirically. It is also known from GB-A-1104827 for the size of an outlet opening in a mixer to be varied in response to the instantaneous value of the power consumption of the mixer in order to maintain a desired value for the consistency of granular solids mixed with liquid or thermoplastic binders and discharged through the outlet opening of the mixer. Apparatus, in accordance with a first aspect of the present invention, for continuously processing thermoplastic polymeric material under controllable power, comprises a barrel forming at least one substantially cylindrical material working chamber having a material charging orifice adjacent one end and a material discharge orifice of constant area adjacent the other end, a mixing rotor extending axially in said chamber, means for rotating said rotor, said rotor having axially extending blade portions, means on said rotor between said charging orifice and said blade portions for substantially continuously pushing material fed to said chamber toward said discharge orifice, a discharge control device having a moving surface for engaging material entering said discharge control device through an inlet connecting to said discharge orifice, and means for controlling said discharge control device at a variably controlled rate for applying a variable back pressure to the material fed through said discharge orifice, characterised in that the apparatus further comprises means for adjusting a discharge rate of said discharge control device by adjusting the speed at which said moving surface of said discharge control device moves in the direction of flow of the material to maintain the torque produced by said means for rotating said rotor at a chosen target torque. A method, in accordance with a second aspect of the present invention, for continuously processing thermoplastic polymeric material under controllable power in apparatus comprising a barrel forming at least one substantially cylindrical material working chamber having a material charging orifice adjacent one end and a material discharge orifice of constant area adjacent the other end, a mixing rotor extending axially in said chamber, means for rotating said rotor, said rotor having axially extending blade portions, means on said rotor between said charging orifice and said blade portions for substantially continuously pushing material fed to said chamber toward said discharge orifice, a discharge control device having a moving surface for engaging material entering said discharge control device through an inlet connecting to said discharge orifice, and means for controlling said discharge control device at a variably controlled rate for applying a variable back pressure to the material fed through said discharge orifice, is characterised by the step of adusting a discharge rate of said discharge control device by adjusting the speed at which said moving surface of said discharge control device moves in the direction of flow of the material to maintain the torque produced by said means for rotating said rotor at a chosen target torque. It will be noted that the present invention can reliably control the speed of the discharge control device, and thereby control the energy transmitted to and the consequent temperature of material passing through the apparatus, can easily be adjusted to exert different pressures on the material being mixed and operates as well during start up as during operation at the desired capacity. Preferably, said means for adjusting a discharge rate of said discharge control device comprises: means for measuring torque produced by said means for rotating said rotor; means for comparing said measured torque with a target torque and for generating an adjustment signal indicative of the difference between said measured torque and said target torque; and means for resetting a discharge rate of said discharge control device in response to said adjustment signal. Preferably, said means for rotating said rotor is a rotor drive motor and said means for measuring torque is a means for measuring amperage drawn by said rotor drive motor. In other words, preferably said means for rotating said rotor includes a rotor drive motor and said means for adjusting a discharge rate of said discharge control device comprises: means for measuring actual amperage drawn by the rotor drive motor; means for comparing said actual amperage drawn by said motor with a target amperage to be drawn by said motor; means responsive to said comparing means for producing a control signal indicative of the difference between said actual amperage and said target amperage; and means for sending said control signal to means for re-setting a discharge rate of said discharge control device to adjust said re-setting means in response to said control signal to maintain said amperage drawn by said rotor drive motor at a chosen level. In a particularly preferred arrangement, said means for adjusting a discharge rate of said discharge control device comprises: means for selecting a production rate, a specific energy applied to the material and a speed of operation for said rotor; and processor means for receiving a signal representing the torque produced by said means for rotating said rotor, said processor means calculating said target torque from said selected production rate, specific energy and speed, and said processor means also comparing said target torque to said torque produced by said means for rotating said rotor to provide a signal for adjusting a discharge rate of said discharge control device. Preferably, said processor means calculates a horsepower value from the production rate and the specific energy, and then calculates said target torque from the rotor speed and the calculated horsepower value; said processor means comparing said target torque with a feedback signal indicative of actual torque produced by said means for rotating said rotor, and then generating a correction signal indicative of the difference between said target torque and said actual torque, said processor means applying said correction signal to a proportional, integral and derivative control loop to produce an adjustment signal which is applied to said discharge control device so as to adjust a discharge rate thereof and thereby maintain said actual torque equal to said target torque. Although said discharge control device could be a screw-type extruder, it is preferably a gear-type pump. Apparatus, and a method, in accordance with the present invention, will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a cross-sectional view of the apparatus to be controlled in accordance with the present invention; Figure 2 is a diagrammatic representation of the relationship of input and output signals utilised for controlling the operation of the discharge control device of the continuous mixer shown in Figure 1; Figure 3 is a flow diagram of the method in accordance with the present invention; Figure 4 is a continuation of the flow diagram of Figure 3; and Figure 5 is a continuation of the flow diagram of Figure 4. Appendix 1 is a listing of a computer instruction code for operating the apparatus and method of the present invention. The apparatus according to the present invention is closely based on that taught by U.S. patent 4,310,251, the specification of which is incorporated herein by reference. Briefly, as shown in Figure 1, the apparatus includes a continuous mixer 10 and a discharge control device gear pump 30, which are closely coupled so that the operation of the gear pump 30 can and does affect the operation of the continuous mixer 10 as will be described below. The mixer 10 preferably includes a barrel 12 forming at least one and preferably two parallel, laterally inter-connecting, cylindrical material working or mixing chambers 14 having at one end a common discharge orifice 16. Mixing rotors 18 are located in the chambers 14, and the material to be mixed is conveyed into the chambers 14 at an entrance location spaced from the discharge orifice 16. A vertical port 20 leads to conveying or feed screw blades 22 formed on the rotors 18 for pushing or feeding the material in the direction of the discharge orifice 16. The rotors 18 are inter-geared for either opposite rotation or co-rotation by gears 24 and are supplied with a rotary drive system 26. The exit opening or discharge orifice 16 opens transversely from the chambers 14 and at least mainly in overlapping relation with respect to blade portions 28 of the rotors 18 adjacent thereto. The discharge orifice 16 is connected directly and positively to inlet 32 of gear pump 30. The gear pump 30 comprises a housing 34 forming chambers in which the pump gears 36 are located. The gears 36 may be driven by a motor (not shown). The pump has an outlet 38 connected to an extrusion orifice 40. In operation, polymer material which is to be mixed, is fed continuously through vertical port 20 in the mixer 10, where the rotors 18 convey the material into the mixing chambers 14. Usually, the vertical port is located at the lower end of a hopper to which the polymer material is fed automatically by feeders, whereby it is possible to achieve a substantially continuous feeding of the polymer material to the chambers 14 through the vertical port 20 at a controllable rate. The material is melted in the chambers 14 before it exits through the discharge opening 16 and enters the close coupled gear pump inlet 32. This material, when discharged through the discharge orifice 16, is engaged by the gears 36. When the gears 36 are unpowered, they should normally hold back the material discharged from the mixer 10 because of the friction of the rotating parts. Once pressure is built up in the mixture to the desired value, the gears 36 should be rotated to carry the mixed material away from the mixer at the same rate as the material to be mixed is charged into the mixer. During operation, a momentary slow-down of the gears 36 serves to increase the mixer's internal pressure after which they should be returned to the rate matching the mixer's charging rate. A momentary speed-up of the gears 36 drops the pressure in the mixer. In all instances the material should be discharged from the mixer at substantially the rate at which the mixer is charged with material to be mixed, regardless of the selected mixer internal pressure. According to the present invention, the internal pressure of the mixer 10 is controlled by adjusting the RPM of the gear pump 30 through control based on the torque of the mixer motor 26. The amperage of the mixer drive motor is proportional to the actual torque produced by the motor, and this actual torque signal is monitored and compared with a target or required torque. The required torque is calculated from the following equation with known variables: Work input = HP. hr/kg Production rate = kg/hr RPM = Rev/Min Under normal operation the mixing chambers 14 of the continuous mixer 10 are only partially filled. By adjusting the speed of the pump gears 36, the level of fill inside the mixing chambers changes, affecting the operation of the mixture. An increase or decrease in the amount of material in the mixing chambers results in a corresponding increase or decrease of torque in the continuous mixer for a constant mixer rotor RPM and a constant feed rate. This also corresponds to an increase or decrease in energy. To monitor and control the torque, a microprocessor is connected to the motor in a known fashion. As shown in Figure 2, the microprocessor receives three inputs which can be entered by an operator or stored in memory and fetched by the microprocessor. These three input signals are the production rate 44 in kg/hr, the specific energy 46 in HP. hr/kg and the mixer RPM 48. A feedback signal 58 which is proportional to the actual torque on the mixer is also required. Since torque is proportional to the DC amperage on a DC motor, and to the main motor horsepower in an AC motor/clutch type arrangement, these signals are used as the feedback signals. The processor output signal 60 is used to control the RPM of the pump gears. In order to derive the process output signal 60, the microprocessor multiplies the production rate times the specific energy to obtain a signal 50 representing horsepower. The microprocessor divides this horsepower by the mixer RPM and obtains a value for torque. This calculated value for torque is the torque that the mixer must have in order to satisfy the three input variable requirements, i.e. the production rate, the specific energy and the mixer RPM. The actual torque obtained from the feedback signal from the mixer is compared to the target torque (calculated from three known inputs) in the microprocessor. The difference between these two values is considered an error which is placed into a standard velocity form of a proportional, integral and derivative control mode algorithm. The output of the PID loop minimises the error by adjusting the speed or RPM of the gear pump. This change in speed of the gear pump results in a change in the actual torque of the mixer. Since the actual torque is measured by the microprocessor through the use of the feedback signal, continuous adjustments are made to the gear pump RPM which minimises the difference between actual and target torques. This control strategy is most often used to maintain and/or adjust to different specific energy levels (the amount of energy applied to the material). Since the target torque is calculated by using specific energy, a change to this input by an operator will result in a change in the target torque. This change in target torque causes the microprocessor to adjust the gear pump speed in order to match the actual and target torques. A specific embodiment of the control method set forth above is shown in Figures 3 to 5. The desired production rate, specific energy and mixer RPM are input to the processor, and a signal proportional to the actual torque of the mixer motor is fed to the processor in step 72. By means of an interrupt routine which is executed every 0.1 seconds, several readings of the signal proportional to the torque are made and scaled appropriately. In step 74, the torque signal read is examined to see whether the signal represents an electrical spike, and if so, further processing of that signal ceases. The actual torque signal read in step 72 is added during step 76 to an accumulated torque which represents a running total of torque values taken over a given time interval. A torque signal counter is incremented in step 78, and if the requisite number of actual torque values have been read to provide a reliable reading of the actual torque, step 80 initiates the processing of the torque signal as the feedback signal. If the requisite number of signals representing the actual torque have not been read, the control of the processor switches back to the main scan routine which scans for operator inputs. Once the requisite number of actual torque values have been fed to the microprocessor, the average of the input torque values is computed in step 82. In order to process further signals, steps 84 and 86 re-set both the accumulated torque and the counter to zero. The proportional, integral and derivative (PID) control loop then takes over operation in order to equate the actual torque with the target torque. In step 88, the actual torque is subtracted from the stored target torque, and this integral error value E n is stored. The previous E n , (E n-1 ) is subtracted from E n in step 90 to arrive at change in E which represents the proportional error. E n-1 is doubled in step 92 and is then subtracted from E n in step 94 to arrive at an intermediate calculation which, in step 96 is then added to E n-2 to arrive at the derivative error. Proportional, integral and derivative correction factors are then calculated depending on whether the torque value represents in step 98 that of a start-up system or a steady state system. If the system is in the start-up mode, the proportional, integral and derivative errors are multiplied by appropriate constants for the start-up mode in steps 100a, 102a, 104a. Likewise, if the system is running in the steady state mode, appropriate constants are multiplied by the proportional, integral and derivative errors to arrive at the proportional, integral and derivative correction factors in steps 100b, 102b and 104b. Representative constants for the steady state operation are proportional gain: -50 derivative gain: -5 integral gain: -10 Once the correction factors have been calculated, they are summed in step 106 to arrive at the appropriate RPM correction. This RPM correction factor is added to the actual RPM to arrive at the new target RPM in step 108. At that point, the new target RPM is compared to the minimum possible RPM as well as the maximum possible RPM and adjusted as appropriate. Steps 110 and 112 compare the actual RPM with the minimum and maximum shut down limits, and if either of these limits is detected, appropriate shut down action is taken. In a representative embodiment of the present invention, a Size 4 Farrel Continuous Mixer is connected to a Maag 70/70 Therminox gear pump. A Reliance Automate 35 microprocessor may be used to monitor the actual torque (DC amperage of a DC motor) and control the speed of the gear pump. The foregoing invention has been described as utilizing the motor amperage to measure the rotor torque. While the motor amperage provides a convenient means for measuring motor torque with satisfactory accuracy, any other means for measuring motor torque may be substituted in the present invention. For example, a commercially available torque meter (such as a shaft with strain gauges applied) may be inserted between the motor and the mixer, and such a device will actually provide a more accurate torque measurement.

Claims: Apparatus, for continuously processing thermoplastic polymeric material under controllable power, comprises a barrel (12) forming at least one substantially cylindrical material working chamber (14) having a material charging orifice (20) adjacent one end and a material discharge orifice (16) of constant area adjacent the other end, a mixing rotor (18) extending axially in said chamber (14), means (26) for rotating said rotor (18), said rotor (18) having axially extending blade portions (28), means (22) on said rotor (18) between said charging orifice (20) and said blade portions (28) for substantially continuously pushing material fed to said chamber (14) toward said discharge orifice (16), a discharge control device (30) having a moving surface for engaging material entering said discharge control device (30) through an inlet (32) connecting to said discharge orifice (16), and means (36) for controlling said discharge control device (30) at a variably controlled rate for applying a variable back pressure to the material fed through said discharge orifice (16), characterised in that the apparatus further comprises means for adjusting a discharge rate of said discharge control device (30) by adjusting the speed at which said moving surface of said discharge control device (30) moves in the direction of flow of the material to maintain the torque produced by said means (26) for rotating said rotor (18) at a chosen target torque. Apparatus according to claim 1, characterised in that said means for adjusting a discharge rate of said discharge control device (30) comprises: means for measuring torque produced by said means (26) for rotating said rotor (18); means for comparing said measured torque with a target torque and for generating an adjustment signal (60) indicative of the difference between said measured torque and said target torque; and means for re-setting a discharge rate of said discharge control device (30) in response to said adjustment signal. Apparatus according to claim 2, characterised in that said means (26) for rotating said rotor (18) is a rotor drive motor and said means for measuring torque is a means for measuring amperage drawn by said rotor drive motor. Apparatus according to claim 1, characterised in that said means (26) for rotating said rotor (18) includes a rotor drive motor and said means for adjusting a discharge rate of said discharge control device (30) comprises: means (72) for measuring actual amperage drawn by the rotor drive motor (26); means (88) for comparing said actual amperage drawn by said motor (26) with a target amperage to be drawn by said motor (26); means (90 to 106) responsive to said comparing means for producing a control signal (60) indicative of the difference between said actual amperage and said target amperage; and means for sending said control signal to means (36) for re-setting a discharge rate of said discharge control device (30) to adjust said re-setting means (36) in response to said control signal to maintain said amperage drawn by said rotor drive motor (26) at a chosen level. Apparatus according to any preceding claim, characterised in that said means for adjusting a discharge rate of said discharge control device (30) comprises: means (44,46,48) for selecting a production rate, a specific energy applied to the material and a speed of operation for said rotor (18); and processor means for receiving a signal representing the torque produced by said means (26) for rotating said rotor (18), said processor means calculating said target torque from said selected production rate, specific energy and speed, and said processor means also comparing said target torque to said torque produced by said means (26) for rotating said rotor (18) to provide a signal (60) for adjusting a discharge rate of said discharge control device (30). Apparatus according to claim 5, characterised in that said processor means calculates a horsepower value (50) from the production rate and the specific energy, and then calculates said target torque from the rotor speed and the calculated horsepower value; said processor means comparing said target torque with a feedback signal indicative of actual torque produced by said means (26) for rotating said rotor (18), and then generating a correction signal indicative of the difference between said target torque and said actual torque, said processor means applying said correction signal to a proportional, integral and derivative control loop to produce an adjustment signal which is applied to said discharge control device (30) so as to adjust a discharge rate thereof and thereby maintain said actual torque equal to said target torque. Apparatus according to any preceding claim, characterised in that said discharge control device (30) is a gear-type pump. A method, for continuously processing thermoplastic polymeric material under controllable power in apparatus comprising a barrel (12) forming at least one substantially cylindrical material working chamber (14) having a material charging orifice (20) adjacent one end and a material discharge orifice (16) of constant area adjacent the other end, a mixing rotor (18) extending axially in said chamber (14), means (26) for rotating said rotor (18), said rotor (18) having axially extending blade portions (28), means (22) on said rotor (18) between said charging orifice (20) and said blade portions (28) for substantially continuously pushing material fed to said chamber (14) toward said discharge orifice (16), a discharge control device (30) having a moving surface for engaging material entering said discharge control device (30) through an inlet (32) connecting to said discharge orifice (16), and means (36) for controlling said discharge control device (30) at a variably controlled rate for applying a variable back pressure to the material fed through said discharge orifice (16), said method being characterised by the step of adjusting a discharge rate of said discharge control device (30) by adjusting the speed at which said moving surface of said discharge control device (30) moves in the direction of flow of the material to maintain the torque produced by said means (26) for rotating said rotor (18) at a chosen target torque. A method according to claim 8, characterised by being performed in apparatus according to any one of claims 2 to 7.

Inventors: MARKHART, GARY T., VALENZKY, DAVID
Applicants: FARREL CORPORATION
Application year: 1986
Main classification: B01F15
IPC codes: B29B7, B01F3, B29C47, B01F5, B01F7, B01F15