BACKGROUND OF THE INVENTION The present invention relates to a concrete mixing system and, more particularly, to a system for precisely mixing liquid and cementitious material in a slurry mixer to form a flowable slurry to be mixed with aggregate in making concrete.

Increasingly strict local, state and federal pollution regulations have become an onerous burden to the operators of concrete mixing plants, particularly small mixing plant operators. Limits on airborne particulates and groundwater runoff and contamination require expensive modifications to existing concrete mixing plant equipment and operating procedures. New equipment that has become available only incidentally addresses these problems, and is complex and generally unsuitable for an existing mixing plant retrofit.

In addition to the need to reduce airborne particulates and groundwater runoff and contamination, there is an increased awareness that water is a very finite resource that needs to be conserved. While water is a minor component in the concrete mixture per se, it is a major component in the cleanup process for the concrete mixing area.

In addition, there are potential quality control issues that can arise when a specific concrete mixture requires a precise ratio of materials. Materials that are carefully measured should also be added together in a precise metered manner and thoroughly mixed to produce complete hydration. Obviously, when a portion of the cement that has been carefully measured according to a ratio for inclusion in a mixture is lost as airborne particulate, the characteristics of the final concrete mixture are altered. Likewise, mixing equipment that relies

primarily on gravity to dispense and meter cement can easily clog, resulting in uneven metering, mixing, and an inferior end product.

Prior art improvements in the field of concrete mixing apparatus have generally been either technically complex attempts to solve particular problems affecting the very specific needs of small segments of the industry, or attempts to increase overall efficiency.

Ono et al. U. S. Patent No. 5,100, 239 discloses a method to produce concrete for mass concrete members by spraying liquid nitrogen onto aggregate (particularly sand) within enclosed conveyor screws prior to combining the nitrogen cooled aggregate with cement, water, and coarser aggregates for the final mixing operation. Ono does not recognize nor address the need to control cement dust pollution in a concrete mixing system by providing an inexpensive retrofittable apparatus.

Raypholtz U. S. Patent No. 2,486, 323 discloses a complicated variable output mixing system for mixing aggregate and bituminous material that operates similar to a pugmill without recognition of the foregoing pollution problem.

Owen U. S. Patent No. 1,753, 716 discloses a screw conveyor mixer particularly suited to producing a grout mixture for cementing oil wells. Owen does not provide nor suggest a final product mixing chamber for mixing a flowable cement slurry with aggregate to form concrete, nor recognize the foregoing pollution problem.

Haws U. S. Patent No. 4,586, 824 discloses a mobile concrete mixing apparatus wherein a conveyor initially carries aggregate from a storage bin.

Dry cement is dumped on top of this aggregate as it travels on the conveyor, and water is sprayed on the aggregate and dry cement as it is dumped into a feed screw for mixing. Nothing in the system prevents cement dust pollution.

Dunton et al. U. S. Patent Nos. 4,904, 089 and 4, 830, 505 disclose a method of mixing particulate cement and water in a primary mixing vessel to form a slurry and delivering the slurry to an auxiliary mixing vessel for mixing with aggregate. The method and apparatus disclosed in Dunton'505 and'089 illustrates the recognized desirability and advantages produced by premixing

concrete and liquid to form a slurry before mixing with aggregate. However, Dunton's solution is very complex and expensive, requiring the use of high velocity pumps and multiple rotary agitators to create the flowable slurry, and lacking easy retrofit adaptability to existing concrete mixing plants.

Macaulay et al., U. S. Patent Nos. 5,352, 035 and 5,427, 448, incorporated herein by reference, disclose a concrete mixing system with a cement/water slurry mixer and a method for mixing concrete with a cementitious material/liquid slurry mixer. Cementitious material is introduced to the slurry mixer at an inlet and moved toward an outlet by a screw conveyor.

Liquid is added to the cementitious material as the cementitious material moves toward the outlet. The liquid and cementitious material are mixed by the screw conveyor forming a flowable slurry that is discharged from the outlet of the slurry mixer into a cement truck or other final product mixing chamber where it is mixed with sand and aggregate to form concrete. Because the cementitious material and liquid are fully enclosed during the flowable slurry mixing step, the amount of airborne cementitious particulate matter, i. e.,"dust," that is usually attendant in such mixing methods is greatly reduced. The need to apply copious quantities of water to wash the dust off the mixing equipment and other surfaces in the mixing area is also proportionately reduced.

Moreover, the time required to mix a given quantity of concrete is likewise reduced using this method, which results in more efficient equipment utilization and greater output.

The strength of concrete is a function of and varies widely with the water-cement ratio. The ratio of water to cementitious material, the type and gradation of aggregate, and the moisture content of the sand are all interrelated factors effecting the quality of the final concrete product. Concrete mixing is typically a batch process where the constituents (aggregate, sand, cementitious material, and liquid) are measured and contemporaneously introduced to a mixing chamber, such as a cement truck, for mixing. In a typical dry batching concrete system the cementitious material, typically cement and fly ash, are released into the mixing chamber from an automated weighing hopper as the aggregate and sand are introduced to the mixing

chamber by a conveyor belt. Typically, the concrete plant's automation system introduces the majority of the water called for by the concrete recipe to the mixing chamber before the introduction of the cementitious material and the remainder at the conclusion of the process.

However, if the cementitious material and the liquid are premixed in a slurry mixer, the flow rates of liquid and cementitious material must be precisely metered if all of the cementitious material is to be properly mixed and the total quantity of liquid is to be correct according to the recipe for the batch of concrete. What is desired, therefore, is a system for controlling a slurry mixer that cooperates with a concrete plant automation system to precisely introduce liquid to dry cementitious material to produce a thoroughly mixed cementitious slurry useful for making high quality concrete.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation view of a slurry mixer of cementitious material and liquid.

FIG. 2 is a top view of the apparatus of FIG. 1.

FIG. 3 is a side sectional view of the apparatus of FIG. 1.

FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 1.

FIG. 5 is a block diagram of a data processing system for controlling a slurry mixing system.

FIG. 6 illustrates an exemplary graphical user interface for a data processing system controlling a slurry mixing system.

FIG. 7 is a block diagram of exemplary program instructions for a data processing system controlling a slurry mixing system including an exemplary set of component routines.

FIG. 8 is a flow diagram of a liquid usage simulation routine for a data processing system controlling a slurry mixing system.

FIG. 9 is a flow diagram of a liquid-cement ratio calculation routine for a data processing system controlling a slurry mixing system.

FIG. 10A is a flow diagram of a first portion of mixing process control routine for a data processing system controlling a slurry mixing system.

FIG. 1 OB is a flow diagram of a continuation of the mixing process control routine illustrated in FIG. 10A.

FIG. 10C is a flow diagram of a continuation of the mixing process control routine illustrated in FIG. 10B.

FIG. 11 is a flow diagram of a nozzle control routine for a data processing system controlling a slurry mixing system.

FIG. 12 is a flow diagram of a load control and manual input routine for a data processing system controlling a slurry mixing system.

FIG. 13 is a flow diagram of a rinse and end process routine for a data processing system controlling a slurry mixing system.

DETAILED DESCRIPTION OF THE INVENTION Referring in detail to the drawings wherein similar parts of the invention are identified by like reference numerals. With reference in particular to FIGS. 1-4, an exemplary slurry mixer 10 is illustrated. The exemplary slurry mixer 10 comprises several major subassemblies including a cementitious material measuring device 12, screw conveyor 14 and final product mixing chamber 16 arranged in-line with each other.

The cementitious material measuring device 12 dispenses (by gravity or otherwise) a measured (by volume or weight) quantity of cementitious material, e. g. cement, fly ash, etc. Typically, the cementitious material measuring device 12 is an automated hopper, as illustrated in FIG. 1, equipped with at least one weighing or volume measuring transducer 200 and a gate 202 responsive to a signal from a control system that may opened and closed by an actuator 204 to permit the contents of the hopper 12 to feed through a material outlet 18 into the screw conveyor 14. The material outlet 18 of the measuring device 12 is in-line with the conveyor inlet 20, which feeds to a first or feed conveyor screw assembly 28 of the screw conveyor 14. The screw conveyor 14, which moves and mixes the cementitious material received from

measuring device 12 with a liquid such as water to form a flowable slurry, has a conveyor outlet 22 in the bottom of a second or blend conveyor screw assembly 56, the outlet 22 being positioned in-line with the inlet 20. Positioned below conveyor outlet 22 is the final product mixing chamber 16 where the flowable slurry emitted from conveyor outlet 22 is mixed together with a measured quantity of aggregate material (such as rock and sand) to form concrete.

In the exemplary slurry mixer 10 the dimensions of the screw conveyor 14 are such that it may be easily and inexpensively retrofitted between the measuring device 12 and mixing chamber 16 already being used in most existing concrete mixing plants. Flanges 24 may be used to sealingly couple measuring device outlet 18 to the conveyor inlet 20. Depending on the particular final product mixing chamber 16 to be used, the conveyor outlet 22 may also be coupled to the top of the mixing chamber 16 by flanges 26. If the final product mixing chamber 16 is a mobile mixer, physical coupling may be unnecessary and the flowable slurry may"free-fall"from conveyor outlet 22 into final product mixing chamber 16 without the danger of any airborne particulate matter being emitted.

Dry cementitious material entering the conveyor inlet 20 of screw conveyor 14 from the cementitious material measuring device or hopper 12 is moved by the rotation of the feed conveyor screw assembly 28 enclosed within a first tubular housing 30. The dry cementitious material is moved in opposite diverging directions away from the conveyor inlet 20 and toward a first outlet 32 and a second outlet 34 located at opposite ends of the first tubular housing 30.

Feed conveyor screw assembly 28 has a pair of opposed single flight, standard pitch conveyor screws 46,48 on a common shaft 50. Suitable conveyor screw assemblies are readily available from such manufacturers as Thomas Conveyor Co., Ft. Worth, Texas. In the exemplary slurry mixer, conveyor screws having a diameter of approximately six inches have been found to be suitable.

Feed conveyor screw assembly 28 can be rotated by any suitable power source, here an electric drive motor 36 is shown coupled to common shaft 50

by a belt 38 and pulleys 52,54. The rate of rotation of the feed conveyor screw assembly 28 should be variable to allow control over the rate that the dry cementitious material is moved to the outlets 32,34. A motor controller 206 responsive to a feed signal from a slurry mixer control system controls the rotation of the drive motor 38. Flanged ball bearings 110 and shaft seals 112 are used with shaft 50.

When the dry cementitious material moved by the feed conveyor screw assembly 28 reaches the first outlet 32 and the second outlet 34 located at the opposite ends of first tubular housing 30, the dry cementitious material falls by gravity through first outlet 32 and second outlet 34 into and through respective first and second inlets 40,42 of the second tubular housing assembly 44 of the blend conveyor screw assembly 56 positioned beneath and substantially parallel to the first tubular housing assembly 30. The blend conveyor screw assembly 56 is a pair of opposed conveyor screws 58,60 on a common shaft 62 which also has flanged ball bearings 110 and shaft seals 112. Unlike the feed conveyor screw assembly 28 which is designed simply to move the dry cementitious material away from the conveyor inlet 20 toward the first outlet 32 and second outlet 34 at a controlled, metered rate, the design and function of blend conveyor screw assembly 56 is different. In the blend conveyor screw assembly 56, the dry cementitious material must be thoroughly mixed with liquid to form a flowable slurry as it is simultaneously moved away from the first and second inlets 40,42 at the opposite ends of the second tubular housing assembly 44 converging inward toward the conveyor outlet 22.

The design of a screw conveyor to mix material as it is being moved is well known in the art, and depending upon such variables as the particular cementitious material to be mixed and the power source (motor 64), the blend conveyor screw assembly 56 could include paddles (not shown) to perform the mixing operation. There are also conveyor screws, well known in the art, of cut flight, cut and folded flight, and multiple ribbon flight design that could be used to move and mix the materials.

The liquid, generally water, necessary to create the flowable slurry may be introduced as a wide angle spray 68 as shown in FIG. 4. The liquid

introduction means may be easily constructed of readily available standard plumbing pipe and fittings. In the exemplary slurry mixer 10, illustrated in FIGS. 1-4, a hose 70 connects a distribution manifold 72 to a pump 214. One or more diversion valves 340,342, 344,346 divert one or more liquids (e. g., chemicals, cold water, hot water, and reclaimed water) from sources 240,242, 244,246, respectively to the pump 214 which supplies the liquid to the manifold 72. The pump 214 is driven by a motor 216 controlled by the slurry mixer control system. The flows of liquid from the sources to the distribution manifold 72 is measured, regulated, and controlled by the pump 214 and the diversion valves 340,342, 344,346 and a plurality of meters 332,334, 336, and 338. The meter is a type that outputs an instance of a signal or a pulse in response to the flow of specified volume of liquid through the meter. For example, the meter 334 may output a pulse to the plant automation system each time a gallon of liquid flows through the meter. Many concrete plants utilize a combination of cold water, hot water and reclaimed water in the concrete recipe. In addition, chemicals may be added to the liquid to improve certain properties of the concrete. Additional sources, meters, and valves may be connected in parallel to divert and meter other liquids to the pump 214 for mixing in the manifold 72. A flow rate of 60-180 gpm at 110 to 130 psi is adequate.

The manifold 72, here made out of iron pipe and fittings, is shown supplying the liquid to a plurality of nozzles 76 which produce a wide angle spray 68 of approximately 100 degrees. The nozzles 76 comprise a port for introducing liquid to the cementitious material being moved toward the outlet 22 by the blending screw 56. Flexible tubing 74 is connected to a plurality of nozzle control valves, for examples nozzle control valves 348 and 350, spaced at regular intervals along the length of the manifold 72. Liquid flows from the manifold through a nozzle control valve 348,350 to a flexible tube 74 connected to the nozzle control valve by appropriate fittings 80. The other end of each length of flexible tubing 74 is connected to a nozzle 76 which is inserted into the second tubular housing assembly 44. The nozzles 76, arrayed along the second tubular housing 44, may have differing flow rates at a

particular drop in liquid pressure. As a result, the total liquid flow rate into the second tubular housing can be controlled by allowing, blocking, or modulating flow through one or more of the nozzle control valves 348,350 comprising the liquid flow control for the port into the blend screw. On the other hand, the liquid flow rate may be varied by varying the displacement of the pump 214.

The overlapping spray 68 pattern is achieved by positioning the nozzles 76 such that they are fairly high up the sides of the second tubular housing assembly 44 and in matched opposed pairs.

The blend conveyor screw assembly 56 can be rotated by any suitable power source, here an electric motor 64 shown coupled to common shaft 62 by a belt 84 and pulleys 86 and 88. The rotational speed of the blend conveyor screw assembly 56 should be variable to allow control of the rate of movement and mixing of the cementitious material and liquid. Blend screw motor speed control is provided by a motor controller 65 responsive to a blend signal from the slurry mixer controller system. A motor 64 rotating at slightly more than twice the rpm of motor 36 has been found adequate. Thus, due to either higher speed or more aggressive screw configuration, or both, the blend conveyor screw assembly 56 agitates the mixture to a much greater degree than does the feed conveyor screw assembly 28. As explained previously, it is desirable that the rates of rotation of both the feed conveyor screw assembly 28 and blend conveyor screw assembly 56 be independently controllable and variable, and that the flow rate of liquid be meterable. By altering these variables relative to each other, the quality and quantity of the flowable slurry may be controlled.

The slurry mixing system comprises the slurry mixer 10 and, referring to FIG. 5, a data processing system 300 that controls the slurry mixer and related equipment in coordination with an automation control system for a concrete plant 302. While the slurry mixer control system 300 may have many configurations and may be installed on a number of types of data processing equipment including a programable logic controller (PLC) and a personal computer (PC) system, FIG. 5 depicts an exemplary block embodiment of the slurry mixer control system 300 as installed on a personal computer system.

The exemplary computer system includes a microprocessor-based, central processing unit (CPU) 304 that fetches data and instructions from a plurality of sources, processes the data according to the instructions, and stores the result or transmits the result in the form of signals to control some attached device.

Typically, basic operating instructions used by the CPU 304 are stored in nonvolatile memory or storage, such as read only memory (ROM) 306. The instructions of application programs and related data are typically stored on a nonvolatile mass storage device or memory 308, such as a disk storage unit.

The data and instructions are typically transferred from the mass storage device 308 to random access memory (RAM) 310 and fetched from RAM by the CPU 304 during execution. Data and instructions are typically transferred between the CPU 304, ROM 306, and RAM 310 over an internal bus 312.

The exemplary slurry mixer control data processing system 300 may also include a plurality of attached devices, including a printer 314; a display 316, and one or more input devices 318, such as a keyboard, mouse, or touch screen. Under the control of the CPU 304, data is transmitted to and received from each of the attached devices over a communication channel connected to the internal bus 312. Typically, each device is attached to the internal bus by way of an adapter, such as the interface adapter 320 providing an interface between the input device 318 and the internal bus 312. Likewise, a display adapter 322 provides the interface between the display 316 and the video card 324 that processes the video data under the control of the CPU 304. The printer 314, and nonvolatile mass storage device 308 are connected to the internal bus 312 by one or more input-output (I/O) adapters 326.

The)/0 adapter 326 includes an analog-to-digital converter (ADC) 328 and a digital-to-analog converter (DAC) 330 to convert analog signals received from several transducers in the slurry mixing system to digital signals suitable for processing by the CPU 304 and to convert the digital signals output by the CPU 304 to analog signals suitable to operate the slurry mixer's various transducers. The slurry mixer control system 300 communicates with the

plant's automation system 302 over a communication channel 303 facilitated by l/O adapter 326 or another data processing network interface.

The CPU 304 receives data from a plurality of transducers or sensors sensing the performance of the slurry mixer 10 and other parameters related to the production of slurry. The CPU 304 processes the data according to program instructions contained in a program stored in the memory (the data storage elements of the mass storage device, ROM, RAM and CPU) of the control system and outputs signals to transducers of the slurry mixing system to alter the operation of the slurry mixer 10 as required by the processed data.

Generally, the slurry mixer control system 300 monitors the rate of delivery of cementitious material by the feed screw 28 from the cement hopper 12 to the blend screw 56 through a feed screw speed sensor 207 and the flows of liquids (chemical, cold water, hot water, and reclaimed water), measured by their respective meters 332,334, 336, and 338, into the slurry mixer 10. As operating conditions change, the control system signals the feed screw 206 and blend screw 208 drive motor controllers to alter the speeds of rotation of the feed 36 and blend 64 screw drive motors and signals the diversion valves 340,342, 344,346 and the nozzle control valving 348,350 to alter the flows of the respective liquids (chemical, cold water, hot water, reclaimed water) to the blending screw to produce a slurry having the properties required by the concrete recipe and at a rate established by the user of the slurry mixing system. For example, the load on the blend screw, as sensed by a blend motor load sensor 209, varies substantially during mixing. In response the control system either alters the rate of introduction of cementitious material or liquid to prevent overloading and keep the mixer operating at the best available slurry production rate. It is desired that liquid be introduced to the dry cementitious material at a rate that provides complete hydration of the cementitious material with a minimum of dust.

The slurry mixing system typically operates in series with the equipment of an automated dry batching concrete system of a typical concrete plant. In a typical dry batching concrete system, the cementitious material is measured in an instrumented cement hopper. The dry powder is then dumped into a final

mixing chamber, e. g. a concrete truck, through an automated gate in the bottom of the cement hopper. Sand and aggregate are simultaneously added to the mixing chamber. The majority of the liquid called for by the concrete recipe is typically introduced to the final mixing chamber before the dry cementitious material is added to the chamber. The remainder of the liquid is added after the sand and aggregate are introduced to the mixing chamber.

When the slurry mixer 10 is used to mix a slurry of cementitious material and liquid before introduction to the final mixing chamber 16, the cementitious material is diverted to the slurry mixer 10 from the cementitious material measuring device or cement hopper 12 and the liquid is diverted from the final mixing chamber to the slurry mixer.

Referring to FIG. 6, to initiate slurry production, a concrete plant operator causes a graphical user interface 400 to be displayed on the control system's display 316. Typically, the charging port of the final mixing chamber 16 has a limited capacity to accept the flow of the sand, aggregate, and slurry. By manipulating an input device 318, the operator can specify a "target rate"402 for slurry production that satisfies the limitations of the final mixing chamber's charging port. The slurry mixer control system 300 determines the quantity cementitious material to be used in the batch of concrete, the quantity of liquid to be mixed with the cementitious material, and the rates of consumption of cementitious material and liquid to produce slurry at a production rate limited by the target rate 402, as it may be manually modified during the mixing operation.

Referring to FIG. 7, the CPU 304 of the slurry mixer control system 300 executes instructions included in a slurry mixer control program 500 stored in memory to prepare for and control the mixing process. The slurry mixer control system 300 initializes itself 502 and reads certain data from the plant automation system 504. In parallel 506, the control system 300 may prepare for mixing operations by executing instructions of an automatic nozzle sizing routine 508, a liquid-cement ratio calculation routine 510, and a liquid usage simulation routine to determine the quantities of liquids to be used and divert the liquids to the slurry mixer 512. During mixing the slurry mixer control

system 300 monitors the performance of the slurry mixer and the consumption of cementitious material and liquid and adjusts the operation of the mixer according to instructions in a process control routine 516, a motor load and manual input routine 514, a nozzle control routine 518, and a rinse and end process routine 520 so that the specified quantities of dry and liquid ingredients are consumed simultaneously.

In a dry batching process, the concrete plant's automation system typically introduces liquid to the final mixing chamber 16 by opening one or more liquid valves and monitoring the output of a liquid meter. The liquid meter typically outputs an electrical pulse when a specified volume or mass of liquid flows through the meter. The plant automation system counts the pulses output by the meter and compares the number of pulses in the meter output with a number of pulses equivalent to the quantity of liquid specified by the recipe for the batch of concrete. Several liquids may be used in the production of concrete and the exemplary slurry mixing control system 300 includes liquid valves and meters for, respectively, cold water 340,332 ; hot water 342,334 ; reclaimed water 344,336 ; and chemicals 346,338.

To facilitate the operation of the slurry mixer with an existing concrete plant automation system, the slurry mixer control system 300 interfaces with an existing plant automation system 302 through the communication channel 303 and simulates the operation of a dry batching process for the plant automation system. Referring to FIG. 8, to determine the quantity of liquid to be used in a batch of slurry and to simulate the delivery of the liquid to the final mixing chamber, the slurry mixer control system 300 executes a plurality of instructions 512 simulating liquid usage in a dry batching process for the plant automation system 302. Initially, the control system 300 determines whether the slurry mixing process is operating 602 and whether the gate 202 of the cement hopper 12 is open 604. If the either of these conditions exist, indicating that the mixing operation is in process, the slurry mixer control 300 exits the liquid usage simulation routine 605. If not, the slurry mixer control system reads the status of liquid request signals 606,608, 610 from the plant automation system 302 to each of the cold water 342, hot water 344, and

reclaimed water 346 valves. If the request signal from the plant automation system 302 to one of the valves is not active 612,614, 616, the slurry mixer control 300 repeatedly monitors the request signal line 618,620, 622 until a request is issued or the cement gate opens 624 causing the routine to exit 605.

If a request signal is sent by the plant's automation system 302 to open one of the diversion valves 342,344, 346, the slurry mixer control system 300 intercepts the signal and transmits echoing pulses 632,634, 636 simulating flow through the corresponding meter 332,334, 336,338 back to the plant automation control system. For each instance of an echoing pulse, the slurry mixer control system 300 increments a pulse counter for the specified liquid 626,628, 630. The echoing pulses from the slurry mixer control system 300 simulate an output pulse of the appropriate liquid meter 334,336, 338 indicating a flow of a specified quantity (volume or weight) of liquid through the respective diversion valve. When the plant automation system 302 reads a number of pulses corresponding to the quantity of a liquid that is to be included in the batch of concrete, it will turn off the request signal that would open the appropriate diversion valve. To determine the quantity of each liquid to be added to the slurry for a batch of concrete, the slurry mixer control system 300 reads the count of echoing pulses 626,628, 630 sent to the plant's automation system 302 before the automation system signals that the quantity of liquid is sufficient by failing to acknowledge the echoing pulse 638,640, 642. To introduce liquid to the slurry mixer 10, the slurry mixer control system 300 will transmit a diversion valve signal to the appropriate liquid diversion valve 340,342, 344,346 opening the valve and diverting the liquid that was to be sent to the final mixing chamber 16 to the slurry mixer 10.

Since the plant's automation system 302 has received a signal simulating the introduction of the liquid to the mixing chamber 16, it typically continues with the dry batching routine and signals the appropriate plant equipment to begin loading the sand and aggregate into the final mixing chamber 16.

Referring to FIG. 9, before the mixing process is initiated 702 by the opening of the cement hopper gate 704, the slurry mixer control system 300 determines the appropriate rates for introducing the cementitious material and

the liquids to the slurry mixer 10 by executing program instructions of a liquid- cement ratio calculation routine 510. The slurry mixer control system 300 reads the quantity of cementitious material to be used in the slurry by reading 706 the weight or volume transducer 200 on the cement hopper 12 in which the cementitious material is accumulated prior to mixing. If there is no cement in the cement hopper 12, the slurry mixer control system 300 will continue to monitor the transducer 200. If there is cement in the hopper 708, the control system 300 will display the amount 422 on the user interface 400 and proceed with calculating the initial flow rates. The slurry mixer control system 300 divides the detected quantity of cementitious material by the target rate specified by the operator to determine a delivery time for the cementitious material 710. The meter pulse counts are read from memory for the various liquids 712,714, 716 and the counts are added to determine the total liquid for the batch of slurry 408. A quantity of liquid 404 specified by the operator at the user interface 400 is reserved for rinsing the slurry mixer after the batch of slurry is mixed. The rinse water quantity is subtracted from the total liquid 720 to determine the quantity of process water 406 to added to the cementitious material during the slurry mixing process. If the quantity of process water 722 and a delivery time for the cement 724 have been determined, the slurry mixer control system 300 calculates 726 and displays 728 the cement liquid ratio 410 on the user interface 400. The cement-liquid ratio is a gauge of the consistency and stiffness of the slurry and is used by the control system to determine an order in which the nozzles 76 are turned"ON"to introduce water to the mixer. The control system also divides the process water meter count by the delivery time to determine a liquid flow rate 730 which is displayed 412 on the screen of the user interface 400. If the flow rate of water in the plant water system is likely to pose a limitation on the liquid flow rate, a process liquid flow rate adjustment can be specified by the operator to either extend or shorten the liquid introduction period. If a process water flow rate adjustment is specified, the flow rate is multiplied by the adjustment 734 and the adjusted rate is displayed 736. The slurry mixer control system 300 continues to update the calculations until the cement gate opens 704 initiating the mixing process

and causing the control system to exit 738 the cement-liquid flow rate calculation routine 510.

Referring to FIG. 10A, the program instructions of the process control routine 516 initiate slurry mixing 800 when the control system is signaled that gate 202 on the outlet of the cement hopper 12 is opened. When hopper gate opening is signaled, the slurry mixer control system 300 reads the target rate 804 input by the operator and calculates a feed screw speed to deliver the cementitious material to the final mixing chamber 16 at the target rate 806.

The slurry mixer control system 300 reads the current cement weight or volume 808 from the transducer 200 on the cement hopper 12 and signals the motor controllers 206,208 to start the feed 36 and blend 64 screw motors and operate the feed screw at a rate that will produce slurry at the target rate. If there is no process liquid available, the system exits the mixing process routine to a rinse and end process routine 815.

If process water is available for mixing 814, the mixing process continues and the control system checks the nozzle direction 816. The slurry mixer 10 includes a plurality of nozzles 76 arranged along a length of the blend screw 56 between the first and second inlets 40, 42 and the outlet 22. In the forward mode of operation 820, flow is enabled first at the nozzles 76 (nozzles 1) closest to the inlets 40,42 by signals from the control system 300 to the nozzle 1 control valve 348. As additional liquid flow is required, flow is successively enabled in nozzles 76 progressively nearer the outlet until flow through the last nozzle (nozzle N) is enabled by signaling the nozzle N control valve 350. However, if the liquid-cement ratio is low, the blend screw 56 is susceptible to plugging. The order in which the nozzles 76 are enabled may be reversed 822 to reduce this susceptibility. If automatic nozzle direction selection has been elected by the operator, the control system will determine the selected nozzle direction 816 and automatically reverse the direction in which nozzles are enabled, if the liquid-cement ratio for a batch of slurry is below a set point (SND) 818.

Referring to FIG. 10 B, in the forward nozzle mode 820, the flow rating of nozzle 1 (the first nozzle enabled) is compared to the liquid flow rate 824. If

the flow rating of nozzle 1 is less than the liquid flow rate, the nozzle 1 control valve 348 is signaled to turn nozzle 1 fully"ON"826. The control system then subtracts the nozzle 1 flow rating from the flow rate 828 to determine a remaining flow rate to be supplied by nozzle 2. The process of comparing the nozzle flow rating to the remaining liquid flow rate is repeated. This process is repeated for each successively enabled nozzle in the forward direction until the flow rating of the nth nozzle is compared to the remaining liquid flow 830. If the flow rating of the last nozzle is less than the remaining flow rate 830, the last nozzle (nozzle N) is enabled to turn fully"ON"832. However, if the flow rating for the last nozzle to be enabled (nozzle 1 or any succeeding nozzle) exceeds the remaining liquid flow rate at that nozzle 824,830, a modulation time is calculated by dividing the remaining flow rate at the nozzle by the nozzle flow rating 834,836. The modulation time (T%) equals a portion of a period of time between revisions of the operating regime of the nozzles or nozzle reset period in which flow will be enabled through the last enabled nozzle. If the reverse mode is set 822, the process proceeds in the same manner with the flow rating of nozzle N first compared to the liquid flow rate 838, followed by nozzle N-1 and so forth until the last enabled nozzle (may be nozzle 1) 840.

The control system 300 enables liquid flow with a diversion valve signal 846 and starts 848 the pump 216. Referring to FIG. 10 C, the control system 300 directs the flows of liquids from their sources to introduce the specified quantities liquids to the slurry during mixing. If reclaimed water is to be used, the reclaimed water pulse count will be greater than zero 850 and the reclaim water valve 346 is turned"ON"854 by the control system 300. If the stored pulse count for reclaimed water is zero 850, the reclaimed water valve will be left in an"OFF"or closed condition 852. The control will check the count of pulses stored for the hot water 856 to determine the quantity of hot water be used in the batch of concrete. If hot water is to be used the hot water valve 344 will be turned"ON"860. If no hot water is to be used 858 the valve will remain"OFF"and the control will check the cold water count 862 and enable or disable flow through the cold water valve 342 as appropriate 864, 866. With liquid flow enabled, the control system 300 begins

execution of program instructions in the nozzle control routine 870.

Referring to FIG. 11, the nozzle control routine 518 controls and periodically adjusts the operation of the nozzles 76 introducing liquid to blend screw 56 of the slurry mixer 10. Initially, the control system 300 resets a nozzle reset timer 902 that times the updating of nozzle operation. When it is time to set the operation of the nozzles for the next reset period, the control system 300 checks the enablement of each of the nozzles as established during the process control routine (steps 824-836). If for example, nozzle 1 is to be turned"ON"full time 904, the control system transmits a flow enablement signal to the nozzle control valve 348 to open permitting flow through nozzle 1 908. However, if nozzle 1 is not to be turned"ON"full time because its flow rating would produce flows exceeding the remaining flow rate, the control system 300 checks the status of the modulation time (T%) 906. If the modulation time 906 is greater than zero, the control system sets and starts a modulation timer for nozzle 1 910. As long as the modulation time is greater than zero 912, the nozzle control valve 348 will be turned"ON"908. When the modulation timer reaches zero 912, the nozzle control valve 348 is turned "OFF"914 and flow through nozzle 1 is interrupted for the remainder of the nozzle reset period. This process is repeated for each of the N nozzles 916 because any of the nozzles may be fully"ON"or modulating flow to produce the liquid flow rate required for complete mixing of the slurry at the appropriate target rate and liquid-cement ratio.

If process liquid remains to be mixed 928, the state of the nozzle timer is checked. If the time to reset the nozzle timer has not expired, the control system 300 performs the nozzle control routine 518 again. Otherwise, the control executes 930 instructions of the load control and manual input routine 514 and the process control routine 516.

The process of mixing dry cementitious material and liquid is subject to substantial variability. As a result, the loading of the blend screw drive motor can change significantly during the production of a batch of slurry. The increased load on the blend screw motor caused by plugging of the blend screw is typically sensed by an ammeter measuring the blend screw motor

current or another motor torque sensor. In response to an increase in load, the control system 300 incrementally increases the liquid flow rate by enabling a new combination of liquid control valves or by extending a modulation timer and incrementally slows the feed screw by signaling the feed screw drive motor to reduce speed. On the other hand, as the current in the blend screw motor decreases indicating relief of an overload condition, the slurry mixer control system 300 enables another combination of nozzle control valves or reduces modulation time to decrement the liquid flow rate to achieve the desired cement-liquid ratio and increases the speed of the feed screw drive motor to incrementally increase the slurry flow rate toward the target flow rate.

Referring to FIG. 12, the load control and manual input routine 514 permits the operator to increase 1002 or decrease 1004 the liquid flow rate from the user interface 400. If the operator signals an increase in the liquid flow rate 1002, the control system increments the mixer's flow rate 1006 and if the operator signals to decrease the flow rate 1004, the control system decrements the mixer's flow rate 1008. Likewise, an operator input to increase the rate of slurry production 1010 increments the speed of the feed screw 1014 and a command to decrease the target rate 1012 decrements the feed screw speed 1016.

The control system 300 periodically (following a time delay 1018) reads the blend screw motor current 1020 which is displayed 416 along with the feed screw motor current 418 and feed speed 420 on the user interface 400. The blend screw motor current is compared to an over current limit (lo) 1022. If the blend screw motor current exceeds the over current limit 1022 for a specified period over current period (TDO) 1024, the feed screw speed is decremented 1016 and the liquid flow rate is incremented 1006 to reduce the load on the blend screw motor 64.

On the other hand, if the blend screw motor current does not exceed the over current limit 1022, the current is compared to an under current limit (lu) 1026. If the motor current is less than the under current limit 1026 and remains below that limit for a period (TDU) 1028, the control system will increment the speed of the feed screw 1014 to increase the rate of slurry

production if the feed screw speed would not exceed the speed for the target rate 1030. If the feed speed is incremented 1014 or decremented 1016, the control system calculates a new slurry production rate 1032 and a new liquid flow rate 1034 and returns these new rates to the process control routine 1036,1038 to trigger an adjustment of the enablement or modulation of the nozzles or the feed screw speed. If the flow rate is incremented 1006 or decremented 1008 as a result of manual input or motor loading, the new flow rate is calculated and used by the process control routine 1038 to adjust the operation of the mixer 10. The slurry mixer control 300 compares the flow rate of cementitious material from the cement hopper 12 to the slurry production rate and signals the feed screw motor controller to incrementally adjust the rotational speed of the feed screw toward a speed that will satisfy the target flow rate or reduce the load on the blend screw motor.

While the hopper gate is open, the slurry mixer control system periodically and repeatedly checks the liquid flow rate and the cement weight to determine the actual flow rate and to determine if adjustment is required as a result of the actual flows in the mixer. When the nozzle control reset timer signals expiration of the reset period 930, the control executes program instructions of the process control routine 516 to check the status of pulsing indicating liquid flow 872. If liquid is flowing, the control determines whether the reclaimed water valve 874 or hot water valve 876 is"ON"and decrements the pulse count for the appropriate liquid 878,880, 882. The control reads the current cement weight or volume 886 from the hopper transducer 200 and determines whether the hopper 12 is empty, that is remaining weight or volume is less than or equal to a minimum 886.

If cementitious material remains in the hopper 886, the control determines whether the quantity of cementitious material is less than a dust reduction limit 890. If not, the control system 300 determines the liquid cement ratio exceeds a set point 818 and operation continues in the appropriate nozzle mode 820,822. The flow from some cement hoppers decreases substantially as the quantity of cementitious material in the hopper decreases. This extends the time required to empty the hopper and necessitates a reduction in liquid

flow rate to avoid consuming the total quantity of process liquid before the hopper is completely emptied. If the total quantity of liquid is consumed before the supply of cementitious material is exhausted, substantial quantities of dust can be produced as the dry material is added to the final mixing chamber. If the quantity of cementitious material remaining in the hopper is less than the dust reduction limit 890, the control reduces the flow rate 892 to extend the supply of liquid and reduce the likelihood of dust production.

Referring to FIG. 10A, when the control system 300 determines that the cementitious material in the hopper 12 has been depleted 866, the control system shifts to the rinse mode and directs that the nozzles be turned fully "ON"894. The control system 300 shifts 870 to the program instructions of the nozzle control routine 518 and turns the nozzles fully on to begin the rinse process. The control system 300 detects that the rinse mode has been selected 922 and decrements the pulse count representing the water reserved for rinsing 923. The control system then begins executing instructions in the rinse and end control routine 932 to prepare the mixer 10 for shut down.

During rinsing, the feed and blend screw drive motors continue to operate and all nozzles are enabled. As the supply of rinse liquid is depleted, the numbers of enabled nozzles are reduced to increase pressure in the remaining nozzles to clean the least frequently used nozzles. The feed and blend screw drive motors continue to run for a set period and then are stopped by the slurry mixer control, completing the slurry mixing operation.

Referring to FIG. 13, the steps directed by the program instructions of the rinse and end process routine 520 include reading the weight of the cement in the hopper 1102 and determining whether the cement in the hopper has been depleted 1104. If cementitious material remains in the hopper 12, the control system 300 repeats reading the transducer 200. If the hopper 12 is empty, the control system 300 begins rinsing the mixer 1106. The pulse count representing the quantity of rinse water is read 1108. If rinse water is available 1110 and the quantity exceeds a first limit 1112, all nozzles remain fully"ON."If the quantity of rinse water available is less than the first limit 1112 but greater than a second limit 1114, the control system turns"OFF"

nozzle 1 1118. The control system proceeds in this fashion, comparing the remaining quantity of rinse water to second 1112, third 1114 and additional limits and progressively turning off valves 1118 until all of the rinse water is depleted 1116.

When no cementitious material remains in the hopper 1104 and the rinse water supply is depleted 1110, the mixer continues to operate for a period 1120 to make sure that the cementitious material is expelled from the mixer 10 and a shut down period 1122. Following the delays, the control system 300 stops the feed screw 1124, stops the blend screw 1126 and closes all liquid valves 1128 to end the slurry mixing process 1130.

The detailed description, above, sets forth numerous specific details to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid obscuring the present invention.

All the references cited herein are incorporated by reference.

The terms and expressions that have been employed in the foregoing specification are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow.