adaptive control

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M.S.RAMAIAH INSTITUTE OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING BANGALORE COMPUTER CONTROL OF MANUFACTURING SYSTEMS SEMINAR ON: Microcomputers in Adaptive Control System   Shahin Shafiee USN : 1MS08MCM12 2nd Sem , M.Tech , CIM 2009

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ADAPTIVE CONTROL SYSTEMS Is a logical extension of the numerical control and computer control Improvement of the cutting process by automatic online determination of the feed and/or cutting speed Basically a feedback system in which the cutting speed and feed automatically adapt themselves to the actual conditions of the process The cutting speed and feed have to be varied in such a way as to improve the performance level Can be classified into two types: Adaptive Control with Optimization (ACO) Adaptive Control with Constraints (ACC) The use of AC systems is mostly justified when extremely hard materials have to be machined Basic block diagram of adaptive control system

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ADAPTIVE CONTROL SYSTEMS ACO SYSTEMS Most of them are of the laboratory type used for Research and Development (R&D) Very costly Major problems with such systems have been: difficulties in developing comprehensive indexes or performance Sensor to measure the necessary parameters A control system that optimizes performance index subject to various constraints Basically a sophisticated closed-loop control system The input of the adaptive controller is the Data Reduction Subsystem (DRS) The DRS produces two signals: Tool Wear Rate (TWR) Metal Removal Rate (MRR) Where Θ is the tool temperature and dT/dt is the torque rate of change. K1,K2 and K3 are constants prescribed by the tool and workpiece; b and a are the width and depth of cut, respectively and s is in ipm

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ADAPTIVE CONTROL SYSTEMS ACO SYSTEMS The TWR and MRR signals are fed into a performance computer unit which calculates the performance index, or figure of merit φ: Where C1 = Cost of machine and operator per unit time C2 = Cost of tool and regrind per change = Tool changing time W0 = Terminal allowable width of flank wear β = Constant which establish the performance criterion If β = 1 the criterion is cost per unit If β = 0 the criterion is production rate i.e. 0 ≤ β ≤ 1 φ is fed into an optimization computer unit (OCU) OCU contains the strategy according to which the optimization is made Objective of OCU is to continually maintain the value of Φ at the highest possible value

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ADAPTIVE CONTROL SYSTEMS ACO SYSTEMS Adaptive controller

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ADAPTIVE CONTROL SYSTEMS ACC SYSTEMS Simpler and lower cost Retrofitted with relative ease to existing machines The most useful constraints in practical ACC systems are: The cutting force F The cutting power P The torque T The principle of ACC system is to sense the constraint variables, F, T or P and to adjust the cutting feeds and speed The adaptive control system

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ADAPTIVE CONTROL SYSTEMS ACC SYSTEMS Figure illustrates the general block diagram of an ACC system which uses eight constraints: Maximum permissible spindle speed, in rpm Minimum spindle speed Maximum allowed torque Maximum allowed chip load, in in/rev Minimum chip load Maximum permitted feedrate, in ipm Maximum allowed vibration Impact chip load, in in/rev The adaptive controller is fed by signals of two sensors: 1. Spindle torque sensor 2. Tool vibration sensor Mechanical values for a lathe operation

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COMPUTER EFFECTS ON NC Duplicates the traditional capabilities of hardwired NC The CNC reads four or more blocks of data with one command Facility to store the entire part program Capability to allow editing of tapes or stored programs Offset storage to compensate for tool length and cutter diameter: Traditionally, this compensation has been implemented by thumbwheel switches, limited by: The cost of the switches The physical space required to mount them Overcomed by storing offsets in the computer's memory, can be easily inserted and displayed on the system's CRT The actual computations for tool length offset and cutter compensation are performed in software Axis calibration in software vibration sensor

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THE MICROPROCESSOR MICROPROCESSOR The biggest new force for change of CNC is the arrival of the microprocessor A complete microcomputer can be formed on a single printed circuit board at very low cost Three dominant characteristic of the microprocessor : Increased flexibility The low cost The reliability that comes from using a small number of Ics The potential applications of microprocessors include: Industrial process controllers Small-business machines Terminal controller Machine-tool controller A microprocessor is the control and processing unit of a small computer Can handle both arithmetic and logic data under control of a program Usually works with two types of memories: Read Only Memory (ROM) in which the control program is stored Random Access Memory (RAM) which is a read-write memory used for volatile data storage The principle of ACC system is to sense the constraint variables, F, T or P and to adjust the cutting feeds and speed

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THE MICROPROCESSOR MICROPROCESSOR The cost savings areas are: Component costs Connectors can be decreased in number Cabling can be simplified Cooling requirements are decreased THE MICROPROCESSOR- A FLEXIBLE BUILDING BLOCK APPROACH TO RETROFITABLE AC Offers a dimension to control flexibility and retrofittability unobtained with hard-wired systems The only hardware change required in the unit would be for any difference in the NC control interfacing encountered MICROPROCESSORS IN NC EQUIPMENT Removing NC disadvantages: expensive and complex The principle of ACC system is to sense the constraint variables, F, T or P and to adjust the cutting feeds and speed

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SELECTION CRITERIA Many factors to be considered: Capability and risk considerations Hardware design Architecture features CREDIBILITY AND RISK Factors that influence credibility: Process maturity and the chip size If the process is new and not in production by other major semi-conductor manufacturers, it is a high risk consideration HARDWARE DESIGN Design factors can be separated from the others in terms of speed, power, power supplies The most critical question to be asked is, "Will the machine be fast enough to perform the operations that are needed?" This depends on the way the system is structured

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SELECTION CRITERIA ARCHITECTURE The effect of the microprocessor architecture is a function of the system design The efficiency with which the processor runs the program in term of lines of codes, execution time and the amount of memory can make the choice of a microprocessor a quantitative decision. Centralized or decentralized functions Once the system is decentralized, portions of the system can be cost reduced and modified without a complete redesign, thus reducing the cost of a new design Another advantage of the decentralized system: expandability The decentralized approach allows expansion by increased module usage such as additional axis control The principle of ACC system is to sense the constraint variables, F, T or P and to adjust the cutting feeds and speed

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CASE STUDY A general purpose microprocessor has been developed for adaptive control (AC) of NC drills and machining centers. While sensing spindle horsepower and torque The system modulates the feed rate to obtain the highest penetration rate possible under existing drilling conditions Spindle RPM, maximum feed rate, and the drill's torque limit are obtained from a machinability computer program Where APT is used, AC instructions are automatically inserted onto the tape. They also can be edited at the machine Drilling tests comparing AC and conventional feed rate control show time savings of 35% in steel and 28% in cast iron for the same drill life The principle of ACC system is to sense the constraint variables, F, T or P and to adjust the cutting feeds and speed

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CASE STUDY INTRODUCTION In drilling process, torque requirements can vary widely from hole entry to hole exit Jammed flutes, excessive wear, hard spots or unusual entry/exit conditions can produce torques sufficient to break the drill Torque controlled drilling ensures optimum feed rate and torque performance under all circumstances For a torque controlled drilling system key requirements are: Supply only information describing type of operation ( i.e. drilling or milling ), cutter geometry, workpiece material type and hardness, and the hole entry conditions A tape-edit feature that permits modification of adaptive control tape instructions at the machine without changing the integrity of the NC tape itself The MAC40 torque controlled drilling system, using the Motorola 6800 processor, was built to meet all these requirements

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CASE STUDY GENERAL DESCRIPTION OF THE MAC40 The MAC40 torque system electronically senses horsepower and torque exerted on a drill A precision horsepower measuring sensor is used to sense the variation in horsepower and torque at the spindle A block diagram of the MAC40 control strategy Spindle RPM, maximum feed rate and the drill's torque limit are computed by a machinability computer program The programmer needs only to describe the workpiece material type and strength, drill material and size, type of coolant flow

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CASE STUDY GENERAL DESCRIPTION OF THE MAC40

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CASE STUDY RESULTS OF COMPARATIVE TESTS BETWEEN CONVENTIONAL AND TORQUE-CONTROLLED DRILLING Hole drilling times in mild steel and cast iron Note the greater reduction in drilling time as the drill gets larger Extensive drilling tests were conducted on an NC drill to compare drill life as well as hole drilling times to get a measure of the productivity improvement of MAC40 under equal drill life conditions for conventional and torque controlled drilling

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CASE STUDY RESULTS OF COMPARATIVE TESTS BETWEEN CONVENTIONAL AND TORQUE-CONTROLLED DRILLING Extensive drilling tests were conducted on an NC drill to compare drill life as well as hole drilling times to get a measure of the productivity improvement of MAC40 under equal drill life conditions for conventional and torque controlled drilling

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CASE STUDY RESULTS OF COMPARATIVE TESTS BETWEEN CONVENTIONAL AND TORQUE-CONTROLLED DRILLING Note the lower spindle RPM's prescribed by the MAC40 preprocessor

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Thank You