Chapter: Automation, Production Systems, and Computer Integrated Manufacturing - Automotion and Control Technologies

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Advanced Automation Functions

Advanced Automation Functions: Safety Monitoring, Maintenance Rapair Diagnostics, Error Detection and Recovery.

       ADVANCED  AUTOMATION   FUNCTIONS

 

In addition to executing work cycle programs, an automated system may be capable of executing advanced functions that are nOI specific to a particular work unit. In general, the functions are concerned with enhancing the performance and safety of the equipment. Advanced automation functions include the following: (1) safety monitoring, (2) maintenance and repair diagnostics, and (3) error detection and recovery.

Advanced automation function> are made possible by special subroutines included in the program of tnsttuctluns. In some cases, the runcnons provide information only and

do not involve any physical actions by the control system, An example of this case includes reporting a list of preventive maintenance tasks that should be accomplished. Any actions taken on the basis of this report are decided by the human operators and managers of the system and not by the system itself 11\ other cases, the program of instructions must be physically executed by means of the control system using available actuators. A simple example of this case is a safety monitoring system that sounds an alarm when a human worker gets dangerously close to the automated system.

 

          Safety Monitoring

 

One ot the significant reasons for automating a manufacturing operation is to remove workcr{s) from a hazardous working environment. An automated system is often installed to perform a potentially dangerous operation that would otherwise be accomplished manually by human workers '.However, even in automated systems. wo~kers arestill needed to service the system. at periodic time intervals If not fulltime. Accordingly. it is important that the automated system be designed to operate safely when workers arc in attendance. In addition.rt is essential that the automated system carry QUills proCI;:SS in a way that is not selfdestructive. Thus. there are two reasons for providing an automated system with a safety monitoring capability: (1) to protect human workers in the vicinity of the system ami (2) to protect the equipment associated with the system.

 

Safety monitoring means more than the conve.ntionalsafety measures til ken in a ill: a ufacturing operation, such as protective shields around the operation or the kinds of manual devices that might be utilized by human workers. such as emergency stop buttons. Safety moniroringin an automated system involves the use or sensors to track the system's operation and identify conditions and events that are unsafe or potentially unsafe. The safety monitoring system is programmed to respond to unsafe conditions in some appropriate way. Possible responses to various hazards might include one or more of the following:

 

   complete  stoppage  of the automated   system

 

   sounding  an alarm

 

   reducing  the operating  speed  of the process

 

   taking corrective  actions  to recover  from the safety  violation

 

This last response is the most sophisticated and is suggestive of an intelligent machine performing some advanced strategy, This kind of response is applicable to a variety of possible mishaps, not necessarily confined to safety issues, and is called error detection and recovery (Section 3.2.3).

 

Sensors for safety monitoring range from very simple devices to highly sophisticated systems. The topic of sensor technology is discussed in Chapter 5 (Section 5.1). The following list suggests some of the possible sensors and their applications for safety monitoring:

 

   Limit switches to detect proper positioning of a part in a workholding device so that the processing cycle can begin.

    Photoelectric sensors triggered by the interruption of a light beam; this could be used to indicate that a part is in the proper position or to detect the presence of a human intruder into the work cell.

   Temperature sensors to indicate that a metal workpart is hot enough to proceed with a hot forging operation. If the workpart is not sufficiently heated, then the metal's duetutty may be too low, and the forging dies might be damaged during the operation.

   Heat  or smoke  detectors  to sense fire hazards.

 

   Pressuresensitive   floor pads to detect  human  intruders  into the work celL

 

   Machine  vision systems  to supervise  the automated  system  and its surroundings.

 

It should be mentioned that a given safety monitoring system is limited in its ability to respond to hazardous conditions by the possible irregularities that have been foreseen by the system designer. If the designer h as not anticipated a particular hazard, and consequently has not provided the system with the sensing capability to detect that hazard, then the safety monitoring system cannot recognize the event if and when it occurs.

 

       Maintenance  and Repair Diagnostics

 

Modem automated production systems are becoming increasingly complex and sophisticated, thus complicating the problem of maintaining and repairing them. Maintenance and repair diagnostics refers to the capabilities of an automated system to assist in the identi fication of the source of potential or actual malfunctions and failures of the system. Three. modes of operation are typical of a modern maintenance and repair diagnostics subsystem

 

    SWills monitoring, In the statu, monitoring mode, the diagnostic subsystem moniton and records the status of key sensors and parameters of the system during norma. operation. On request, the diagnostics subsystem can display any of these values and provide an interpretation of current system status, perhaps warning of an imminert failure

 

    Failure diagnostics. The failure diagnostics mode is invoked when a malfunction or failure oCCUTS. Its purpose is to interpret the current values of the monitored van. ables and to analyze the recorded values preceding the failure so that the cause of the failure can be identified

 

3 Recommendation of repair proceaure.uv the third mode of operation. the subsystem provides a recommended procedure to the repair crew as to the steps that should be taken to effect repairs. Methods for developing the recommendations are sometimes based on the use of expert systems in which the collective judgments of many repair experts arc pooled and incorporated into a computer program that uses artificial intelligence techniques.

 

Status monitoring serves two important functions if"!machine diagnostics: (I) providing information for diagnosing a current failure and (2) providing data to predict a future malfunction or failure. first, when a failure of the equipment has occurred, it is usually difficult for the repair crew to determine the reason for the failure and what steps should be taken 10 make repairs. It is often helpful to reconstruct the events leading up to the failure. The computer is programmed to monitor and record the variables and to draw logical inferences from their values about the reason for the malfunction, This diagnosis helps the repair personnel make the necessary repairs and replace the appropriate components.

 

This is especially helpful in electronic repairs where it is often difficult to determine on the basis of visual inspection which components have failed

The second function of status monitoring is to identify signs of an impending failure, so that the affected components can be replaced before failure actually causes the system to go down. These part replacements can be made during the night shift or other time when the process is not operating. with the result that the system experiences [10loss of regularoperation

 

          Error Detection  and Reovery

 

In the operation of any automated system, there are hardware malfunctions and unexpected events that occur during operation. These events can result in costly delays and loss of production until the problem has been corrected and regular operation is restored. Traditionally. equipment malfunctions are corrected by human workers, perhaps with the aid of a maintenance and repair diagnostics suhroutine. With the increased use of computer control for manufacturing processes, there is a trend toward using the control computer not only to diagnose the malfunctions but also to automatically take the necessary corrective action to restore the system to normal operation. The term error detection and recovery is used when the computer performs these functions,

Error Detection. A, indicated by the term. error detection lind recovery consists of two steps: (1) error detection and (2) error recovery. The error defection step uses the automated svstem's available sensor systems to determine when a deviation or malfunction has occurred, correctly interpret the sensor signal(s), and classifythe error. Design of the error detection subsystem must begin with 11 classification of the possible errors that can occur during system operation. The errors in a manufacturing process tend to be vcry application specific. They must be anticipated in advance in order to select sensors that will enable their detection

 

In analyzing a given production operation, the possible errors can be classified into one of three general categories: (1) random errors, 1'2)systematic errors. and (3) aberrations, Random errors occur as a result of the normal stochastic nature of the process. These errors occur when the process is in statistical control (Section 21.1). Large variations in part dimensions, even when the production process is in statistical control, can cause problems in downstream operations. By detecting these deviations on a partbypart basis, corrective action can be taken in subsequent operations, Systematic errors are those that result from some assignable cause such as a change in raw material properties or a drift in an equipment setting. These errors usually cause the product to deviate from specifications so as to be unacceptable in quality terms. Finally. the third type of error. aberrations, results from either an equipment failure or a human mistake. Examples of equipment failures include fracture of a mechanical shear pin, bursts in a hydraulic line, rupture of a pressure vessel, and sudden failure of a cutting tool. Examples of human mistakes include errors in the control program, improper fixture setups, and substitution of the wrong raw materials,

 

The two main design problems in error detection are: (1) to anticipate all of the possible errors that can occur in a given process and (2) to specify the appropriate sensor systems and associated interpretive software so that the system is capable of recognizing each error. Solving the first problem requires a systematic evaluation of the possibilities under each of the three error classifications. If the error has not been anticipated, then the error detection subsystem cannot correctly detect and identify it.

 

EXAMPLE        3.2    Error Detection     in an Automated Machining  Cell

 

Consider an automated cell consisting of a CNC machine tool, a parts storage unit, and a robot for loading and unloading the parts between the machine and the storage unit. Possible errors that might affect this system can be divided into the following categories: (1) machine and process, (2) cutting tools, (3) workholding fixture, (4) parI storage unit, and (5) load/unload robot. Develop a list of possible errors (deviations and malfunctions) that might be included in each ofthcse five categories.

 

Solution: A list of possible  errors    in the machining   cell is presented    in Table 3.3.

 

Error Recovery. Error reCIJVPfY is concerned with applying the necessary corrective action to overcome the error and bring the system back to normal operation. The problem of designing an error recovery system focuses on devising appropriate strategies and procedures that will either correct or compensate for the variety of errors that can occur in the process. Generally, a specific recovery strategy and procedure must be designed for each different error, The types of strategies can he classified as follows:

 

l , Make adjustments at the end of the current work cycle. When the current work cycle is completed. the part program branches to a corrective action subroutine specifically


designed for the error detected, executes the subroutine, and then returns to the work cycle program. this action reflects a low level of urgency and is most commonly associated with random errors in the process.

 

Make udjusnnents during IIII' curreW cycle. I his general1y indicates a higher level of urgency than the preceding lype. In this case, the action to correct or compensate for the detected error is initiated a~ soon as the error is detected. However, it must be possible to accomplish the designated corrective action while the work cycle is still being executed

 

Stop the prnc<'ss {O invoke corrective action. In this case, the deviation or malfunction requires that the execution of the work cycle be suspended during corrective action. It i~assumed that the system is capable of automatically recovering from the error without human assistance. Atthe end of the corrective action, the regular work cycle Is continued.

 

Stop (III:process and call for help. In this case, the error requiring stoppage of the process cannot he resolved through automated recovery procedures. This situation arises because: (I) the automated cell is not enabled to correct the problem or (2) the error cannot be classified into the predefined list of errors. In either case, human assistance is required to correct the problem and restore the system to fully automated operation.

 

Error detection and recovery requires an interrupt system (Section 4.3.2). When an error in the process is sensed and identified, an interrupt in the current program execution is invoked to branch to the appropriate recovery subroutine, This is done either at the end of the current cycle (type 1 above) or immediately (types 2, 3,and 4).At the completion of the recovery procedure, program execution reverts back to normal operation.

 

EXAMPLE        3.3 Error        Recovery    in 811Autom8ted Machining   CeU

 

lor the automated cell uf Example 3.2. develop a list of possible corrective actions that might be taken by the system to address certain of the errors.

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