Fuzzy control of robotic manipulator and mechanical systems.

The robotic Institute of America gives the following definition of robot: "A robot is a reprogrammable multifunctional manipulator designed to move material, parts, tools or specialized devices through variable programmed motions for the performance of a variety of tasks". Based on this definition it is apparent that a robot must be able to operate automatically. This means that in most of the robots it is possible to distinguish the following major subsystems: a manipulator (mechanical unit which can be compared to the skeleton of living beings), a controller (the brain), appropriate power supplies, and very often a computer system which takes care of the monitoring and control functions relative to the robot operation and which allows exchange of data between the robot and human operators and/or other parts of the manufacturing process in which the robot is performing some specified tasks. The motions of the manipulator must be controlled and the control system obey the same basic principles as for control of motions of any mechanical system from simple servomechanisms up to complex machines or vehicles. It means that positions and velocities or displacements of the various parts of the mechanical system must be transmitted to the control system. Then this system is able to determine the driving forces and/or torques (момент вращения) which must be applied to the mechanical system in order to force the actual positions and displacements to track the desired ones.

Fuzzy control is a natural extension of multilevel discontinuous control. The advantages of simplicity and reliability of discontinuous control are conserved by fuzzy control, but not its main drawback which is continuous cycling between different types of operation. Therefore, fuzzy control can be viewed as an intermediate class between discontinuous and linear control systems, resulting in an acceptable compromise between advantages and drawbacks of both. Positions and velocities or displacements are the usual state variables defining the state of a mechanical system. Control laws based on measurements of those variables allow some changes in the dynamics of the system, in particular the stabilization of unstable or neutrally stable systems. These laws give them the capability of reproducing desired motions with an accuracy which depends on the gain of the control system. However, in such control systems static errors due to steady-state loading forces cannot be avoided. The only way to cope with such disturbances and reject their effect on the system is the introduction of a reset action in the control system. This can be achieved through parallel controllers using control laws based on triples instead of pairs of data. However, it may result in some deterioration of the dynamic performances and lead to more difficulties in the design of fuzzy controllers. Self-organizing controllers are a possible solution to the latter problem.

Nevertheless, there is another way since an indirect reset action can be introduced via model-based control schemes. These allow a neat separation of the two basic tasks of the control system: following the desired trajectory (tracking) on one hand and reducing the effect of disturbances (regulation or disturbance rejection) on the other hand. Such control schemes consist of two control loops, one of them including a basic position + velocity controller and the other one a model of the system. The basic controller and/or the model can be implemented as numerical or fuzzy systems,

Position or displacement control is not the only type of control which may be required in the performance of robotic tasks. In some of them one has to control contact forces between the robot-end-effector and the robot environment. Often the complete control system of a robotic manipulator is hybrid: displacements are controlled directly along some directions in the robot workspace and forces along others. Then force control is generally implemented as an external control loop around usual displacement control loops. Here again use of fuzzy logic is possible.

Safety systems

Safety systems, sometimes also called monitoring systems in industry, are on-line diagnosing systems used to prevent break-downs or to minimize the damage caused by catastrophic tool failure. These systems usually monitor the process, and the control action that is taken is a simple on/off signal. A safety system may be considered as a "safety net" for the machine tool, the tool or the workpiece. A safety system is a system that monitors the process and stops execution at dangerous level.

Of all the supervision systems used in industry today, safety systems are the most common, especially in turning (токарные работы) and boring (сверлильные работы). The existing systems all belong to the emergency type of safety systems.

The first systems appeared on the market in the mid-1970s. Since then, the systems have developed to such an extent that one could even speak of different generations.

Safety systems of the first generation usually consist of a monitoring device made up of a transducer, an amplifier and electronic devices that analyse the measured signal. They also work with teaching techniques. It means that information about the measured process quantity is recorded and memorized together with NC information. This in turn means that the system records the process parameter for each NC block used for machining a component.

After the information related to all the machining involved in making a completed component is stored, the actual monitoring phase can take place for the next workpieces. As soon as the instantaneous measured process parameter exceeds the unit calculated on the recorded value, the process is stopped and an alarm signal is activated. Since any process is a more-or-less stochastic, the measured values will always differ from the recorded values. In order to handle this, an appropriate tolerance band has to be defined, i.e. the process is stopped only when the measured value is outside this tolerance of, for instance, +/-20%.

One big disadvantage of this kind of system is that the operator must calibrate it by using the first workpiece as a calibrating device. The time and memory used for this can be enormous especially for more complex workpieces using long PC programs. Furthermore, in modern small-batch production, the number of workpieces can be so small that even just one workpiece can be a considerable percentage of the whole batch.

In the second generation of safety systems signal processing and signal evaluation of the system have become more advanced. Also, the use of more advanced transducers is typical. Most systems now used in industrial applications belong to this generation. These systems may still have the teaching strategy, meaning that at least one initial workpiece has to be used as a calibrating instrument.

The third generation of safety systems has eliminated the necessity for the teaching mode. This means that such systems can work adequately from the first small-batch production. These systems also have a higher level of intelligence that is they can distinguish between different operational situations. At the same time, they all retain the safety features of the earlier generations of systems. Yet, another advance is their computational ability, which makes it possible to store historic information that can help the operator.

The safety system usually has two different operational modes. In the first, maximum cutting forces are memorized for each NC-block during the machining of the first component. From these values, minimum and maximum cutting-force limits are created for each NC-block. Three different limits are established: tool wear limit, tool breakage limit and minimum limit. The minimum limit is used for the checking of a missing workpiece or tool, or completely broken tools. When components are machined, actual cutting forces are monitored and checked to ensure that they are within the established limits. Another global maximum force limit for the machine tool is determined by the operator according to the size of the machine tool and motor power available.

The tool-wear function assumes that the cutting force will increase with tool wear. Since this is not always the case, one should be aware of the misinterpretation that may occur. The force limit is decided, for instance, as 130% of the recorded force value. As soon as the tool-wear force limit is exceeded after a steady increase of the force, a signal from the system will tell the NC system that a new tool is needed. Of a new tool should be available, a change can be activated. Otherwise, an alarm signal will alert the operator to a malfunction.

The breakage force limit is decided, for instance as 150% of the recorded force value. When the breakage force limit is exceeded for a certain pre-determined time after a fast increase of the force, the spindle (вал) and the feed are immediately stopped and an alarm will alert the operator to investigate the machine.

The minimum force limit with the typical value will usually not stop the machine tool immediately, but only after the on-going (продолжающий работать) NC block has come to an end.

One common problem with these systems is that machine tool friction will vary with time. The system therefore must be calibrated during idle running (холостой ход) of the NC program. The stored values must then be deducted from the measured values in order to obtain the true machining values. While this may be a nuisance (вред), the recorded values will also yield information about the general machine tool condition.

The main advantage of this type of system is that integration with the NC system is very easy and, especially when current-measuring systems are used, the installation can easily be carried out. The disadvantage, as many industrial installations will show, is the frequent number of false alarms, often forcing the operator to disconnect the system.

Because of the nature of the measured values, safety systems of this type are usually only used for medium and heavy cuts. Cutting forces for finishing operations will usually be too close to the friction forces of the system.

Perhaps the most advanced safety system on the market is the US Montronix system, originally developed and marketed by Kennametal. This system can be considered as belonging to the third generation of safety systems, mainly because no learning process is necessary. This means that the first component can be monitored and the system is thus also suitable even for batch production down to a single component.

The transducer installation is somewhat similar to that of the Prometec transducer installation, and three-component piezo-electric transducers are used.

The system is capable of discriminating (распознавание) between tool wear, tool breakage, collision and missing tool. Tool wear is sensed by monitoring the relatively changes between the three cutting-force components. A new tool signifies a relative wear index of 100%, and a worn out tool signifies an index of 0%. The relative change in tool wear can be pre-set (установить заранее) by the operator, that means that various wear magnitudes of the tool can be used.

Tool breakage is sensed by using advanced pattern recognition of changes in the cutting force. This is done by simultaneously comparing the cutting force to stored cutting force patterns. Several different patterns are stored in the system, each signifying a different tool breakage event, or typical patterns for different tool materials or work materials. Standard installation of 16 different patterns are made, but customer (заказчик) design of new patterns is possible. As soon as a pattern is recognized, an alarm indicating tool breakage is activated.

The collision detection will activate the alarm when the cutting force exceeds a pre-set value. In order not to generate false alarms arising from sudden high and very short cutting-force levels, a collision time delay may be pre-set. As soon as the force level has exceeded the force limit for a time period longer than this collision delay, an alarm signal will be sent to the emergency shutdown of the CNC system. The response times of the monitoring system are fast enough to allow for this check without any serious damage.

Cutting tests performed at KTH, Sweden, have shown that the Montronix system is able to detect tool wear and tool breakage with extremely good reliability.

 

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