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Experimental determination of dynamic parameters of an industrial robot

In an industry increasingly used are industrial robots. Commonly used are two basic methods of programming, on-line programming and off-line programming. In both cases, the programming consists in getting to the selected points record this position, and set the order of movement of the robot, and the introduction of logical tests. Such a program is easy to write, and it is suitable for most industrial applications. Especially when the process is known, respectively slow and unchanging. In this case, the program is being prepared for a universal model of the robot with the appropriate geometry and are checked only collisions. Is not taken into account the dynamics of the robot and how it will really behave while in motion. For this reason, the robot programmed to be tested at a reduced speed, which is raised gradually to the final value. Depending on the complexity of the move and the proximity of the elements it takes a lot of time. It is easy to notice that the robot at different speeds have different trajectories and behaves differently.


Introduction
It is easy to determine in theory, geometric and mass parameters of robot arm separately but the determination of the parameters for the whole of robot is not so easy and requires a series of experiments. When determining mass parameters it is necessary to know the material from which it was made [16,17] These experiments were carried out on the robot Fanuc Arc Mate using a very fast real-time computer -PXI [3,4]. The robot is a typical welding robot but equipped with appropriate sensors to enable testing. Sensors measure the movement parameters and electrical parameters of work of individual drives. The results are sent to the real-time computer and stored there. After the recording is analyzed of movement parameters of each arm and drives the robot. When this is done in a variety of configurations robot is possible to determine the dynamic parameters of individual drives and robot arm. The vision system could support this process [13,14].
It is very difficult to carry out such experiments. Firstly, when the robot does not move automatically, the brakes are activated, which makes it impossible to perform the experiments. It is very difficult to carry out such experiments. Firstly, when the robot does not move automatically, the brakes are activated, which makes it impossible to perform the experiments. Secondly, the robot moves all axes in motion while it controls its speed and changes all the time [8]. Thirdly, each servo drive has speed ramps to make the movements smooth, but this makes it difficult to analyze. The best for analysis would be with impulse forcing. Such an impulse is during an emergency stop, but is triggered by the brakes.

Measurements of dynamic parameters of a robot
After many tests, method of forcing similar to the impulse was found. Many measurements were taken, because the difficulty of the forcing method, not all results were satisfactory.
Of the correct tests, one was selected for further analysis. Figures 1-6 shows the results of measurements on all joints of a robot.  It is easy to note in figure 1 that the angular displacement has a plot similar to the damped vibrations but the asymptote is not centered and the lower part of the amplitudes is cuted. Voltage graphs show peaks that protrude over the voltages over time. The graph of current changes is more interesting because of the nature of the waveform.   In figures 2, 4, 5 and 6 the angular displacements have nonlinearities that are probably caused by joints clearance. On these charts it is hard to notice the change in voltage. During experiments the biggest problems were joints 4 and 6 because of their construction.
After analysing graphs figures 1-6, the angles and currents were taken into the analysis.

Analysis of waveforms
The research was done in Labview the same way in which data acquisition was done. Because the analysis was made from the data stored in the file, it was not necessary to use a real-time computer and could use an ordinary PC, but the program could also be apply on a PXI computer [14]. Sampling time is 0.25 ms that is 4000 samples per second. During this time, the PXI computer makes measurements and writes them to the file. This causes high frequencies to dominate the system, so data preparation was necessary [6,7]. The part of the program where pre-processing is performed followed by the model analysis is shown in figure 7.    It is very difficult to obtain the same characteristics as obtained from the real system. These are just approximations and the choice between the accuracy of the mapping and the degree of the equation of the transition function. In CAD modelling and simulation, machine mechanisms are more important than lower frequencies [10,11].       The obtained Bode characteristics and transfer function in the same position in different experiments were similar, so the results obtained from one of the experiments. This enables better CAD [12,13] robot modelling. The model, which better describes the operation of the production machine, supports modelling the whole system [5,15].

Conclusions
The obtained characteristics are for a particular robot position, but even their designation is a complex process. When determining the dynamic parameters of a whole robot it is difficult to isolate the influence of each element of the robot. Research each item separately facilitate disassembly robot arm robot. Another option is to unlock only one brake and carry out the test successively for all drives. Another option is to unlock only one brake and carry out the test successively for all drives. Both methods require significant interference in the construction and control of the robot. The research will be developed and their results published.