Path planning for five-part robots

Industrial robots are often used today for repetitive production tasks such as assembly, welding and painting work. With their 6 degrees of freedom, these devices offer complete mobility in space. For simpler sorting or placement tasks, however, mobility in the plane is completely sufficient. In addition to gantry systems or X-Y tables, a five-unit crank mechanism (see kinematic diagram, Fig. 1) with two controlled drives can also perform such positioning tasks in the plane. This arrangement has the advantage that the drive motors can be permanently mounted and are not moved with the machine. This makes this structure suitable for highly dynamic positioning tasks.

Figure 1: kinematic diagram of a five-unit crank mechanism

Analysis of mechanism characteristics

Figure 2: Range of movement of a five-unit crank mechanism

The output point K, which is connected to the coupling element 2 (where a gripper or a cutting tool can be attached as a technical application, for example), can be positioned in a certain working area (see Fig. 2) depending on the position of the drive cranks 1 and 4. It should be noted that for one crank position, the two-stroke made up of links 2 and 3 can be mounted in two positions. The second possibility is shown in grey in picture 2.

When using such a crank gear, singular positions (e. g. top or stretch positions of the crank and coupling) must be avoided, as only limited mobility is present in these positions. In addition, the characteristics of the gearbox within the working area are very different. For example, there are areas where the gear unit can exert a particularly high force at point K at a specified maximum torque on the cranks in a certain direction. The same applies to the achievable speed. These properties can be investigated using the program FB/analysis developed at the Institute (FB stands for Five Bar) (Fig. 3).

The output forces and speeds achievable in the working area can be displayed as a color diagram (Fig. 4a, b). One can see, for example, that the achievable speeds are very small (shown in blue) at the limits of the working area. A gear unit that is well suited for handling tasks is characterised by a relatively even distribution of the achievable forces and speeds. This analysis is helpful when selecting a gear unit for a specific task and determining the areas of the working area in which the movement is to be programmed.

Figure 3: Interface of the FB/analysis program

Figure 4a): Attainable forces

Figure 4b): attainable speeds

Programming the path

Figure 5: Path programming

The movements required in technical practice can be divided into two groups: On the one hand, there are movements in which only the start and end points are fixed. The path selected between these points is not defined (point-to-point control). On the other hand, there are also completely defined paths (track control). The FB/motion program was developed to program such paths for five-unit crank gears. To program a path (Fig. 5), first of all support points are defined, which are then connected by geometric elements (e. g. straight line, circular arc, splines). Finally, the velocity profile is defined along the path.

With the analysis function available with the program FB/analysis, the speed profile can be adjusted very precisely to the possibilities of the gearbox.

Practical testing on the test bench

Fig. 6: Test stand for five-link crank gears

A test bench (Fig. 6) is available at the institute for practical testing of the results. In this test bench, the gearbox is made of lightweight aluminium profiles. The cranks are driven by high-dynamic servo motors via toothed belt drives. It is controlled by the real-time control system XPC-Target, which uses control algorithms programmed under Matlab/Simulink. During the test runs, a large number of measurement signals (e. g. courses of angular errors, motor current, feedforward control, friction compensation, etc.) can be used. can be viewed and recorded. The control parameters can also be changed during the run. The aim is to improve the attainable accuracy and speeds by means of improved control and a path that is better adapted to the handling device, while at the same time reducing the loads on the drive system.

Current focus of work

Main research topics within this project are

• Improvement of the analysis possibilities for crank gears
• Extension of the trajectory planning program to include advantageous point-to-point movements
• Filtering of the acceleration curve to avoid disturbing vibrations
• Further development of the programmed control system