05 May 2016
This section is addressed to developers of robots and robot modules for RCML environment. When selecting robot axes to control, one should remember of the so-called “alpha-5” problem true for industrial robots using six-axis kinematics, when a minor robot movement in Cartesian coordinates is accompanied by a significant movement in the coordinates of motor axes. And often it is true for the fifth axis of robot motor, whence the name. However, the principle of this problem can be extended to a fairly wide range of robots, including even such a simple robot crawler as described above.
This principle states that a minor change in the target value of the axis along which the robot must take a new position, should cause proportional small changes in the axes of rotation of robot motors and units, possibly causing only one phase of operation of motors involved in one direction, that is, the same motor does not re-accelerate and re-decelerate.
Otherwise, abrupt accelerations of robot parts and mechanisms are possible, and small changes suggest greater frequency of their occurrence and ultimately it can cause excessive wear and even robot failure or damage to objects of its environment and people.
In the above crawler robot example, this principle is observed. However, if we violate it, and for robot axes select, for example, standard Cartesian X and Y axes describing the plane in which the robot moves (see Figure 6), we can get a number of negative effects.
Figure 6 Example of robot axes selection
For example, a minor positive change in X axis in the current position would require the robot mechanics first to turn the robot to the right for a quarter turn and then move it forward. Each motor would have to perform two operation sessions, one for turn and one for movement, and right track motor would have to change the direction of rotation after the turn, which significantly increases the time when the robot executes the command received and, therefore, reduces robot response speed.
Additionally, selection of such axes of motion for the robot leads to uncertainty, since the robot can move to the new position in different paths:
- Make a quarter turn right and move forward;
- Make a three-quarter turn left and move backward.
In this case, the path selection algorithm is required for the robot, which should be executed whenever a new axis value is received, and each time consuming portion of the CPU time, increasing the robot response speed.