Robot Control Meta Language (RCML)

4.1 General Information

RCML environment can provide capability of hand robot control by means of a certain control device when calling hand_control system function with the appropriate parameters.

The basic operating principle of RCML environment when switching to hand control mode is to match robot axes along which it can somehow move, with control device axes along which this device can detect changes, see Figure 5. 

Figure 5 Example robot axes and control device axes

This example includes a robot crawler (shown to the left), which can move into its new absolute position of the plane through a series of changes in its position along two axes: R axes of movement (forward or reverse) and A axis of rotation A (left or right). And there is a simple control device (a joystick) (shown to the right) which can be deflected in the plane from its initial position along two axes – X and Y. Accordingly, by means of RCML, you can relate joystick and robot axes so that joystick deflection results in robot movement. For example, joystick deflection along Y axis in the positive direction causes movement forward, and joystick deflection along X axis in the negative direction causes robot turn to the left. Let’s assume that this robot is specified in RCML environment with tarakan module, and the joystick, respectively, with joy control module. RCML code to link them in hand control mode to obtain the above effect, is as follows:

@r = robot_tarakan;
hand_control(@r, “joy”, “R”, “Y”, “A”, “Y”);

4.2 Value Transfer Principle

Thus, control device axis is a source of values, and robot axis is a receiver, and the robot itself is the executive device. Of course, the scale of values or ranges of robot and control device axes may not coincide. RCML environment automatically brings the values from control device axis to the range of the values of robot axes, maintaining their ratio.

RCML environment gets information on robot axes and the ranges of their values through API from robot module, and thus they must be declared to the documentation for this module, in order for RCML programmers to use them. Similarly for control devices.

This principle can be extended not only to real axis of movement of a robot or a joystick, but to all its functions, which anyhow can have values and change robot status in real time (within the OS capabilities) for example, turning on light indicators, blocking robot movements, etc. The only requirement is that they must be declared in robot module and documentation to it.

Additionally, you can create the so-called discrete axes, which take a small range of values, and each value may correspond to some status of the robot or its mechanism. A special case is – “binary” axes producing or having a value of 0 or 1, similar to “on” or “off”. A striking example may be an ordinary computer keyboard with 101 keys and, respectively, 101 binary axes. In case of a robot, such axis can be switching on of lamps and lights, or making some sound or enabling voice communication mode. Variations are limited only by imagination of designers of robots and control devices, as well as programmers in RCML, which can link the axes of certain devices in any order. Due to the fact that values can be brought to integers, it is possible to link binary axes of both robot and control device with “non-binary” axes of a certain device.

It is important to note that RCML environment has no control over the frequency and variability of transfer of values from control device to the robot. If control device each time sends axis value, then even if it has not changed, this value will be transferred to the robot each time. Module developers are recommended to take this factor into account. In certain cases, developers of modules for control devices should pay attention to the factor of value transfer frequency, because each transfer reaches the robot, and it should have time to respond. Developers of modules for robots are recommended not to lose sight of the fact that it is possible to transfer a series of the same values for a given axis only because it (the series) was transferred by control device.

4.3 Recommendations on Robot Axis Selection for Hand Control Mode

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.

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