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Add description of a gpio-tag (#38)
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‎design_drafts/non_joint_command_interfaces.md‎

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# Definition of command interfaces for non-joint related information
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## Motivation
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Already multiple times, users raised the question about the handling of non-joint-related interfaces.
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Such interfaces are not considerd during [design of the ros2_control URDF-tag structure](./components_architecture_and_urdf_examples.md).
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The design proposes two possibilities to classify (group) the interfaces, using `<joint>` and `<sensor>` tags.
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`<joint>`-tag groups the interfaces associated with the joints of physical robots and actuators.
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They have command and state interfaces to set the goal values for hardware and read its current state.
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`<sensor>`-tag groups multiple state interfaces describing, e.g., internal states of hardware.
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Most modern robots, especially industrial manipulators, also have other physical ports to control external hardware.
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The most usual ones are General Purpose Inputs/Outputs (GPIOs).
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Although the strict definition considers only digital ports, the term GPIO in this document describes any (one-dimensional) input/output port of physical hardware, e.g., digital or analog input/output.
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## Problem
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To use GPIOs in the ros2_control framework, they have to be listed in the robot's URDF: inputs (state interfaces) under a `<sensor>`-tag and output (command and state interfaces) under a `<joint>`-tag.
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This could lead to confusion since non-existing (virtual) joints have to be defined in the ros2_control part of the URDF structure.
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"Joint" is a well-defined term in robotics. Therefore this naming should be avoided, i.e., another keyword should be used describing GPIOs, semantically separating them from the kinematics joints.
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## Proposed solution
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Use of keyword "gpio" when describing input and output ports of a robotic device that cannot be associated with any joint or sensor.
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Parsing of `<gpio>-tag` is similar to this of a `<joint>`-tag having command and state interfaces.
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The tag must have at least one `<command>`- or `<state>`-tag as a child.
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The keyword "gpio" is chosen for its generality.
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Although strictly used for digital signals, in this document describes any electrical analog, digital signal, or physical value.
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The `<gpio>` tag can be used as a child of all three types of hardware interfaces, i.e., system, sensor, or actuator.
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Semantically, they describe values that can not be associated with a joint or a sensor.
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### Examples
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1. Robot with multiple GPIO interfaces
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- RRBot System
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- Digital: 4 inputs and 2 outputs
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- Analog: 2 inputs and 1 output
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- Vacuum valve at the flange (on/off)
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```
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<ros2_control name="RRBotSystemMutipleGPIOs" type="system">
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<hardware>
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<plugin>ros2_control_demo_hardware/RRBotSystemPositionOnlyHardware</plugin>
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<param name="example_param_hw_start_duration_sec">2.0</param>
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<param name="example_param_hw_stop_duration_sec">3.0</param>
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<param name="example_param_hw_slowdown">2.0</param>
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</hardware>
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<joint name="joint1">
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<command_interface name="position">
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<param name="min">-1</param>
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<param name="max">1</param>
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</command_interface>
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<state_interface name="position"/>
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</joint>
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<joint name="joint2">
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<command_interface name="position">
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<param name="min">-1</param>
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<param name="max">1</param>
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</command_interface>
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<state_interface name="position"/>
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</joint>
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<gpio name="flange_digital_IOs>
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<command_interface name="digital_output1"/>
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<state_interface name="digital_output1"/> <!-- Needed to know current state of the output -->
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<command_interface name="digital_output2"/>
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<state_interface name="digital_output2"/>
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<state_interface name="digital_input1"/>
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<state_interface name="digital_input2"/>
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</gpio>
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<gpio name="flange_analog_IOs>
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<command_interface name="analog_output1"/>
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<state_interface name="analog_output1"/> <!-- Needed to know current state of the output -->
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<state_interface name="analog_input1"/>
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<state_interface name="analog_input2"/>
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</gpio>
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<gpio name="flange_vacuum>
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<command_interface name="vacuum"/>
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<state_interface name="vacuum"/> <!-- Needed to know current state of the output -->
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</gpio>
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</ros2_control>
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```
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2. Gripper with electrical and suction grasping possibilities
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- Multimodal gripper
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- 1-DoF parallel gripper
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- suction on/off
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```
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<ros2_control name="MultimodalGripper" type="actuator">
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<hardware>
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<plugin>ros2_control_demo_hardware/MultimodalGripper</plugin>
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</hardware>
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<joint name="parallel_fingers">
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<command_interface name="position">
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<param name="min">0</param>
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<param name="max">100</param>
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</command_interface>
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<state_interface name="position"/>
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</joint>
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<gpio name="suction>
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<command_interface name="suction"/>
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<state_interface name="suction"/> <!-- Needed to know current state of the output -->
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</gpio>
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</ros2_control>
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```
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3. Force-Torque-Sensor with temperature feedback and adjustable calibration
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- 2D FTS
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- Temperature feedback in °C
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- Choice between 3 calibration matrices, i.e., calibration ranges
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```
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<ros2_control name="RRBotForceTorqueSensor2D" type="sensor">
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<hardware>
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<plugin>ros2_control_demo_hardware/ForceTorqueSensor2DHardware</plugin>
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<param name="example_param_read_for_sec">0.43</param>
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</hardware>
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<sensor name="tcp_fts_sensor">
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<state_interface name="fx"/>
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<state_interface name="tz"/>
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<param name="frame_id">kuka_tcp</param>
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<param name="fx_range">100</param>
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<param name="tz_range">100</param>
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</sensor>
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<sensor name="temp_feedback">
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<state_interface name="temperature"/>
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</sensor>
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<gpio name="calibration">
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<command_interface name="calibration_matrix_nr"/>
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<state_interface name="calibration_matrix_nr"/>
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</gpio>
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</ros2_control>
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```
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