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Skill Creation

Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime.

Preparation

Familiar with upload the firmware (Chapter 3 of the Nybble/Bittle User Manual), the assembly process (Chapter 4 of the Nybble/Bittle User Manual) and calibrate the servo (Chapter 6 of the Nybble/Bittle​ User Manual). Use an IR remote to verify that the following functions work as expected.
  • Press the button on the IR remote's 2nd row, 2nd column (as shown in the picture below). Later it will be expressed by (2, 2). You can also connect a USB adapter and enter the serial command "kbalance" in the serial monitor. The robot will stand up.
  • Press the (7, 3) button on the IR remote. You can also connect a USB adapter and enter the serial command "kzero" in the serial monitor. All the servos of the robot are turned to the 0-degree position, which is the "Zero" skill (as shown in the figure below).

Understand the code

All skill arrays of the robot are defined in the Instinct***.h file.
  • Nybble: InstinctNybble.h
  • Bittle: InstinctBittle.h
Here is an abbreviated example of an Instinct***.h file:
//a short version of Instinct***.h as example
const char rest[] PROGMEM = {
1, 0, 0, 1,
-30, -80, -45, 0, -3, -3, 3, 3, 75, 75, 75, 75, -55, -55, -55, -55,};
const char zero[] PROGMEM = {
1, 0, 0, 1,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,};
​
const char crF[] PROGMEM = {
36, 0, -3, 1,
61, 68, 54, 61, -26, -39, -13, -26,
66, 61, 58, 55, -26, -39, -13, -26,
...
51, 81, 45, 72, -25, -37, -12, -25,
55, 76, 49, 68, -26, -38, -13, -26,
60, 70, 53, 62, -26, -39, -13, -26,
};
​
const char pu[] PROGMEM = {
-8, 0, -15, 1,
6, 7, 3,
0, 0, 0, 0, 0, 0, 0, 0, 30, 30, 30, 30, 30, 30, 30, 30, 5, 0,
15, 0, 0, 0, 0, 0, 0, 0, 30, 35, 40, 29, 50, 15, 15, 15, 5, 0,
30, 0, 0, 0, 0, 0, 0, 0, 27, 35, 40, 60, 50, 15, 20, 45, 5, 0,
15, 0, 0, 0, 0, 0, 0, 0, 45, 35, 40, 60, 25, 20, 20, 60, 5, 0,
0, 0, 0, 0, 0, 0, 0, 0, 50, 35, 75, 60, 20, 30, 20, 60, 6, 0,
-15, 0, 0, 0, 0, 0, 0, 0, 60, 60, 70, 70, 15, 15, 60, 60, 6, 0,
0, 0, 0, 0, 0, 0, 0, 0, 30, 30, 95, 95, 60, 60, 60, 60, 6, 1,
30, 0, 0, 0, 0, 0, 0, 0, 75, 70, 80, 80, -50, -50, 60, 60, 8, 0,
};
​
const char* skillNameWithType[] = {"crFI", "puI", "restI", "zeroN",};
#if !defined(MAIN_SKETCH) || !defined(I2C_EEPROM)
const char* progmemPointer[] = {crF, pu, rest, zero, };
#else
const char* progmemPointer[] = {zero};
#endif

Data structure

The meaning of the data structure of the skill array is shown in the figure below:
Data structure

Total # of Frames

The 1st element in the skill array indicates the total number of all action frames contained in the skill. If there is a minus sign (-) before the value, it means that the skill is a behavior.
In the above data structure figure, the rest is a posture. It has only 1 action frame (1 row of data).
the crF (Crawl forward) is a gait. It contains 36 action frames (36 rows of data).

Expected Body Orientation

The 2nd and 3rd elements in the skill array represent the expected value of the body direction, that is, the inclination angle of the robot's body when performing skill actions, corresponding to the body's roll angle (Roll) and pitch angle (Pitch) respectively. When the robot is performing a skill action, if the body tilt deviates from the expected value, the balance algorithm will make some adjustments to the angle value of the relevant leg servos to keep the body tilt as close as possible to the expected value.
The pitch angle (Pitch) and roll angle (Roll) of the robot shown in the figure above are both at 0 degrees. In the picture on the left, the robot body rotates counterclockwise from the 0-degree position around the center point, and the pitch angle is positive; when it rotates clockwise, the pitch angle is negative. In the picture on the right, the robot body rotates counterclockwise from the 0-degree position around the center point, and the roll angle is positive; when it rotates clockwise, the roll angle is negative.
Take a look at the following sample code, standing:
const char balance[] PROGMEM = {
1, 0, 0, 1,
0, 0, 0, 0, 0, 0, 0, 0, 30, 30, 30, 30, 30, 30, 30, 30,};C
Sitting:
const char sit[] PROGMEM = {
1, 0, -30, 1,
0, 0, -45, 0, -5, -5, 20, 20, 45, 45, 105, 105, 45, 45, -45, -45,};
The 2nd and 3rd elements in the array represent the expected value of the body direction (corresponding to the roll angle and pitch angle of the body), and the unit is degree.
When the gyroscope is activated, Bittle's body is slightly rotated, and when the body tilts away from the expected value of the body direction, the balance algorithm will keep the body in this posture as much as possible.

Indexed Joint Angles

The index numbers of the servo are shown in the figure below:
Please control the rotation angle range of the joint servo between [-125~125]. For the leg servo, when viewed from the left side of the robot, the leg rotates counterclockwise from the 0 degree position around the joint center point (the screw fixing position) , the angle is a positive value; clockwise rotation, the angle is a negative value; viewed from the right side of the robot, the leg rotation angle is mirror-symmetrical to the left side (rotating clockwise from the 0 degree position around the joint center point, the angle is a positive value; Rotate counterclockwise, the angle is negative). For the robot's neck servo, looking down from the top of the robot's head, the neck rotates counterclockwise from the position of 0 degrees around the joint center point (the position where the screw is fixed), and the angle is a positive value; when it rotates clockwise, the angle is a negative value.
For the Nybble head servo (No. 1 servo) observed on the right side of the robot, the head rotates counterclockwise from the 0-degree position around the joint center point (screw fixed position), and the angle is positive; when it rotates clockwise, the angle is negative.
For the Nybble tail servo (No. 2 servo) facing the tail and looking down, the tail rotates counterclockwise from the 0-degree position around the center point (screw fixing position), and the angle is positive; when it rotates clockwise, the angle is negative.
Each action frame in the skill array contains the rotation angle values of multiple servos.
In the above data structure figure, the rest action frame contains 16 joint servo angle values, and the corresponding servo index numbers are arranged from small to large starting from 0.
Each action frame in the crF (crawling forward) contains 8 joint servo angle values, and the corresponding servo index numbers are arranged from small to large starting from 8.

Angle ratio

The 4th element in the skill array represents the angle ratio. The angle ratio value can be increased when it is necessary to store angle values outside the range of -128 to 127. For example, if the angle ratio is set to 2, the angle value of all joint servos in the skill array multiplied by 2 is the actual rotation angle of the servo.
Take a look at the following example skill array rc (return to standing after being on all fours):
const char rc[] PROGMEM = {
-3, 0, 0, 2,
0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, -88, -43, 67, 87, 42, -35, 42, 42, 15, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, -83, -88, 87, 42, 42, 42, 42, -40, 15, 0, 0, 0,
-8, -20, -11, 0, -1, -1, 0, 0, 18, 18, 18, 18, -14, -14, -14, -14, 10, 0, 0, 0,
};
The 4th element (2) in the array represents the angle ratio. This means that the actual angle value of all joint rotations is equal to the angle value of each joint in the array (starting from the 8th element) multiplied by this angle ratio.

Posture array

The posture array contains only one action frame. Taking Bittle as an example, find the zero posture array in InstinctBittle.h:
const char zero[] PROGMEM = {
1, 0, 0, 1,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,};
Modify some of the joint servo angle values:
const char zero[] PROGMEM = {
1, 0, 0, 1,
70, 0, 0, 0, 0, 0, 0, 0, -60, 0, 0, 0, 60, 0, 0, 0,};
Save the modification and upload the main function program OpenCat.ino to the Bittle mainboard. After the upload is complete, click the button (7, 3) on the IR remote to see the modified zero skill, and the new posture is shown in the figure below:
The first element (1) in the array represents the total number of frames of the skill, and 1 means it is a posture skill.
The 4th element (1) in the array represents the angle ratio. This means that all index joint angles below are actual angles (because each of them is multiplied by 1).
The 5th to 20th elements in the array represent the respective angle values of the 16 joints in the current frame.
For Bittle, the 5th element in the array (70, corresponding to the No.0 servo) means that the servo on Bittle's neck rotates 70 degrees counterclockwise. Bittle's head turned to the left side of the body.
The 13th element in the array (-60, corresponding to No.8 servo) indicates that Bittle's left front thigh rotates 60 degrees clockwise around the joint center point.
The 17th element in the array (60, corresponding to No.12 servo) indicates that Bittle's left front calf rotates 60 degrees counterclockwise around the center of the joint.
The other joint angles remain unchanged at 0 degrees.

Gait array

The gait array contains multiple coherent action frames, and these action frames are executed repeatedly in a sequential cycle, unless the robot receives a new skill command, it will stop executing. For example, the bk(draw back) gait array is defined as follows:
const char bk[] PROGMEM = {
35, 0, 0, 1,
46, 54, 46, 54, -5, -23, -5, -23,
43, 58, 43, 58, -5, -24, -5, -24,
......
52, 43, 52, 43, -5, -21, -5, -21,
50, 48, 50, 48, -5, -22, -5, -22,
47, 53, 47, 53, -5, -23, -5, -23,
};
The 1st element in the array (35) means the skill has 35 action frames. Starting from the second line of data, each line is an action frame, which contains the angle values of 8 joint servos, and the corresponding servo index numbers are arranged from small to large starting from 8 (a total of 35 lines).
For gait skills, each action frame in the future may contain 12 joint servo angle values, and the corresponding servo index numbers will be arranged from small to large starting from 4, depending on the number of leg servos participating in the movement.

Behavior array

The behavior array also contains multiple coherent action frames, and all the action frames are only executed for one round in order, but some of the continuous action frames can be executed multiple times in a loop. For example, the pu(push-up) behavior array is defined as follows:
const char pu[] PROGMEM = {
-8, 0, -15, 1,
6, 7, 3,
0, 0, 0, 0, 0, 0, 0, 0, 30, 30, 30, 30, 30, 30, 30, 30, 5, 0, 0, 0,
15, 0, 0, 0, 0, 0, 0, 0, 30, 35, 40, 29, 50, 15, 15, 15, 5, 0, 0, 0,
30, 0, 0, 0, 0, 0, 0, 0, 27, 35, 40, 60, 50, 15, 20, 45, 5, 0, 0, 0,
15, 0, 0, 0, 0, 0, 0, 0, 45, 35, 40, 60, 25, 20, 20, 60, 5, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 50, 35, 75, 60, 20, 30, 20, 60, 6, 0, 0, 0,
-15, 0, 0, 0, 0, 0, 0, 0, 60, 60, 70, 70, 15, 15, 60, 60, 6, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 30, 30, 95, 95, 60, 60, 60, 60, 6, 1, 0, 0,
30, 0, 0, 0, 0, 0, 0, 0, 75, 70, 80, 80, -50, -50, 60, 60, 8, 0, 0, 0,
};
This data structure contains more information than pose and gait:
The meanings of the 4 elements in line 1 are as mentioned above, and the first element (total number of frames) is a negative number, indicating that this skill is a behavior.
The 3 elements in line 2 indicate the loop structure contained in this behavior: start frame, end frame, and loop times:
6, 7, 3 in the example means that this behavior is executed 3 times from the 7th frame to the 8th frame (the frame index number starts from 0). The motion sequence of the entire behavior is performed in one round, rather than in a continuous loop-like gait.
Each action frame contains 20 elements. The first 16 elements represent the angles of the joint servos as mentioned above, and the corresponding servo index numbers are arranged from small to large starting from 0. The last 4 elements have the following meanings:
  1. 1.
    The 1st represents the speed factor. The default speed factor is 4, which can be changed to an integer from 1 (slow) to 127 (fast). Units are degrees per step. If set to 0, the servo will rotate to the target angle at maximum speed (about 0.07 seconds/60 degrees). Values greater than 10 are not recommended unless you understand the risks.
  2. 2.
    The 2nd represents the delay time. The default delay is 0. The range that can be set is 0 to 127, and the unit is 50 milliseconds (if it is set to 2, the actual delay is 100 milliseconds).
  3. 3.
    The 3rd represents the trigger axis. It is used to set the body rotation direction when the robot triggers the next frame of action. There are the following 5 setting options:
    • 0 means there is no trigger axis and the trigger angle condition setting
    • 1 means positive pitch, the robot body is bent forward and rotated in the downward direction
    • -1 means negative pitch, the robot body rotates backwards
    • 2 indicates positive roll, the robot rolls to its left side
    • -2 means negative roll, the robot rolls to its right side
  4. 4.
    The 4th represents the trigger angle. Angle values have the same positive and negative meanings as body orientation expectations. Trigger angle setting range: an integer value between -125 and 125.
Note:
  • The trigger axis and trigger angle need to be used together: only when the robot completes the frame of motion and rotates in accordance with the set body rotation direction to exceed the trigger angle, the next frame of motion will be triggered. If the trigger axis is set to 0, the trigger angle is meaningless. Therefore, when the trigger axis is set to 0, the trigger angle is generally also set to 0.
  • If a delay time is also set in the action frame, then when the robot runs this action frame, it must not only meet the trigger conditions set by the trigger axis and trigger angle but also reach the delay setting time before triggering the next frame of action.

Skill storage type

The erasing and writing times of EEPROM are limited (1,000,000 times). In order to minimize write operations, two skills are defined: Instinct and Newbility. Their addresses are stored in the built-in EEPROM (1KB) of the chip (ATmega328P) as a lookup table, but the data of the main body exists in different storage units:
  • I2C EEPROM (8KB) memory Instincts. Instincts are fixed skills (or occasional fine-tuning), which can be thought of as "muscle memory".
  • Flash (shares 32KB storage space with Arduino program code) Storage Newbilities. Newbilities refer to user-defined skills (possibly modified, added, or deleted). They will not be written into the static EERPOM but uploaded to the flash memory (Flash) together with the Arduino program code. Their addresses are assigned in real-time when the code is executed, and this value rarely changes as long as the total number of skills (including all instincts and newbilities) remains the same.
The specific sample code is as follows:
const char* skillNameWithType[] = {"crFI", "puI", "restI", "zeroN",};
#if !defined(MAIN_SKETCH) || !defined(I2C_EEPROM)
const char* progmemPointer[] = {crF, pu, rest, zero, };
#else
const char* progmemPointer[] = {zero};
#endif
Add a suffix to each skill array name in the character pointer array skillNameWithType, "N" means Newbility, and "I" means Instinct.
const char* progmemPointer[] = {crF, pu, rest, zero, }; This part of the code is active when uploading the configuration mode sketch. It contains data and pointers for all abilities.
const char* progmemPointer[] = {zero}; This part of the code is active when uploading the major functionalities sketch. Since the instincts are already saved in the external I2C EEPROM, their data is omitted here to save space. If you only need to adjust the actions of existing new skills (such as zero), you don't need to re-upload the configuration mode sketch.

Create new skills

Debug the skills

The following two methods can be used when authoring or debugging skill actions:
  • Control the robot to make actions by Python scripts. For details, please refer to the serialMaster User Guide.
  • Use the Skill Composer in the Desktop App to control the robot to do actions, and then use the "Export the skill" function to copy and paste the contents of the debugged skill array to the Instinct***.h file for use. Please refer to the data structure for the specific format.
This GitHub repo is a good starting point if you want to develop a customized gait. If you are going to do some inverse kinematics calculation, you may use the following key dimensions to build your model:
​
​
The "SkillLibrary" folder in GitHub repo is a collection of new skills that OpenCat robots can perform, which can be used for your reference.
You are also welcome to share your new skills by sending merge requests to this folder.
Another direction is to set up a simulation for Bittle then test the model in reality. You may use this Unified Robot Description Format (URDF) file for Bittle to set up an NVIDIA Omniverse simulation.

Add skill array

Add Instinct

To add a new skill(testI) array ending with I, you need to add the skill array variable name (test) to the first branch of the sample code macro judgment. The specific code modification is as follows:
const char* skillNameWithType[] = {"crFI", "puI", "restI", "zeroN", "testI"};
#if !defined(MAIN_SKETCH) || !defined(I2C_EEPROM)
const char* progmemPointer[] = {crF, pu, rest, zero, test};
#else
const char* progmemPointer[] = {zero};
#endif

Add Newbility

To add a new skill(testN) array ending with N, you need to add the skill array variable name (test) to both branches of the sample code macro judgment. The specific code modification is as follows:
const char* skillNameWithType[] = {"crFI", "puI", "restI", "zeroN", "testN"};
#if !defined(MAIN_SKETCH) || !defined(I2C_EEPROM)
const char* progmemPointer[] = {crF, pu, rest, zero, test};
#else
const char* progmemPointer[] = {zero, test};
#endif
After adding the skill array In the Instinct***.h file, for NyBoard, use the data cable to connect the USB uploader and the computer, use the Arduino IDE to upload the configuration mode sketch, open the serial monitor when the serial port prompts the following content:
Reset joint offsets? (Y/n):
After inputting 'Y' or 'n', all instinct arrays will be stored in the I2C EEPROM at one time, and the index addresses of all skills will also be generated and stored in the built-in EERPOM of the chip, and then upload the major functionalities sketch. For the specific operation process, please refer to the relevant chapters in uploading the sketch for NyBoard.
When verifying the skill action, you can open the serial monitor and call through the serial port command with 'k' token. For example, "ksit" can be transformed into a sitting posture by Bittle.

Intelligent

By adding some extensible modules (such as a gesture sensor), it can help the robot to better perceive the environment and even make decisions. By accumulating these automatic behaviors and designing a decision tree, robot's fully automatic intelligent operation is finally realized!