Calibrate the joints with Arduino IDE
Prepare to Enter the Calibration State
Please refer to the preparation section in the Joint Calibrator of the Petoi Desktop App according to the robot's mainboard.
Calibration process
1. Enter the calibration state
You must plug the servos and external batteries into the mainboard and check the position and direction of all servos.
Send the serial command ‘c’ in the serial monitor to enter the calibration state. Depending on their initial shaft direction, some may travel larger angles until stopping at the middle point. There will be noise coming from the gear system of the servos. You will see a calibration table like the following:
The first row is the joint indexes; the second row is their calibration offsets:
Index
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Offset
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
-1
Initial values are “-1” or “0” and should be changed by later calibration.
2. The rationale for calibration
2.1 Understand the zero state and the coordinate system
After typing ‘c’ in the serial monitor, with all servos rotated to their zero angles, attach the head, tail, and legs prepared in the previous section to the body. They are generally perpendicular to their linked body frames. The calibration pose is shown below:
Note: Insert the servo-related components directly into the servo output shaft; do not turn the output shaft during this process.
Rotating the limbs counter-clockwise from their zero states will be positive (same as in polar coordinates). Viewed from the left side of the robot's body, the counter-clockwise rotation of the joint is defined as the positive direction.
2.2 Discrete angular intervals
If we look closer at the servo shaft, we can see it has a certain number of teeth. That’s for attaching the servo arms and avoiding sliding in the rotational direction. In our servo sample, the gears divide 360 degrees into 25 sectors, each taking 14.4 degrees(offset of -7.2~7.2 degrees). That means we cannot always get a perfect perpendicular installation.
2.3 Attach body parts to the servos
Install the head
In the calibration state, place the head as close to the central axis as possible and insert its servo shaft into the servo arm of the neck.
Press down on the head so it is firmly attached to the neck.
Install the legs
Install upper leg and lower leg components to the output teeth of the servos after the Bittle is powered on and in the calibrated neutral position. Please keep the torso, upper leg, and lower leg installed vertically as much as possible, and do not install the lower leg backward, as shown in the picture.
3. Fine-tune the calibration using the serial monitor
3.1 Joint Control Commands
The command for fine-tuning calibration (refer to the serial protocol) is formatted as cIndex Offset. Notice that there’s a space between cIndex and Offset. The index number of the robot's joints is shown in the pictures below:
For example :
c8 6This means giving the 8th servo an offset of 6 degrees.c0 -4This means giving the 0th servo(the head) an offset of -4 degrees.
The resolution of the correction amount is 1 degree; do not use decimals.
Find the best offset that can bring the limb to the zero states. It's a process of trial and error.
After calibration, remember to type ‘s’ to save the offsets. Otherwise, they will be forgotten when exiting the calibration state. You can even save every time after you’re done with one servo.
3.2 Use ‘L’ shaped joint tuner
When watching something, one's observations will change from different perspectives. When measuring length, one always wants to read directly above a referencing ruler.
You must keep a parallel perspective when calibrating Bittle. Use the 'L'-shaped joint tuner as a parallel reference to avoid reading errors. Align the tips on the tuner with the center of the screws in the shoulder and knee joints and the little hole on the tip of the foot. Look along the co-axis of the centers. For each leg, calibrate the shoulder servos (index 8~11) first, then the knee servos(index 12~15). When calibrating the knee, use the matching triangle windows on both the tuner and shank to ensure parallel alignment.
Nybble
Bittle
3.3 Testing and validation
After calibration, type ‘d’ or ‘kup’ to validate the calibration. This will result in Bittle / Nybble symmetrically moving its limbs between the rest and stand states.
You may need to do a few rounds of calibrations to achieve optimal states.
Take Bittle, for example, as follows:
3.4 Install the screws
After completing the joint calibration, install the center screws to fix the leg parts and servo gears.
3.5 Center of mass
Try to understand how the robot keeps balance even during walking. If you add new components to the robot, distribute its weight symmetrically about the spine. You may also need to slide the battery holder back and forth to find the best balance spot. Because the battery is heavier in the front, you can insert it in a reversed direction to shift the center of mass more toward the back.
Please do not force the robot to add heavy objects, which may cause the servos to sweep or get stuck.
Last updated
Was this helpful?