104 lines
5.0 KiB
Markdown
104 lines
5.0 KiB
Markdown
# DJI Gimbal FOC
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The aim of this project is to be able to use the 3-axis DJI gimbal with a custom open source controller like [SimpleFOC](https://docs.simplefoc.com/). This high quality gimbal is very tiny and easy to find as a replacement part which makes it very suitable for DIY projects.
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## Description
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## Pinout identification
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The Gimbal is composed of a flex PCB with a main connector and 3 smaller for each motor. The main end connector is a 40-pin mezzanine board to board connectors. In order to work easily I have designed a breakout board which open to a 2.54" header.
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Here is the strategy I followed to find the pinout:
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1. Find all equipotential pins with a multimeter set to continuity tests, and test all the combinations
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2. Group remaining pins by motor With the multimeter find all the pins connected to the motor connector. (Reapeat 3 times for the other connectors)
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### Open-loop control
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Each motor has its own drivers a MP6536. Which makes it easy as no additional hardware is necessary to drive the motors.
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There are 4 pins from the MP6536:
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1. PWM1
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2. PWM2
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3. PWM3
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4. Fault : Output. When low, indicates overtemperature, over-current, or under-voltage.
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Connected directly to a MCU and with the Simple FOC Library, open-loop control works quite well. However due to open-loop control, it cannot know when a "step" is missed so misalignment can occur. Also, the motor tends to become quite hot due to the continuous current sent to the coils.
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## Position estimation with the integrated linear hall sensors
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### 1. Setup
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Each motor is composed of two ratiometric linear hall sensors. (Texas Instrument DRV5053 Analog-Bipolar Hall Effect Sensor) They are placed at around 120º from each other (eyes measured) and measure the magnetic field of the rotor.
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Ratiometric means that the output signal is proportional to the voltage supply to the sensor. In this setup, with 5V supply, the output measured is between 520mV and 1.5V, so a 1V amplitude.
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### 2. Measures
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These oscilloscope traces are the sensor output when rotating the rotor forth and back. (a bit less than 180º on the 3rd motor)
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The channel 0 (Yellow) is the Hall 1 and the Channel 1 (Green) is the Hall 2
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We can see that in the first movement (positive rotation), the green is out of phase of π/2.`
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### 3. Encoding the position
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1. Get the absolute angle within a period
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Since the 2 signals correspond to a cos and sin signals, it is possible to compute the angle inside the period using arctan2 function. However, we have more than one period, it is so necessary to increment a position.
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$$\theta= atan2(a,b)$$
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2. Incremental position
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To increment the position, it is necessary to start from 0 at a known postion. For that the motor is moved in open loop to one end and the position is set to 0.
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Then we need to sum all the delta of movement at each measure sample.
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$$\phi_t=\phi_{t-1} + (\theta_t - \theta_{t-1})mod(-\pi;\pi)$$
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## Coding the solution
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1. Get the angle in the perdiod
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In order to compute the angle from the cos and sin with atan, it is necessary to remap the values of the analog readings from -1 to 1.
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Beforehand, the maximum and minimum peak of the signals need to be found. It can be done by swiping the motor on startup in open-loop mode.
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Then the arctan function can be applied. It is preferable to use arctan2 as it will give an angle within the 4 quadrants (-π,π). Whereas arctan give an angle between (-π/2,π/2). [Wikipedia](https://en.wikipedia.org/wiki/Atan2)
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```C++
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float LinearHallSensor::Callback() // Return the estimated position of the sensor
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{
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A = norm(analogRead(CH1),minCh1, maxCh1); //read analog values and normalise between [-1;1]
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B = norm(analogRead(CH2),minCh2, maxCh2);
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theta = atan2(A,B); // Compute the absolute angle in the period
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phi = phi + dist_angle(theta, theta_prev); // increment the difference
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theta_prev = theta; // save fot nex time
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return phi;
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}
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float norm(float x, float in_min, float in_max) //return the input value normalised between [-1;1]
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{
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return (float)(x + 1.0) * (2.0) / (float)(in_max - in_min) -1.0;
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}
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float dist_angle(float newAngle, float prevAngle) // return the difference modulo [-pi;pi]
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{
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float diff = newAngle - prevAngle;
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while (diff < (-M_PI))
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diff += 2 * M_PI;
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while (diff > M_PI)
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diff -= 2 * M_PI;
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return diff;
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}
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```
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## Tuning the PIDs
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To achive position control it is necessary to have first, a velocity controller well tuned, as they are in cascade. (SimpleFOC implementation and diagram)
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However, the motors of the gimbal have hard stop and can only rotate of around a half turn. It was so necessary to remove these mecanical stops. I drilled with a 1.6mm drill the two little holes to remove it. Then the motor was able to rotate freely.
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