Bot PID troubles

Hi!

I have been working on a 2-wheel balancing robot for the last few weeks and I have been having some problems getting the PID controller to operate properly. I am using Brett Beauregard's PID library which I have seen many other people use in similar projects. This is paired with an MPU 6050 accel/gyro to read the angle.

When I first put the project together, I tried to run my code using Brett's PID library. I am pretty sure that I had everything coded properly, but the output was acting weird. When the gyro reads an input greater than my setpoint, the PID responds how I would expect and turns the motor to move it in the direction that stabilizes it. However, when the gyro reads an input less than my setpoint, the output oscillates very quickly at one specific amplitude, no matter what the input is (as long as it is less than my setpoint). I attached a picture of my serial monitor during one of my runs. The input is the first column and the output is the second column.

To me, it seems like the code is interpreting anything below the setpoint as being stable and it is trying to oscillate around that setpoint. I am sure it isn't a problem with my hardware because I implemented my own sort of PD controller in a separate file and that file works but is not all that stable. I am hoping that with Brett's PID library implemented correctly, that my robot will be more stable.

Anyone have any suggestions or has experienced something similar? I've looked around but have not found someone with the same problem. I am at a total loss, so any advice would be appreciated.

I have attached my code for reference. Quick note: my setpoint is -10 because that is level for my bot.

swagbot_auto_PID.ino (5.57 KB)

Maybe the problem is this.....

Serial.begin(9600);

Set your comms speed to max.... eg 115200 or something like that. Otherwise it could slow you down a lot. Something to do with buffering and waiting at slower baud rates, that can slow down processes......possibly affecting control processes.

Ok I can check that later this afternoon and get back to you about that. I do not think that that will be the problem though. I think that it has something to do with my setpoint somehow. The controller behaves perfectly fine when I move it to one side of the setpoint but behaves incorrectly when I move it is on the other side of the setpoint.

n8dunk:
Ok I can check that later this afternoon and get back to you about that. I do not think that that will be the problem though. I think that it has something to do with my setpoint somehow. The controller behaves perfectly fine when I move it to one side of the setpoint but behaves incorrectly when I move it is on the other side of the setpoint.

Ok. If processing lags isn't the issue, then the issue is not the 'setpoint'. The issue will be in the control algorithm.

It is essential for the algorithm to be well understood, so that the output does what it is expected to do. This just means having a clear picture of the algorithm to be used --- not the code yet....but just the algorithm for the automatic controlling technique. If the controlling technique is sound, then the coding simply implements that technique.

This sort of means..... if the gyro 'angle' gives a reading that is below the setpoint, then your control algorithm needs to reliably handle that. Also need to make sure that accelerometer angle readings and gyro angle readings are all good (sensible, and relatively stable) and keep in mind effects of noisy readings.

I set the serial rate to 115200 and the problem still persists.

With respect to gyro readings, they are sensible and stable. I can manually move the bot to any angle and I see the appropriate angle displayed on my serial monitor at a relatively constant (+/- less than a degree) rate.

Additionally, I am very familiar with PID controllers, if this is what you meant by "It is essential for the algorithm to be well understood". My bot behaved correctly (just not very stable) on my own implemented PID controller, but not with the library that I downloaded. I have also attached my code where I implemented my own controller if that is helpful. I am not sure what the difference is between my implementation and the one that is in the library.

Thanks

swagbot_auto.ino (5.94 KB)

I'm rather suspicious of this code which seems to be attempting to compensate
for backlash or motor friction or something:

  if(Output > 0){
    Output = Output + wheel_offsets;
  }
  
  else if(Output < 0){
    Output = Output - wheel_offsets;
  }

Basically this makes your control loop very non-linear and likely to be impossible to
stabilise unconditionally, as small deviations of input produce +/- 50 excursions which is
a massive gain compared to the large signal case - this means there will be some
minimum amplitude of fluctuation below which the loop has excess gain, ie it will always
oscillate.

It may be the case that some sort of compensation is useful here, but it should be
much gentler and not a large discontinuity in the response I suspect.

[BTW what decay-mode are you using in the H-bridge? Do you know that this
strongly affects linearity of response, and that synchronous rectification mode
is the most linear mode. There is also the question of whether to use the more
complex current control (with an inner current-control loop and current sensors),
typically the way industrial servo motors work as torque is directly controlled.]

hye i need help......i'm now working on project robot cycling bicycle...the arm of robot is used to control the handlebar by using servo......now i need help on how to implement PID in my servo.....

this is my code for mPU6050 and servo

//Servo
#include <Servo.h>

Servo myservoY; // Roll
Servo myservoX; // Pitch

#include "I2Cdev.h"
#include "MPU6050_6Axis_MotionApps20.h"
#if I2CDEV_IMPLEMENTATION == I2CDEV_ARDUINO_WIRE
#include "Wire.h"
#endif

// class default I2C address is 0x68
// specific I2C addresses may be passed as a parameter here
// AD0 low = 0x68 (default for SparkFun breakout and InvenSense evaluation board)
// AD0 high = 0x69
MPU6050 mpu;
//MPU6050 mpu(0x69); // <-- use for AD0 high
#define IN1 6
#define IN2 5
#define ENA 10

#define OUTPUT_READABLE_YAWPITCHROLL

#define LED_PIN 13 // (Arduino is 13, Teensy is 11, Teensy++ is 6)
bool blinkState = false;

// MPU control/status vars
bool dmpReady = false; // set true if DMP init was successful
uint8_t mpuIntStatus; // holds actual interrupt status byte from MPU
uint8_t devStatus; // return status after each device operation (0 = success, !0 = error)
uint16_t packetSize; // expected DMP packet size (default is 42 bytes)
uint16_t fifoCount; // count of all bytes currently in FIFO
uint8_t fifoBuffer[64]; // FIFO storage buffer

// orientation/motion vars
Quaternion q; // [w, x, y, z] quaternion container
VectorInt16 aa; // [x, y, z] accel sensor measurements
VectorInt16 aaReal; // [x, y, z] gravity-free accel sensor measurements
VectorInt16 aaWorld; // [x, y, z] world-frame accel sensor measurements
VectorFloat gravity; // [x, y, z] gravity vector
float euler[3]; // [psi, theta, phi] Euler angle container
float ypr[3]; // [yaw, pitch, roll] yaw/pitch/roll container and gravity vector

// packet structure for InvenSense teapot demo
uint8_t teapotPacket[14] = { '$', 0x02, 0,0, 0,0, 0,0, 0,0, 0x00, 0x00, '\r', '\n' };

// ================================================================
// === INTERRUPT DETECTION ROUTINE ===
// ================================================================

volatile bool mpuInterrupt = false; // indicates whether MPU interrupt pin has gone high
void dmpDataReady() {
mpuInterrupt = true;
}

// ================================================================
// === INITIAL SETUP ===
// ================================================================

void setup() {
// join I2C bus (I2Cdev library doesn't do this automatically)
#if I2CDEV_IMPLEMENTATION == I2CDEV_ARDUINO_WIRE
Wire.begin();
TWBR = 24; // 400kHz I2C clock (200kHz if CPU is 8MHz)
#elif I2CDEV_IMPLEMENTATION == I2CDEV_BUILTIN_FASTWIRE
Fastwire::setup(400, true);
#endif

//Attach servo

myservoY.attach(9); // Attach Y servo to pin 9
myservoX.attach(10);// Attach X servo to pin 10

// initialize serial communication
// (115200 chosen because it is required for Teapot Demo output, but it's
// really up to you depending on your project)
Serial.begin(115200);
while (!Serial); // wait for Leonardo enumeration, others continue immediately

// NOTE: 8MHz or slower host processors, like the Teensy @ 3.3v or Ardunio
// Pro Mini running at 3.3v, cannot handle this baud rate reliably due to
// the baud timing being too misaligned with processor ticks. You must use
// 38400 or slower in these cases, or use some kind of external separate
// crystal solution for the UART timer.

// initialize device
Serial.println(F("Initializing I2C devices..."));
mpu.initialize();

// verify connection
Serial.println(F("Testing device connections..."));
Serial.println(mpu.testConnection() ? F("MPU6050 connection successful") : F("MPU6050 connection failed"));
/*
// wait for ready
Serial.println(F("\nSend any character to begin DMP programming and demo: "));
while (Serial.available() && Serial.read()); // empty buffer
while (!Serial.available()); // wait for data
while (Serial.available() && Serial.read()); // empty buffer again
*/
// load and configure the DMP
Serial.println(F("Initializing DMP..."));
devStatus = mpu.dmpInitialize();

// supply your own gyro offsets here, scaled for min sensitivity
mpu.setXGyroOffset(220);
mpu.setYGyroOffset(76);
mpu.setZGyroOffset(-85);
mpu.setZAccelOffset(1788); // 1688 factory default for my test chip

// make sure it worked (returns 0 if so)
if (devStatus == 0) {
// turn on the DMP, now that it's ready
Serial.println(F("Enabling DMP..."));
mpu.setDMPEnabled(true);

// enable Arduino interrupt detection
Serial.println(F("Enabling interrupt detection (Arduino external interrupt 0)..."));
attachInterrupt(0, dmpDataReady, RISING);
mpuIntStatus = mpu.getIntStatus();

// set our DMP Ready flag so the main loop() function knows it's okay to use it
Serial.println(F("DMP ready! Waiting for first interrupt..."));
dmpReady = true;

// get expected DMP packet size for later comparison
packetSize = mpu.dmpGetFIFOPacketSize();
} else {
// ERROR!
// 1 = initial memory load failed
// 2 = DMP configuration updates failed
// (if it's going to break, usually the code will be 1)
Serial.print(F("DMP Initialization failed (code "));
Serial.print(devStatus);
Serial.println(F(")"));
}

// configure LED for output
pinMode(LED_PIN, OUTPUT);

pinMode(IN1, OUTPUT);
pinMode(IN2, OUTPUT);
pinMode(ENA, OUTPUT);

}

// ================================================================
// === MAIN PROGRAM LOOP ===
// ================================================================

void loop() {
// if programming failed, don't try to do anything
if (!dmpReady) return;

// wait for MPU interrupt or extra packet(s) available
while (!mpuInterrupt && fifoCount < packetSize) {
// other program behavior stuff here
// .
// .
// .
// if you are really paranoid you can frequently test in between other
// stuff to see if mpuInterrupt is true, and if so, "break;" from the
// while() loop to immediately process the MPU data
// .
// .
// .
}

// reset interrupt flag and get INT_STATUS byte
mpuInterrupt = false;
mpuIntStatus = mpu.getIntStatus();

// get current FIFO count
fifoCount = mpu.getFIFOCount();

// check for overflow (this should never happen unless our code is too inefficient)
if ((mpuIntStatus & 0x10) || fifoCount == 1024) {
// reset so we can continue cleanly
mpu.resetFIFO();
Serial.println(F("FIFO overflow!"));

// otherwise, check for DMP data ready interrupt (this should happen frequently)
} else if (mpuIntStatus & 0x02) {
// wait for correct available data length, should be a VERY short wait
while (fifoCount < packetSize) fifoCount = mpu.getFIFOCount();

// read a packet from FIFO
mpu.getFIFOBytes(fifoBuffer, packetSize);

// track FIFO count here in case there is > 1 packet available
// (this lets us immediately read more without waiting for an interrupt)
fifoCount -= packetSize;

#ifdef OUTPUT_READABLE_YAWPITCHROLL
// display Euler angles in degrees
mpu.dmpGetQuaternion(&q, fifoBuffer);
mpu.dmpGetGravity(&gravity, &q);
mpu.dmpGetYawPitchRoll(ypr, &q, &gravity);
Serial.print("ypr\t");
Serial.print(ypr[0] * 180/M_PI);
Serial.print("\t");
Serial.print(ypr[1] * 180/M_PI);
myservoY.write(int(ypr[1] * -180/M_PI)+90); // Rotation around Y
Serial.print("\t");
Serial.println(ypr[2] * 180/M_PI);
myservoX.write(int(ypr[2] * 180/M_PI)+90); // Rotation around X
#endif

// blink LED to indicate activity
blinkState = !blinkState;
digitalWrite(LED_PIN, blinkState);

digitalWrite(IN1,HIGH);
digitalWrite(IN1,LOW);
}

}