Lab 5

Open Loop Control

0. Objective

Change from manual to open loop control, make the car able to execute a pre-programmed series of moves, using the Artemis board and two dual motor drivers.

1. Prelab

Pins use for control on the Artemis

Since I need the Pulse-width modulation (PWM) to control motor drivers, according to the Artemis Nano Schematic, those pins with ~ in front of it support PWM, therefore I pick A2, A3, 4 and A5 for my motor control pins. Note: those pins start with A support Analog to Digital Converters (ADC).

We recommend powering the Artemis and the motor drivers/motors from separate batteries. Why is that?

Each motor driver can deliver 1.2 A per channel continuously (2 A peak) to a pair of DC motors. In this lab, I have two motor drivers connecting in parallel, if attach the Artemis in parallel together, there may not have enough amps to support motor drivers which can cause the battery be drink out faster.

Hook up the motor drivers

To get the maximum amps, need parallel couple the two inputs and outputs on each motor driver.

For inputs, I use a tiny shunt jumper to connect B2_IN and A2_IN, then use a zero-ohm resistor to connect B1_IN and A1_IN. The reason for not using a bare wire here is to prevent a short circuit with B2/A2_IN when hooking motor drivers into the car and future debug. To make connections between the motor driver and Artemis Nano, I soldered a right-angle male pin on the B channel.

For outputs, there are two wires on each motor, green and white. I define the green wire to be connected to B1_OUT, and the white wire to B2_OUT and A2_OUT. Do the same thing for both motors. Also, use a zero-ohm resistor to connect B1_OUT and A1_OUT.

In the end, use a board-to-board connector pass through both VIN and GND pins to make two motor drivers as a stack.

Schematic diagram

Wiring

In order to pass sensor wires and two power wires (Artemis and motor drivers), I drilled a hole on the side of the battery holder, then pass all necessary wires to the battery holder by squeezing between two motors.

After wires passed the hole.

Finally, hook up sensors by using 3M 9448A Double Coated Tissue Tape.

2. What are reasonable settings for the power supply?

The DRV8833 dual motor driver datasheet states the operating voltage is 2.7‌‌ V to 10.8 V. I use the DC power supply and set 2.70 V along with maximum 2.0 A, connect it to one of the motor drivers. Found out the lowerbound is around 2.63 V. Video Demo

Next is the test on 3.70 V, this is the LiPo battery voltage. Video Demo

Also, I tested 5.00 V, but I don’t want to go over 5 V in case to protect motor driver overheat, although the datasheet states maximum is 10.8 V. Video Demo

3. Use analogWrite commands to generate PWM signals and show using an oscilloscope

The analogWrite() function ranges from 0 to 255, and I set it to 200.

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void setup()
{
  pinMode(A2, OUTPUT);
  pinMode(A3, OUTPUT);
}

void loop()
{
  analogWrite(A2, 200);
  analogWrite(A3, 0);
}

To detect PWM signals, I attached the probe to the motor driver input pins. Video Demo

4. Run the motor in both directions

I wrote three helper functions to perform forward, backward and stop. And wrote a sequence loop to turn wheels in both directions. Video Demo

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#define L1 A2
#define L2 A3
#define R1 4
#define R2 A5

void setup()
{
  pinMode(L1, OUTPUT);
  pinMode(L2, OUTPUT);
  pinMode(R1, OUTPUT);
  pinMode(R2, OUTPUT);
  neutral();
}

void loop()
{
  drive(255);
  delay(1000);

  neutral();
  delay(1000);

  reverse(255);
  delay(1000)
}

void neutral()
{
  analogWrite(L1, 0);
  analogWrite(L2, 0);
  analogWrite(R1, 0);
  analogWrite(R2, 0);
}

void drive(uint8_t speed)
{
  analogWrite(L1, 0);
  analogWrite(L2, speed);
  analogWrite(R1, speed);
  analogWrite(R2, 0);
}

void reverse(uint8_t speed)
{
  analogWrite(L1, speed);
  analogWrite(L2, 0);
  analogWrite(R1, 0);
  analogWrite(R2, speed);
}

5. The lower limit for each motor still turns while on the ground

To test the lower limit, I use analogWrite starting from 0 and going up to 255, increment 1 step every sec.

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for (int speed = 0; speed < 255; speed++)
  {
    Serial.println(speed);
    analogWrite(R1, speed);
    delay(1000);
  }

The lower limit for left wheels is 97. Video Demo

The lower limit for right wheels is 107. Video Demo

6. Move in a fairly straight line

I put my car on the ground, measured approximately 2.520 meters distances, then powered both motors using analogWrite(100) without a calibration factor and it moves in a pretty straight line. Video Demo

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#define L1 A2
#define L2 A3
#define R1 4
#define R2 A5

void setup()
{
  pinMode(L1, OUTPUT);
  pinMode(L2, OUTPUT);
  pinMode(R1, OUTPUT);
  pinMode(R2, OUTPUT);
  neutral();
}

void loop()
{
  drive(100);
  delay(2000);

  neutral();
  delay(10000);  
}

void neutral()
{
  analogWrite(L1, 0);
  analogWrite(L2, 0);
  analogWrite(R1, 0);
  analogWrite(R2, 0);
}

void drive(uint8_t speed)
{
  analogWrite(L1, 0);
  analogWrite(L2, speed);
  analogWrite(R1, speed);
  analogWrite(R2, 0);
}

void reverse(uint8_t speed)
{
  analogWrite(L1, speed);
  analogWrite(L2, 0);
  analogWrite(R1, 0);
  analogWrite(R2, speed);
}

7. Open loop, untethered control

Unplug the USB type-C cable, install two 850mAh batteries to power motor drivers and Artemis board separately, then I write a program that performs forward then left turn. The robot runs autonomously. Video Demo

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void loop()
{
  drive(100);
  delay(400);

  neutral();
  delay(1000);

  left(255);
  delay(450);

  neutral();
  delay(1000);
}

8. Additional Tasks

What frequency analogWrite generates?

By using analogWrite(200), the oscilloscope shows the frequency is roughly 183 Hz (bottom right cornor) and 78.4% duty cycle. One of the benefits of manually configuring timers, is a faster PWM signal can have much more resolutions and controls motor drivers more precisely. e.g. (0 to 255) V.S. (0 to 4095).

Write a program that ramps up and down in speed slowly.

The analogWrite range is from 0 to 255, I start from 0, add 15 each time the program executes the loop, when it reaches 255, decrease by 51 each time until it reaches 0. There are two states in this machine, either ramp_up or ramp_down, defined a boolean variable called “ramp_up” to indicate which state.

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analogWrite(L1, 0);
analogWrite(R2, 0);
analogWrite(L2, spd);
analogWrite(R1, spd);

if(spd == 255)
  ramp_up = 0;
else if(spd == 0)
  ramp_up = 1;
  
if(ramp_up)
  spd += 15;
else
  spd -= 51;

Reporting the values to the computer using Bluetooth when ramp up procedure is over.

Based on previous code, I plan to measure the ramp up procedure, to perform this, I use the ToF sensor and record the distance, current time and current speed, wrap up everyting into a EString object and send it over the BLE.

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void get_tof()
{
  distanceSensor.startRanging(); //Write configuration bytes to initiate measurement
  while (!distanceSensor.checkForDataReady())
  {
    delay(1);
  }
  int distance = distanceSensor.getDistance(); //Get the result of the measurement from the sensor
  distanceSensor.clearInterrupt();
  distanceSensor.stopRanging();

  Serial.print(millis());
  Serial.print(", ");
  Serial.print(spd);
  Serial.print(", ");
  Serial.print(distance);
  Serial.print("\n");
  tx_estring_value.clear();
  tx_estring_value.append(millis());
  tx_estring_value.append(", ");
  tx_estring_value.append(spd);
  tx_estring_value.append(", ");
  tx_estring_value.append(distance);
  
  tx_characteristic_string.writeValue(tx_estring_value.c_str());
}

On the Jupyter Lab side, first I create a new characteristics called RX_MYSTRING, then use notify callback function to receive bytearray and convert to string.

Then I put the car to the wall, and let it speed up to the opposite wall. On my laptop, give a start signal to the Artemis and starting the notify on jupyter lab to capture data. Video Demo

After the experiment, I got the time versus distance data, below the x-axis is time in millisec, y-axis is distance in millimeters.

Clean the dirty data captured by ToF, since the ToF sensor has a measurement range.

Calculate the top speed based on two sample points (0.041-1.758) / (16.753-16.121) = -2.716772 m/s. The range of speeds roughly [0, 2.716772] m/s.

This experiment could be inaccurate, depending on the surface of the floor, friction could be different, battery level may also affect the top speed of motors.