Tracking Raspberry Pi Robot's Distance with Encoders | Twitter Controlled Robot Ep 4

Hello and welcome back to a robotic series. In our last video, we figured out how fast the motor is spinning and what direction it was moving. But we don't know how far it's traveling. To do that you might think that we could set a throttle value and just give it a certain duration, track how far it's gone, and use that as a calibration for the distance traveled. But the problem is that the battery voltage is going to change. If you remember from our last video, that's what dictates how fast our motor is moving. So as our robot is driving around, it's draining the battery and the voltage is decreasing. And if we look at these PWM graphs on the oscilloscope, we can see that a fresh pack of batteries gives us about 6 volts for our peak voltage. And a drained battery is going to drop down to about 5 volts. So we can't use that to measure the distance. But we can use an encoder. And that's going to track the number of rotations that a wheel makes. And these encoder disks go on the outside of the motor. And as the motor rotates, so does the encoder disk. And I'm gonna call them encoder disks, because I don't want to get them confused with wheels. And as we're tracking these with sensors, we'll read high and low values. And these state changes will tell us how far we've rotated. And the wheel's rotations times its circumference will give us the distance that it's traveled. These encoder disks have two states that are going to cause a sensor to go between a high and a low. And if we had a phrase of the video it would be "state changes." We need to really track the number of states that we're going through in order to know how far our wheel has rotated. For this encoder disk, there are 12 state changes for every one rotation. And if we're tracking it and we see 15 state changes, we know that we've moved one and a quarter turns. Let's look at different types of encoder discs and how to use them to track state changes. We have an optical encoder and a magnetic encoder. For the optical encoder, there's a plastic disk with twenty slits and twenty blockages. The magnetic disk has four North and four South magnets around the outside. The magnet types alternate, which is why the permanent magnet jumps as we're moving it around. Each encoder has a sensor that reads these state changes. The optical encoder uses an IR light sensor. The slots in the disk allow the light to travel from the transmitting diode to the receiver. And this is just like the break beam sensors that we used in our giant ramp. The metal ball was able to trip the sensors as it rolled down. The receiving half of the break beam sensor had three wires a power, a ground, and a signal The signal wire went to the digital input of the Arduino. And depending on whether the light was received or blocked, we either got a high or a low. The optical encoder has a five volt power pin, ground pin, and signal pin. When wired to the Raspberry Pi, a red LED turns on. This lets us know that the board is powered and that the IR light is being received. And if we block this light, the LED turns off and the signal on the oscilloscope jumps from zero volts up to five volts. So we've gone from a low to a high. For the magnetic encoder disk, a Hall effect sensor is used. It also has a five volt power wire, a ground wire, and a signal wire. The oscilloscope jumps from a low to a high as the Hall effect sensor moves across the magnetic disk. Attaching the optical encoder disk the motor, we can see that there are a series of state changes. This disk is attached to the gearbox that's opposite the wheel. There is a one to one relationship between the rotation of the encoder disk and the wheel. When we look at the magnetic disk, we see there are many more state changes. This disk is attached to the top part of the motor and is not on the geared down shaft. The gearing means that the motor turns 48 times for every one time that the wheel turns. This provides a higher torque value to the wheel. This means we have to multiply the eight state changes that the motor seized by 48. So we know how many use state changes that the wheel sees. And when we do this we get 384 state changes for every one rotation of the wheel. The optical encoder has a resolution of 40 and the magnetic encoder has a resolution of 384. And we wanted to be able to control these encoders. So we added some lines of code to our existing motor program. This is not at all what would use to control the robot. It's just for playing around with the encoders and getting a feel for how they work. So at the top we defined our GPIO pins and set is a read value. So we want to know if it's a high or low from pin 17. then we need to track some things, the last state, we saw the current state, the number of rotations, the number of steps. And we want to know the distance traveled per step (state change). So we took the circumference of the wheel and divided it by the number of steps per rotation. And then I set up a try and accept part, because when I didn't have this and I exited a program the motor just kept running. So when we hit control-c now it's gonna set the throttle value to zero for the motor, and it's gonna stop. Then we have a while loop. We're asking a couple of questions. First we want to know what is our value that we see from pin 17. And if this value is different from the last value, that we read then we need to increment the number of state changes. And if the number of state changes is equal to the number of states per rotation, then we need to increment the number of rotations we've seen so far and set our tracker back to zero. To figure out the distance we've traveled and not use whole rotations, we can look at the total number of state changes we've seen and multiply it by the distance we have for every state change. And this is the basic program we have for our encoders. If you want to learn more about encoders, we have a few more videos where we talk about quadrature encoders and that will tell you what direction you're rotating in. And absolute encoders that tell you a unique position that you're at with your motors rotation. If you've any questions or comments you can leave them down below. In our next video we're going to actually put the motors and two encoders onto this 3d printed chassis and start wiring everything up. So thank you guys so much for learning with me today, and I'll see you next time.

Channel: SciJoy

Views: 38,639

Keywords: encoder, incremental encoder, raspberry pi encoder motor, raspberry pi, circuitpython, python, oscilloscope, scijoy, raspberry pi robot, robot encoder, motor controller, raspberry pi zero, encoder tutorial, sparkfun, tinysine, hall effect, optical encoder, magnetic encoder, digital signal, electronics, tt motor, pi robot, python robotics, python (software), raspberry pi (computer), python (programming language), circuitpython raspberry pi, tinysine encoder, sparkfun encoder

Id: cLtMcqRetO0

Length: 5min 48sec (348 seconds)

Published: Thu Jan 24 2019