Category: robots

Project Hexapod Part 3: No plan survives contact with the enemy

Post author By bob
Post date June 13, 2020

Design change 1:

Originally, I was going to control the hexapod from an Arduino pro mini. Partly because that’s how I originally started the project. But also, because I’d heard ropey things about raspberry pi’s and Python controlling servo’s in real time. I was however planning to use a raspberry pi zero-w as a master. The control system would connect to the Bluetooth adapter in the PI, some of the more complex calculations could be done on the PI, and then the PI would send messages to the Arduino which was in charge of moving the legs.

When I’ve seen people on YouTube do this (Tom Stanton, James Bruton), they often take this approach. But that’s partly because they seem to have a remote-controlled background and are using devices like RC car controllers or drone controllers (and the associated electronics on the robot end). I don’t have those devices and don’t have that experience.

After playing with the Arduino code a bit, I’m starting to think that idea is a pain in the ass. There’s the physical weight of the Arduino, but also the additional power draw it will take. The calibration of the servo’s and the interfacing between the pi and the Arduino seems like extra work. And configuring the servo’s end stops on the Arduino seems like a massive kludge.

Most of my coding experience is with mobile apps. My thought process was to write a smart phone app to connect to the device over Bluetooth. Therefore, I’m not going to be going down the physical handheld controller route.

In addition, I have more experience with UNIX based systems. So once the remote is walking, I’m more interested in doing client / server / web stuff where the robot is interacting with both it’s environment but also the web. Whilst I have very little experience in Python, I have more experience in that kind of UNIX based scripting language than the C based Arduino language. I’m not a natural developer so I need to minimise the number of language’s I’m learning. I expect I’ll have more usage out of Python from a professional and personal projects perspective

For all of that, I’m now planning on ditching the Arduino and seeing just how difficult it is to get the pi zero-w to control the servo’s using the PCA985 PWM controllers over I2C.

projects robots Uncategorized

Project Hexapod Part 2: More walking design thinking

Post author By bob
Post date June 13, 2020

Each leg on this robot has 3 motors; a shoulder, an elbow, and an ankle

To make things easier I labelled them up like this:

There are 18 separate motors to control:

			Board	Servo				Board	Servo
Left	Front	Shoulder	Pwm1	8	Right	Front	Shoulder	Pwm1	7
Left	Front	Elbow	Pwm1	10	Right	Front	Elbow	Pwm1	5
Left	Front	Ankle	Pwm1	13	Right	Front	Ankle	Pwm1	2

Left	Middle	Shoulder	Pwm1	9	Right	Middle	Shoulder	13	6
Left	Middle	Elbow	Pwm1	14	Right	Middle	Elbow	14	1
Left	Middle	Ankle	Pwm1	15	Right	Middle	Ankle	15	0

Left	Back	Shoulder	Pwm2	8	Right	Back	Shoulder	Pwm2	7
Left	Back	Elbow	Pwm2	9	Right	Back	Elbow	Pwm2	6
Left	Back	Ankle	Pwm2	10	Right	Back	Ankle	Pwm2	5

Design Thoughts

First thought was to create a function that controls each limb. At the least every limb should move in the same way. However, whilst limbs move together from the gait perspective (Figure 2) I don’t think we can do that from an Arduino perspective. I don’t think I can create a function that says move limbs L1, R2, L3 at the same time. I think I’ll need a function that does that. But then separately what it’s doing is moving “Left Front Shoulder, Right Middle Shoulder, Left Back Shoulder” together, then “Left Front Elbow, Right Middle Elbow, Left Back Elbow”, etc. until that function is complete.

I think I’ll have a multidimensional array with degrees in it for each joint. What I might do though is write a routine which sets each joint to 0. Then unscrew and reattach the joints so I know where zero is on each motor. Either way, I probably need a few lists to define:

the start and end position of each joint for a forward motion

the start and end position of each joint for a circular motion

the start and end position of each joint for being sat still.

That currently looks like this:

Name	Board	Servo	servo_min	servo_max
LeftFrontShoulder	Pwm1	8	500	500
LeftFrontElbow	Pwm1	10
LeftFrontAnkle	Pwm1	13
LeftMiddleShoulder	Pwm1	9
LeftMiddleElbow	Pwm1	14
LeftMiddleAnkle	Pwm1	15
LeftBackShoulder	Pwm2	8
LeftBackElbow	Pwm2	9
LeftBackAnkle	Pwm2	10
RightFrontShoulder	Pwm1	7
RightFrontElbow	Pwm1	5
RightFrontAnkle	Pwm1	2
RightMiddleShoulder	13	6
RightMiddleElbow	14	1
RightMiddleAnkle	15	0
RightBackShoulder	Pwm2	7
RightBackElbow	Pwm2	6
RightBackAnkle	Pwm2	5

Maybe a piece of code that takes serial input and resets the device to zero’s.

Control System:

Due to its built in Bluetooth, I’m thinking about having a raspberry pi zero-w connected to an Arduino nano over USB Serial:

https://maker.pro/raspberry-pi/tutorial/how-to-connect-and-interface-raspberry-pi-with-arduino

ser=serial.Serial(“dev/ttyACM0”,9600)

Then I can both ssh into the robot to tell it what to do. And I can also write a phone app to act like a remote control, connecting over Bluetooth. The v5 of the robot car did that.

robots

Project Hexapod Part 1: Walking Design

Post author By bob
Post date June 13, 2020

The design of the hexapod looks like this. Each “limb” comprises of a shoulder, elbow and a foot. The shoulder moves in a horizontal plane, left and right. The elbow and feet move in vertical plans, up and down.

To control the hexapod, I need to consider stability. My current plan is to move three legs at a time. Something like this

This is an example gait I’ve found online. It resembles how an ant walks. So either side always has at least one foot on the ground and the robot should be balanced. And this should be quicker (and easier) than moving each foot at a time.

#Problem – how do I make walking asynchronous? If I have a single function to move a limb, how do I move two limbs at once?

#Answer

/* Sweep by BARRAGAN <http://barraganstudio.com> This example code is in the public domain. modified 8 Nov 2013 by Scott Fitzgerald http://www.arduino.cc/en/Tutorial/Sweep */ #include <Servo.h> Servo servo1; // create servo object to control a servo Servo servo2; // create servo object to control a servo // twelve servo objects can be created on most boards int pos = 90; // variable to store the servo position void setup() { servo1.attach(8); // attaches the servo on pin 9 to the servo object servo2.attach(7); // attaches the servo on pin 9 to the servo object } void loop() { for(pos = 90; pos >= 19; pos --){ servo1.write(pos); servo2.write(180-pos); } //delay(5000); for(pos = 19; pos <= 90; pos ++) // goes from 19 degrees to 90 degrees { servo1.write(pos); // tell servo to go to position in variable 'pos' servo2.write(180-pos); // tell servo to go to position in variable 'pos' delay(15); // waits 15ms for the servo to reach the position } }

Note that this is my design process as I try and plan how I’m going to make this work, and part of that is trying to understand what’s possible. So this isn’t my code, but an example of how to control the servos I found here .

projects robots

Project Hexapod: Introduction

Post author By bob
Post date June 13, 2020

In 2018 I was building lots of Arduino projects. One of which was a robot car which sense walls using ultrasound to sense when it was about to drive into a wall. In the video below I connect via bluetooth, then enter some commands into the Arduino serial console

https://www.youtube.com/watch?v=-6rlZxKwQQU

To build on this I decided to make a hexapod – a walking spider.

https://www.instagram.com/p/B-UYLZ4gD9uStzXugmaBnCfFzCwmCFCYKgSnoE0/

The parts are still available here, although there isn’t a codebase to use the robot – you need to make that yourself. And that’s what this new project is about.