Robot Head Mark 2

Around 3 months ago I started a new robot project. I decided that I wanted to build another robot head that could sit on a desktop. I had a few goals in mind when starting the project. I wanted to 3D print the parts and try out some finishing techniques on the parts. Nothing too fancy, just sand them smooth and paint, to see how good a finish I could achieve. I also wanted the robot to be low-cost and simple. I mean really simple, just two servos and a couple of sensors. This was very intentional. I want to expand on the work of my previous project, the desktop robot head and arm, by concentrating on the software. I found I was too often dealing with mechanical issues or limitations and these were an all to welcome distraction from the software. I like making things and having something physical to show for the hard work, whilst I find that with software there isn’t always that sense of satisfaction of having created something. However, the software is where the robot really comes to life and I need to get better and more focused at writing it and implementing the ideas that I have. To that end I wanted a simple robot with very little to go wrong that can serve as a platform to develop some interesting functions. This project will hopefully pull together a lot of the work from previous projects into one robot, and I intend to craft the software with care. I want to squeeze as much functionality out of this simple robot platform as possible. I also wanted to take the opportunity of a new project to switch from using TKinter for the GUI to using wxPython.

I started the process by designing the robot in FreeCad. I did more design work on this than in previous projects and really spent my time on the modelling. I even put the parts together into an assembly to help decide on the colour scheme. I have started another youtube series following the design and build of the robot. Part 1 is below.

This video shows the design and printing of the robot and at the end of it I had a pile of 3D printed parts ready for finishing. The design is a simple pan/tilt robot head, with a camera and a sonar sensor mounted in the head. I want to build on my previous work with point clouds so I wanted a sonar mounted on the head. I don’t think I will end up creating point clouds with this robot but I will likely use the techniques to come up with a different way to represent the environment. However, I didn’t want the sonar sensor looking like the robots ‘eyes’ as they often do, so I tried something new with this project. I printed a mesh to mount in front of the sonar and, combined with a piece of filter fabric, made a cover for the sensor. Later testing would reveal if this was successful or not.  The only other feature of the head was the robots actual ‘eye’. In place of the TFT screen used in the last project I opted for an RGB LED in a custom enclosure that would furnish the robot with a single colour changing eye that it can use to attract interest or convey emotions.

Part 2 shows the finishing of the 3D printed parts.

This involved a lot of sanding and priming before finally painting the parts. I was overall really pleased with the finished parts. It become clear from this process that the quality of the final finish is directly proportional to the amount of finishing work put in. I also took away a few other lessons from this process when designing the parts for finishing. Sharp corners and deep recesses are difficult to sand and remove all of the build lines without removing too much material. Smooth, rounded surfaces or large flat surfaces are easier to sand and look good when painted.

Part 3 of the video series documents the assembly of the robot.

With the robot assembled I was able to do some testing. I decided to use an Arduino Nano for this project, in-keeping with the minimalist and simplicity goals of the project. I knocked up a bread board circuit for testing and set about testing the RGB LED eye and the sonar sensor. Good news was that the sonar still seemed to function just fine from behind its cover, and the RGB LED worked as expected.

I initially connected the RGB LED to digital outputs, thinking that the few colours that this yields would suffice. I subsequently decided to use PWM outputs, so that a wider range of colours could be generated. Given that the robot was so simple, I thought this was a reasonable extravagance.

Part 4 shows testing of the RGB connected to PWM outputs and the first test of the servos.

This video also represents my first simple programs written using wxPython. So far, I like it. The GUI’s look better than their TKinter counterparts and overall it is not much more complex, if at all. I created a couple of test programs, one that sends commands via serial to set R,G and B values for the LEDs and another that sends servo positions. Code for the Arduino has also been written to receive these commands and set the RGB outputs and servo positions accordingly. I can also send get commands to the Arduino and it will return values as appropriate.

This brings us almost up to date on this project. There will be a new video soon, so be sure to subscribe to my youtube channel. I have been working on embedding OpenCV images and MatPlotLib plots in wxPython windows, as well as making a more permanent circuit for the robots electronics.

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Youtube series Part 2 and 3 now available

Part 2 of my Youtube series following the development of my latest robot project, a desktop robot head and arm, has been up for a week or so now and I have just finished Part 3. Part 3 covers the testing of the hand switch and how I am starting to develop the code for both the Arduino and the controlling PC.

 

 

 

 

 

If you enjoy the videos, please subscribe as I plan to continue making these as often as time allows. If you would like more information on any aspects of the robot, drop me a message and I can go into more detail in a future video.

YouTube video series

Happy Robot

 

I am managing to find a bit more time lately to progress some of my projects, in particular the desktop robot. I have started work on a robot arm to accompany the robot head as I suggested in the last post. So far the base and lower joint of the robot arm have been designed in Freecad and 3D printed. I am currently working towards completing the robot arm. At the moment I don’t intend to fit a gripper to the end of the arm but instead use a custom designed touch sensor to allow the arm to get feedback from the environment. But you can bet that a gripper will be on the cards at some point in the future! To document and share the work on the project, I am making a series of YouTube videos to show you what I am up to. I admit, I would have liked to have started doing this at the beginning of the project but better late than never, right? I will also admit that I am not a fan of talking in my videos, or the sound of my own voice, but its the easiest way to explain what I am doing. I will get used to it and I’m sure I will get more comfortable as time goes on.

If anyone out there would like more information on any part of the build, let me know via a comment on here or on YouTube. I do intend to focus on particular parts of the project in future videos, with a sort of tutorial feel to show you how I implemented various features.

Below is the first video of the series, which gives an introduction to the project and shows some of the work that I have been doing. My aim is to release a video every week, but time will tell how realistic this aim is. I hope you enjoy the videos and I am working on the next one now, which should be available tomorrow.

 

Latest robot project – A desktop social robot

I have been working on a new robot project for the last few months.  I like desktop robots and having read about social robots like Jibo and Buddy I decided I would like to try and create a low cost version of one of these that can sit next to my computer and maybe interact with me and my children. I wanted to try and design some more complex components for 3D printing and thought that this project would be a good opportunity. I decided to give FreeCAD a shot as it looked like a promising open source 3D design package.  After negotiating the expected learning curve I was impressed with the range of functionality that FreeCAD has and I was able to design some cool looking parts for my new robot. The idea was to design a desktop robot with a display and a camera integrated into the head and use servos to pan and tilt the head to enable to robot to look around the room. As the project progressed, as is often the case, it evolved and the robot ended up with an extra servo that enabled the head to roll as well. I wanted to incorporate an accelerometer into the head to track its position and went with a cheap MP6050.  For the display I used an adafruit 2.2″ tft screen that I have used in previous projects. I had a webcam, that I stole from my mobile robot that I mounted in the head as well. I also used this project as an excuse to learn how to use kiCAD for pcb design. I used this to design a shield for the arduino mega to interface all of the sensors and servos.

Below is a picture of the finished robot.

Social robot

Social robot

I was particularly pleased with the lettering on the bottom servo housing, easy to do with FreeCad. I was also pleased with the bracket that connects the roll and tilt servos, as shown below.

Roll to tilt head bracket

Roll to tilt head bracket

The accelerometer mounts to the head and, using this library, is able to spit out roll, pitch and yaw angles.

MP6050 mounted to head

MP6050 mounted to head

After some calibration I have been able to control the servo positions using the accelerometer readings very reliably. A simple control loop for each servo is all that is required to position the head at any combination of roll, pitch and yaw angles.

I have a lot of work to do on the software, but I wanted to share the project now that the mechanical and electronic build is complete. I was going to use the raspberry pi for this project but I have decided to use my desktop computer for now, but I may decide to use the pi at a later date. Previously I was driving the tft screen from the pi but I am now writing to the screen from the Arduino. The screen will display a face, probably just simple shapes for now, to allow the robot to display some emotions.

I am also planning to design a robot arm it some point, and I would like this robot to be able to control the robot arm. I am thinking of possibly having a few modular parts, like arms or other sensors, that can work together. I am not sure how this will happen at that moment but its fun to think about.

Come back soon as I hope to have a video of the head moving around and controlling its position up here when its working.

 


 

New battery and some design revisions. BFR4WD MK2

Work was progressing with BFR4WD and I was experimenting with software for object/landmark recognition as well as developing BFR-Code for controlling the robot. However, I started having problems with the old NI-MH battery pack I was using to power the servos.  It wasn’t holding its charge for long and the servos were constantly struggling to move the robot. This battery pack had sometimes struggled to deliver the current required when all the wheel servos were working hard when it was working well. I decided it was time for a new battery. This led to a succession of design revisions and many parts of BFR4WD have been redesigned and rebuilt.

Lets start with the choice of a new battery. I looked into the various options available  and settled on a 6V SLA battery to power all the servos. SLA batteries are cheap, easy to charge and able to supply a lot of current if required. The downside is that they are heavy! During the original design of BFR4WD I made the decision to gear the servos to increase the top speed of the robot at the expense of torque to the wheels. This was fine using the previous battery but the additional weight of the SLA battery means this gear ratio would no longer be suitable. I decided to redesign the drive train for each wheel so that the required torque could be delivered. This meant designing some more 3D printed gears with a ratio that increased torque at the expense of speed. I ended up with a 20 tooth gear attached to the servo and a 24 tooth gear on the wheel axle.

I used this redesign as an opportunity to review the design of the wheel encoders as well. Whilst the previous design worked well, the encoders were mounted in a place the meant most of the robot had to be dismantled in order to adjust, repair or replace them. Looking into various options I settled on sticking with an optical encoder but instead of a slotted disk I would use a printed pattern. This design involves printing the encoder disk pattern onto transparency film using a laser printer. A slotted opto-interrupter looks at this disk and switches depending on whether a printed line is present or not. The final design ended up with the patterned disk attached to the servo gear meaning I could also do away with the third gear/slotted disk of the previous design. This has the advantages of fewer parts to print, fewer holes to drill and fewer moving parts. This all adds up to a simpler design which is lighter and runs more quietly. The picture below shows an encoder disk mounted to the servo gear.

Encoder disk

Encoder disk

With the alterations to the gear ratio and the new design for the encoders I was also able to increase the encoder resolution from 180 pulses per revolution to 218.

A redesign of the encoder circuit was also required and I decided to use Sharp GP1S093HCZ0F opto-interrupters. These devices are very small and allow a very compact encoder to be built. A test on a breadboard allowed me to find the correct resistor values to use (10K and 220 Ohm) and test that the device could read the encoder disks as required. The pictures below show the breadboard version and the final encoder circuit which was built on stripboard.

Encoder circuit on breadboard

Encoder circuit on breadboard

Encoder circuit

Encoder circuit

To allow the new design drive train to fit in the aluminium box section I’m using to house it, I had to space the servos off from the box section by 3mm. I designed and printed a spacer that would hold both of the servos required for one side of the robot chassis.

Servo Spacer

Servo Spacer

A 3D printed bracket was designed that would mount the encoder circuits in the correct position and allow for some adjustment. These encoders are mounted in the centre of the chassis rail as opposed to the ends as in the previous design. This configuration makes them a lot easier to adjust. Shown below is the finished encoder arrangement mounted in the chassis.

Mounted encoder

Mounted encoder

This encoder design and gear configuration also meant that I could reduce the overall size of the robot. This design is now 40mm shorter in length but 10mm wider. This has the advantage of setting the wheels closer together along the length of the robot but spacing them further apart across the width of the robot. This helps when the robot is turning on the spot.

I had to fabricate all new chassis parts to fit the new design, one of which is shown below.

Chassis rail

Chassis rail

The picture below shows one of the chassis rails with the servos, gears and encoders mounted. I reused the end joining pieces from the previous design. I also changed the bushes that the axles run in from a push fit design to ones that glue into place.

IMG_1608

Due to reducing the overall size of the robot and the addition of a larger battery, space on the chassis to mount everything had decreased significantly. I was forced to start building upwards! I designed some 3D printed stand-offs that would allow me to mount another sheet of HDPE above the first, as shown below.

3D printed stand-offs

3D printed stand-offs

This meant that the two batteries could sit on the bottom layer which allowed good access to the connect / disconnect power to them. The Raspberry Pi, Arduino and USB hub could go on the top layer, again giving good access to connect to them. I also added a compass module to the robot. A CMPS03 that I have had lying around for a long time.  I designed and printed a mount for the compass and used a length of aluminium box section to raise the compass into the air. My experience is the further the compass is from any other components the better as it reduces interference.

The picture below shows the completed robot. So many things have changed with this redesign I think this will have to be referred to as BFR4WD MK 2. Initial testing confirms that I have much more torque at the wheels due to the change in gear ratio and better current delivering capacity of the battery. The encoders are working perfectly and I have increased resolution meaning more accurate positioning and speed control. The compass works and will be used for navigation. I am very keen to try some more map building using a mobile robot with the addition of data from visual images.

BFR4WD - MK2

BFR4WD – MK2

3D printing a new robot

I was very pleased with the way BFRMR1 turned out, but it had some design flaws that I needed to address. The servos used for the drive wheels were a bit too slow. The drive wheels were positioned near the centre of the robot to help limit the size of the turning circle, but meant that the robot would tip forwards when stopping. It also meant that I couldn’t mount anything to the front of the robot, such as a gripper. Wheel encoder resolution was also a bit limited. An idea started forming in my mind for a new robot.

The idea was to make a four wheeled robot, with each wheel driven independently. I wanted to stick with using servos to drive the wheels. I love servos. They are cheap and very easy to control. But they can be slow! My idea evolved to making a gearbox to speed the servos up a bit, whilst taking the hit of losing a bit of torque. However, for this new robot it wouldn’t matter too much as I was doubling the number of drive wheels.  I thought of several ways of gearing the servos. Drive belt and pulleys was the first option but I decided to go for gears instead. I could have bought the required gears but I thought that this project was as good an excuse as any to invest in a new piece of equipment, a 3D printer!

After a bit of research I decided on a prusa i3 printer bought as a kit. I painted the aluminium frame and after a few days and a couple of long nights I had my printer assembled and working.

Prusa i3 3d printer

Prusa i3 3d printer

After calibration and a number of test prints, I set about designing some gears to form a gearbox.  I used OpenSCAD to design all the parts for the robot. To design the gears I downloaded a gear generator from thingiverse http://www.thingiverse.com/thing:3575. I started with a 25 tooth gear that would be connected directly to a servo horn, then a 14 tooth gear that would be connected to the wheel drive shaft. Attached to the 14 tooth gear is a 45 tooth gear with a finer pitch to drive an encoder disc with a 14 tooth gear attached. All of this together would increase the top speed of the servo and give me an encoder resolution of 180 pulses per wheel revolution. It took a few tries to get each of these gears right and some of the prototypes are shown in the picture below.

3D printed gear prototypes

3D printed gear prototypes

With all of the gears designed I made a trial gearbox. I wanted to use aluminium rectangular box section to house the gearbox. This means all of the gears are hidden and contained and also means that the gearbox could form a part of the robots chassis. The prototype gearbox just used a short section of the aluminium box as a test. The picture below shows the gearbox from the end with the encoder disc nearest the camera.

Prototype gearbox

Prototype gearbox

The final gearbox design used a long length of aluminium box section with two servos attached and two gearboxes within. This would form the drive for one side of the robot. Access holes were cut into the box section so allow assembly and adjustment of the gearbox and the picture below shows the view into one of the access holes. You can see the two servos mounted with gears attached and the drive shaft passing through the box section with its gear attached. The encoder discs are hidden.

One completed gearbox

One completed gearbox

I also designed and printed some bushes for the drive shaft to run in that clipped into holes drilled in the aluminium box section. The hole through the middle of these is slightly undersized so that they can be drilled out to exactly the right size for the shaft to fit in.

3D printed bushes

3D printed bushes

The encoders consists of a 28 slot encoder disc and a photo-interrupter to detect each of the slots as the disc turns. I decided on using sharp GP1A52LRJ00F slotted optical switches. These have photologic outputs so only a minimum of external circuitry is required to interface these with the Arduino. In fact only one resistor is needed so I used stripboard to make four encoder circuits that were then mounted inside the aluminium box section with the encoder discs turning between the sensors.

With two gearbox/chassis sections made I had two sides of the chassis. To join these together and make a complete chassis I needed to design some brackets. These brackets attached aluminium box section cross members to the gearbox sections to make a rectangular chassis. These are shown in the picture below.

Chassis bracket

Chassis bracket

One feature I wanted for this robot was the ability to separate the electronics and sensors from the chassis easily. To achieve this I decided to mount the Arduino mega, the Raspberry Pi, the batteries and the USB hub on a sheet of HDPE plastic that would then be bolted to the chassis with four bolts. Should I need to work on the chassis in the future I could just undo these four bolts, disconnect the encoders and drive servos from the Arduino and remove the electronics board. I also decided to mount the head pan/tilt mechanism to this board as well. The picture below shows the chassis with the electronics board attached.

Assembles chassis and electronics

Assembled chassis and electronics

The head pan/tilt mechanism consists of two regular servos and some 3D printed brackets. The picture below shows the bracket that attaches the pan servo to the electronics board.

Servo bracket for head pan/tilt

Servo pan bracket

Attached to the pan servo is the tilt servo via another 3D printed bracket. I designed a further piece that fixes to the tilt servo that the head can be bolted to, all shown in the picture below.

Head tilt servo bracket

Head tilt servo bracket

The head of the robot houses a sonar sensor and a webcam. See the picture below showing the assembled head attached to the pan/tilt mechanism.

3D printed head

3D printed head

With all of this done the robot is almost mechanically complete. I need to design and print some mounts for two IR sensors that will probably mount to the electronics board either side of the pan/tilt mechanism. The other job to do is to design and print a housing for a small screen and some buttons for controlling the robot without having to connect to it with another PC.

BFR4WD almost complete

BFR4WD almost complete

I have been developing software for the new robot alongside the mechanical build. I have modified the wheel control loop software from my previous robot to now control 4 wheels at the same time. A lot of the software from the BFRMR1 can be used in this project but one thing that I knew needed work was the communications between the Arduino and the Raspberry Pi. I was using serial communications but I never really liked the protocol I was using, that I developed so I can’t even blame anyone else for it. I am sticking with serial comms but wanted an improved protocol. Inspired by G-code as used on 3D printers I decided to come up with my own protocol to send commands in the form of strings to the robot. I’m calling it BFR-Code for now! The basic idea is that movement commands or requests for data can be sent to the robot along with some data to determine how to move. So a move command string will start with a capital letter M followed by a number to determine the type of move and then any data required proceeded by a capital D. So the command M1 D200 would drive the robot forward 200 encoder ticks. Error codes and data can be returned to the Raspberry Pi in a similar manner. This whole thing is a work in progress and I will make a blog post in the future with full details if this works out well.

For now I am continuing work on the software but I am near to making a video of the robot in action so check in again soon!

BFRMR1 video

I have finally got around to making a video of BFRMR1 in action. The video shows some of the features of the robot and obstacle avoidance and colour tracking in action.

During testing of the robot I found that at higher speeds, when the robot stops, the front of the robot dips a little bit. This causes the IR sensors to see the floor and leads the robot to believe there is an obstacle where there isn’t one. This was always a possibility as the drive wheels are quite close to the centre of the robot and I was relying on the weight of the batteries to keep the back end of the robot weighted down. I put the wheels where they are so that when the robot turns, it does so within its own footprint. This was to hopefully stop the robot bumping in to things when it turns. To overcome this issue I decided to add some “legs” to the front of the robot that do not contact the ground during normal running but are there to stop the robot tipping forward when it stops from high speed. These are pictured below.

"Legs" at the front of the robot

“Legs” at the front of the robot

I chose this solution as the issue only occurs when running the robot quickly and I didn’t want to add any weight to the rear of the robot after all the effort of lightening the robot with the carbon fibre shell! These “legs” can be easily removed if necessary and I hope to solve the issue in software in the future.

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jimsickstee

Hi, I'm Jim. Read what I write and I'll write more to read.