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Mechanical Robots and Machines
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Mechanical Robots and Machines

Introduction

All ‘machines’ you see daily come into different categories: some are purely mechanical like a swing, some are purely electrical like your mobile phone, and some are even purely optical - like your spectacles. However, most machines we’ll deal with are an effective combination of the first two parts - mechanics and electricity. While there exist more types and subtypes of the categories listed, we will keep our discussion within this boundary so as to not wander into the more complex realms of real-world engineering analysis and machine design.

The topics covered in the following sections will be explained with respect to the physical world, and will directly focus on their applications. They will not cover the mathematical rigour of the derivations but only focus on building a conceptual foundation. In this module, we will only be focussing on the types and designs of ‘mechanical robots’.

Cheers!

Manual Robotics

In the field of primarily ‘mechanical’ robotics, one has to make robots which are manually driven and perform one of the following tasks-

  1. Roborace
  2. Robosoccer
  3. Robowar (Not discussed here)

We will be elaborating on each one of them separately in order to properly understand what goes into making these robots.

Roborace 

Used as slang for ‘Robot Racing’ by roboticists worldwide, it is a field of robotics where, as the name suggests, robots race in order to complete a difficult track in the shortest possible time. The challenge is to create a robot (wired or wireless - both work) within the given dimensions to attain the optimum speed and accuracy to beat other robots on a track and reach the finish line as quickly as possible. The robot starts behind a ‘starting mark’ and is considered to have completed the course when any part of it crosses the finish line after handling all obstacles.

It should be capable of traversing complex terrain and hurdles without going outside the track. Points are awarded for overcoming each hurdle. The bot with the maximum points (or in some cases, least time) will win the race. The competition area has a special place defined for the robot's operation (hereafter called the ARENA).

GETTING TO KNOW THE ARENA

Each path of the arena usually has a specified width within which the robot has to fit, which is why there is a constraint on the dimensions of the robot. The track surfaces will have unevenness, and several hurdles, which will try to slow down the robot. The arena of the race is not very well-defined and has a high probability of being changed at even the last moment. There are a variety of hurdles of varying difficulty which are placed on the tracks. These may include (but are not limited to) -

  1. Ramps (most common, seen in almost every track)
  2. Speed Bumps.
  3. Obstacles (e.g. plastic cups, which the robot cannot disturb).
  4. Oil and soap
  5. Sand and mud

Source-www.technoxian.com

DESIGNING THE BOT

Once we have a general idea of what the arena is going to look like, our next task is to make and build on a plan of how we would design the robot. The robot needs to be fast but also easily maneuverable. Striking the right balance between the two is what usually poses the greatest problem, as an extremely fast robot is generally very difficult to control, whereas an easily controllable robot usually loses out on the speed factor.

The base of the robot is usually composed of a body (which is very simply built using wood / aluminium) and four wheels. Once we have chalked out the basic design of the bot, we move towards improving it so that it is able to counter the obstacles easily. Some of the modifications, one has to usually make to the bot are -

  1. Changing the wheel - The most common problem arises as the wheels sometimes do not allow the bot to perform to its full potential. One needs to choose from wheels of different dimensions and thickness. Thicker and wider wheels are usually used to overcome bigger obstacles , while smaller ones are used to make the bot compact.

Bat Grips and other rubber substances can also be used to improve the overall grip of the wheels on the track (also called the traction) by increasing the frictional coefficient. This allows the bot to move faster and skid less. One can even design their wheels on manual or autonomous lathe machines using specific designs.

  2. Changing the motors - A motor is one of the most important parts of a robot as it is responsible for all the movement the robot makes. So it is very essential that one carefully chooses the motor which is to be used. Some of them are listed here -

  1. Power Requirements - Motors come in different ranges according to their power requirements which are measured in Watts or Horsepower.

 

  1. Torque - Torque of the motor refers to the turning force of the motor with respect to the radius.

  1. RPM (Rotations Per Minute) - It’s the number of rotations that the shaft of the motor completes in one minute.

For example: A 6kg/cm Servo motor should be able to lift 6kg if the load is suspended 1cm away from the motors shaft, the greater the distance the lesser the weight carrying capacity. We usually use a servo to control some special functions the bot has to perform in addition to completing the track. For example, lifting up objects, clearing its path etc.

Servo motors are very widely used for designing pick-and-place mechanisms, because of being easier to control than stepper motors, and performing very accurate movements .

LEARNING PLUS: pick-and-place mechanisms: The most relevant parts of a pick and place robot are its joints. Joints (analogous to human joints) are used to join two consecutive rigid bodies in the robot. They can be rotary or linear. To add a joint to any link of a robot, we use servo motors to control the degrees of freedom for that body part. Degrees of freedom basically refer to the number of different kinds of movements a part can perform (for instance, in a pick-and-place robot, we would want the base of the arm to be on a kind of turntable to allow the arm to pick from anywhere around it, and we would want the main joint of the arm to be able to move up and down to ‘pick’ and ‘place’ objects - one DOF for the rotational movement of the base, and one DOF for the rotational movement of the joint). 

  2. Changing the Centre of Mass

Thought why the robot in the orientation PPT GIF toppled back while going up the steep slope? That’s because of the shift in it’s centre of mass. Centre of mass is that point where the total mass of the body / system is concentrated. While crossing steep ramps, the centre of mass. Usually keeping the Centre of Mass at the centre of the robot is safe but that doesn’t work when the ramps are above 45 degree (it can be lesser than 45 depending upon the frictional coefficient of the surface) or more angle. So let’s take an example. We have taken one case on an inclined planes where the object of mass 60kg is stuck there as the pull force generated by gravity is obstructed by the static friction. The other case we have taken is where the object topples when the angle is increased.  

This means that at 68.2 degree angle, the object will topple. 

In many situations an external force is applied to a body, and that force may cause the body to tip over instead of sliding. This is called toppling. We will consider gravitational force, which acts on the Centre of Mass(CoM).

Let us consider a body of height ‘b’ and length ’a’.

Before we analyse mathematically, consider answering with your intuition first. Which of these two configurations is prone to topple more?

When a force(F) is applied on the CoM, the pressure is not uniform. The Normal reaction shifts to the edge so that it can counterbalance the torque due to friction. For increasing values of F, N keeps shifting until it reaches the edge of the body. First we shall consider the surface is sufficiently rough so that there is no sliding as F increases to Fmax. If the Force is increased any further, then N cannot counter the torque due to friction (f) and the body will topple. The value of Force now is the maximum value at which the body does not topple.

Fmax = f

N = mg

Torque about CoM:

f . b/2 = N . a/2  → f = Na/b = mga/b

Fmax = mg a/b

Thus, toppling depends on the ratio of a/b. Now we shall consider the surface which is not sufficiently rough and the body slides before we reach Fmax = mg a/b. Once the body starts sliding, friction becomes constant, and hence no toppling. This is the case if

Fmax > f →  mg a/b > µmg

→ µ < a/b

Thus, if we choose a and b accordingly, we can prevent toppling.

Robosoccer

Introduction

Robosoccer involves playing soccer with manually controlled robots. This is a team event, usually played in a team of 2 or 3. Each member controls one robot, and your team of robots needs to win the match. Only difference is that there’s no offside, outside or corner in this one. So, what do you think?

The robots can be wired or wireless - and are of two types: attacking robots, and goalkeeping robots. The former should have some shooting mechanism to score and the latter some defense mechanism to block and snatch the ball (usually a Table Tennis ball). Mechanisms could be static wedges or even something with a fan on the top. You can only use batteries (no IC engines or compressors) and the robot needs to be within a specific size and weight constraint. For safety purposes, the robot shouldn’t emit smoke, fire, leak, stain or soil on the arena. Otherwise, your team will get disqualified. Moreover, a robot cannot trap the ball, i.e it can’t keep the ball inside it. The robot either has to shoot it or drag it along.

The field has two goals, centred on each of the shorter sides of the playing field. The goal inner space is different for each event. It has a cross-bar on top (to prevent robots from entering the goal and to allow checking if the ball scored). The goal posts are positioned over the white line marking the limits of the field.

A centre circle will be drawn on the field. It is a thin white marked circle. Referees and Captains use it as guidance during kick-off.

Electronics

1.Wired Communication

Can you identify the kind of switch used here ?

DPDT! What does a DPDT switch look like?

2.Wireless Communication

OR

Commonly Used Machines

4.1 Lathe Machine

A lathe machine rotates a piece of plastic or metal around an axle to perform different tasks, - Fabrication, drilling, cutting, sanding, knurling, etc. Here, the lathe machine will be mostly used for designing custom, efficient wheels (and in Robowars robot body design as well).

4.2 Milling machine

Milling is a drilling machine that could make designs of metal/plastic materials. Milling machining is one of the very common manufacturing processes used in machinery shops and industries to manufacture high precision products and parts in different shapes and sizes. The machine can also be used to remove a part or cut out a part from a unibody system. The material needs to be watered regularly to cool it down and make the process much safer. Plasma and Laser cutting is an advanced form of the same.

4.3 Welding machine

Welding primarily means joining metal surfaces or its alloys. It is like a chemical reaction which causes a lot of heat generation that the metal/alloy melts and stick to the other surface, it fuses and when cooled down, it join the two parts/surfaces.  It is very useful for attaching metal pieces or attaching wedges to robots.

4.4 3D printer

Every 3D printer builds parts based on the same main principle: a digital model is turned into a physical three-dimensional object by adding material a layer at a time. In 3D printing, no special tools are required (for example, a cutting tool with certain geometry or a mold). Instead the part is manufactured directly onto the built platform layer-by-layer, which leads to a unique set of benefits. A 3D printer essentially works by extruding molten plastic through a tiny nozzle that it moves around precisely under computer control. It prints one layer, waits for it to dry, and then prints the next layer on top.

Assignment

For 3D printing anything you need to first make a design on a 3D modeling software. For instance, we can use Blender as it is somewhat similar to Fusion 360 in many ways.

http://www.aeross.org/2019/06/3d-course-prerequisite/

http://www.aeross.org/2019/06/edit-mode-basic-tools/

http://www.aeross.org/2019/06/modifiers-edit-mode-contd/

These resources are present in the Aeross Official website www.aeross.org made by Ashvin Verma. You can enhance your 3D modelling skills from here.

For Solidworks, you could learn the basics from http://www.solidworkstutorials.com/ but since solidworks is too professional, only the seniors are encouraged to try it out. Solidworks doesn’t support animations and is used to design robots in workshops (professional).

You need to answer the following questions in one para, 100 words on what is shown in the 3D render. The doc should have 3 field view rendered images of your design for every question and should also have one diagonal view to make it look original. For juniors, NEAT graph paper designs and 2D designs on Illustrator, etc. Seniors can also provide a 2D design on illustrator or any compatible software.

If you have a self made project related to these questions, you could mail us a video explaining the project at roboknights@gmail.com.

Buona Fortuna!

Question 1 - CRICKET PRACTICE DURING LOCKDOWN

Many sports persons are experiencing a major problem during this covid 19 situation i.e Lack of Sports practice. For cricketers, there are certain machines that can throw balls at certain varying speeds. But you as a cricketer doesn’t have it with you in your house. Your only advantage is that you are a B-Tech Engineer. Hence, you need to make such a machine from scratch for your indoor practice. These are the following instructions-

Question 2 - ROBOWARS

Often known as battle robots, Robowars is now a sport in most robotics competitions in india. It consists of 2 heavyweight robots that battle against each other to disable or knock the other one out of the arena. Your job is to design the weapon of a robowar robot and animate it’s functioning and explain why you chose such a weapon.

(Do not misinterpret the term Robowars, Google for more information on it. You could choose any weapon - Drum, Snail type, Single teeth, etc).

Question 3 - STEPPER MOTOR

What’s a stepper motor? Explain the functioning of the following labelled parts and each of the four coloured wires coming out of it. Where exactly is stepper motor most preferably used?

Question 4 - PROSTHETIC ARMS

A common craze in many high-tech technologic companies and organizations like Boston Dynamics, Hansons Robotics and HackSmiths, etc is the designing of a prosthetic arm. With improvements in technology, this project has been modified using computer vision, various

sensors and using a combination of actuators.

But the only problem is that our arm has grip equivalent to even uphold our own weight whereas in a prosthetic arm, this is not the case.

Can you propose a solution to this?  (Do not misinterpret the term “prosthetic arm”. Here we mean a completely automated robotic prosthesis of an arm. Google it for more information dnd pay attention to the pictures)

you find a solution to this? You may think about pneumatics and hydraulics but then these cylinders are huge and do not give the shape to the arm. It also needs a compressor which makes it more bulky and heavy weight.

Question 5 - Automated BARCODE SCANNER

An extremely important thing for a machine while reading a barcode is that it should be scanned along the same horizontal axis as the code as shown in the figure.

The problem with this barcode is that it is too elongated to be scanned with hand as that would lead to a lot of errors. Instead, I had designed this robot to help but after placing the robot, i realised that the sensor had to be put on the side of the robot rather than at its front, NOW:

  1. WHAT IS THE PROBLEM WITH THE ABOVE SETUP AND WHY WOULD IT HAVE BEEN EASIER IF I HAD PLACED THE SENSOR AT THE SIDE ?
  2. But now I can't rectify what I have done and would like your help in figuring out IF THERE IS ANYWAY THROUGH WHICH THE REQUIRED MOTION CAN BE PRODUCED PROVIDED THAT TURNING OF THE BOT IS NOT POSSIBLE IN SUCH A TIGHT SPACE? 
  3. EXPLAIN HOW DO THESE WHEELS WORK AND HOW CAN YOU MAKE THEM WORK USING SIMPLE DC MOTORS AND DPDT SWITCHES.