Assistive Robotics

ROBOTICS STRUCTURE AND ACTUATION SYSTEMS

LESSON 2: ROBOTICS STRUCTURE AND ACTUATION SYSTEMS

Robot as a Human System - Since we are going to be biomedical engineers, we can do a comparison

between a robot and a human being. If we consider the brain, we can immediately map the

supervision unit in which the intelligent algorithms and control system of the robot are placed.

Obviously, the place, where decisions are taken, needs to be in connection with other different

parts, in particular our brain will have contact with all nerve termination and our senses (vision,

hearing, touching). They can be compared to all the sensor unit of the robot (cameras to simulate

human vision, proximity sensor, or force sensor to simulate touch and so on), which are essential

for the automatic machine to understand what happens and its internal behaviour, and can be

divided in internal and external senses. Then, the robot needs to have a mechanical structure

(human’s body) to allow the robot to perform any kind of tasks,

approach\moving somethings. All sensors, actuators, intelligent

algorithms and control systems need to be in connection thanks to a

bus and communication protocol. The sensor, which is a physical

devise, acquires the various signals (or electric impulses) that need to

be pass to a processing unit (or everything there is inside). The way in which the quantity is passed

from the sensor to the controller is performed through the bus and governed by a communication

protocol, which is the language talked by different part of the robot. If we see a robot as a system,

it has all the units previous described. The robot has some tasks, a set of commands to do in a

working environment: it performed the primary task (what I want the robot to do), but can be other

task (which is the most intelligent part). Example: Roomba - first task is cleaning house, the others

is mapping the house, avoid obstacle etc.

The control architecture is composed by a control system, which provides the correct impulse to

the actuators that performs a mechanic action, typically allowing the motion of the robots. The

electrical motion allows different part of the robot to move but needs to be verify if the motion

happens properly (feedback control). Unit sensor sends back to the control system to find the new

values for the actuators. This loop works until the robot have something to do. The mechanical

structure involves locomotion apparatus (wheels, tracks, legs) and manipulation apparatus

(mechanical arm, end-effector, artificial hand). The actuation system provides the capability to exert

an action (locomotion or manipulation), while the sensory system acquire data on the status of the

robot (e.g., position, joint angles, etc.), thanks to proprioceptive sensors, or

thanks to exteroceptive sensors acquire data on the external status of

environment. At the end the control system it commands the execution of the

action with respect to the goals. 1 ROBOTICS STRUCTURE AND ACTUATION SYSTEMS

Example of human robot where the locomotion is based on wheeled humanoid robot. In this

example we can find different kind of interfaces, LEDs to simulate human like expression, tablet

where we can put the requests and we have a feedback from the robot itself, the human interaction

is a human like way of hand motion. Another example is the NAO: quite common robot used for

education purpose. Nao has a lot of different actuators (right picture), typical connect to a motor

because they are the part that can move.

Furthermore, in the left part we can see all the

sensors and we can observe that the number is

very high. So, we can start to think that when a

robot needs to interact in a human environment

the robot needs to be equipped with many

sensors, not only for security reason, but also to

be more acceptable, to simulate something

closer to the human act (speaking,

communication).

Structure of a dynamic system - A robot is a dynamic system, whose

structure can be separated into two components: Kinematic skeleton: is

based on the kinematics of the robot that includes all properties of the

movement that depend upon the geometry of its mechanical structure

(kinematic chain). Kinetics: is a law of dynamics that permits to study the

conversion of the forces provided by the actuators into robot motion. If we

want to make a comparison with our body we can assume that the kinetics is related to muscles,

while kinematics to our skeleton.

KINEMATIC CHAIN - Kinematics allows to represent positions, velocities and accelerations of

specified points in a multi-body structure, independently from the causes (i.e., forces and torques)

that may have generated the motion. To describe the kinematics of manipulators or mobile robots,

it is necessary to define the concept of kinematic chains. A kinematic chain is a series of ideal links

(rigid static body), connected by ideal joints (rigid mobile components). The ideal condition is

reached when we do not consider mass, friction, elasticity, etc. are considered so we don’t consider

the physical part of the problem.

Links/arms are idealized geometrical bars connecting two or more joints. Joints are idealized

physical components allowing a relative motion between the attached links. Joints allow a single

"degree of motion" (DOM) between the connected links. Joints may be of two types (in the present

context): Revolute (or rotational) joints; they allow a rotation between the connected links;

Prismatic (or translation) joints; they allow a translation between the connected links. Revolute

joints are usually preferred for their compactness and reliability Other types are possible, but will

not be considered. 2 ROBOTICS STRUCTURE AND ACTUATION SYSTEMS

In the left side we can see the revolute joint, the red and blue components are two consecutive

links, so the joint is the rigid component between these two links and allows a relative rotation along

tht axis between the red and blue part. On the right side we can find a prismatic joint: the joint

allows a translation along one axis (in this case along ‘axis’). Revolute joints are the most common

in the world, since they are more compact and reliable.

Kinematic chain is a series of links and joints. In this picture we can observe

two different red part (links), two revolute joints and the blue one is a single

link connecting the two joint and the other two links. So, we have three links

and two joints composing a kinematic chain. Motor actuators will allow the

rotation of joints allowing the motion of robot itself. Typically, all the joints

are actuated, but when it doesn’t happen, they are called passive joint.

There are two chain tipes:

- Open chains (Serial Robots): there is only one link between two joints. The KC has tree-like structure;

- Closed chains (Parallel Robots): there are more than one link between two joints. The KC has the cycle-

like structure. The parallel robots are the most common in industry: example - 3D printers sometimes

have a parallel robot.

Degrees of Motion (DoM) and Degrees of Freedom (DoF)

1. The Degrees of Motion (DoM) is the number of independent motions that can be performed by a system

2. The Degrees of Freedom (DoF) of a mechanical system id the number of independent parameters that

define its position in the Cartesian space.

Suppose a cone with one single point of contact with the plane. The system could perform only the

rotation around the axis between the two links, so the degree of motion is equal to 1. Then, if we

want to define orientation and position in the planar case, we must consider the position x and y in

the point of contact and eventually the angle with respect to one of the axes formed by the cone.

So, we have three DoF, because we need three independent parameters for defining position and

orientation of the cone in the planar case.

In robotics, DoM and DoF are synonymous because for the robot is the same concept.

Degrees of motion calculation – the degree of motion d of a mechanical system composed by l links

(base included) and n joints can be defined in accordance with the Kutzabach formula (for the

Cartesian sopce). 3 ROBOTICS STRUCTURE AND ACTUATION SYSTEMS

In the 3D space instead, six independent parameters are required to describe the orientation and

the position of one point: x,y,z position of the point and three angles that define the orientation. In

the planar case (2D) we used the second formula called Grubler equation (m=3).

Joints are the circles and the links are the segment connecting the

joint including the base and the last element is the end effector. In

the case of the Open chain we will have 5 links and four joints,

computing the formula we will find 4 degrees of motion. In the

closed case both the number of joint and the number of links will be

6. The resultant degrees of motion are 3. In and open kinematic chain , each joint provides the

structure a single Dom so the degrees of motion are equal to the number of joints. For this reason

in robotic DoM and DoF are synonyms.

KINEMATIC REDUDANCY- Going back to the open chain example,

the number of DoM is equal to four while the DoF is three because

we are in the planar case. Since n (DOM) > m (DOF), the robot is

redundant. If the robot is in the 3D space, the DoF is six and DoM is

four, so it is not redundant in this case. The robot with four DoM

will be redundant for some task while not redundant for others. If

we want as a task for the robot the motion of the end-effector in a

particular point in the xyz space, we need to fix the position for this

particular point (x,y and z position). The number of the parameters

required is three and don’t mind the orientation. In this case, the

robot will be redundant considering the task that has to perform.

Redundancy is a relative concept and depends on the specific task and what the robot has to do. So,

the robot is redundant in planar case but is not in the 3D space. But if the task is to move the end

effector only in a particular point, the robot is considered redundant for this specific task. A

redundant robot can have different task. Let’s think about our body, only o the arm we have 7 DoM

(3 wrist, 1 elbow, 3 shoulder) so human body is one of the most redundant robots in the world.

The kinematic redundancy allows to change the configuration of the

manipulator in order to: avoid obstacles close to the robot; avoid joint

position and velocity limits; improve the robot motion performance and

make the robot like a fault tolerant system. For example, if we have 8 DoM

and one of the joints fail (I.e. broken motor) we still have seven joints, so

we can change our control system and perform the same operation

without a joint (avoid losing the robot control).

The robots are classified by the kinematic type. The kinematic chain is a series of joints and links,

where series means one after the other. Considering the series joints, starting from the base and

going on, of the we can name the robot according to the presence of a Prismatic (P) or rotational

(R) joint (remember that one joint means one degree of freedom). Each robot is characterized by

workspace of its arm is the set of all positions that it can reach. This depends on a number of factors

including the dimensions of the arm. 4 ROBOTICS STRUCTURE AND ACTUATION SYSTEMS

1. Cartesian = 3P = P-P-P

The shoulder is composed by three prismatic joints, with mutually

orthogonal axes. Each DOM corresponds to a cartesian task variable. The

task space is a sort of parallelepiped They provide an accurate positioning in the whole task space,

but have a limited dexterity. The most common structures are lateral columns or suspended

bridges. It can reach all the points inside the parallelepiped.

2. Cylindrical = 1R-2P = R-P-P

The shoulder has one revolute joint with vertical axis followed

by two prismatic joints (one vertical the other horizontal). Each

DOM corresponds to one cylindrical coordinate. The workspace is a cylindrical sector, can reach all

the points in the cylinder expect the central axis for mechanical reason. The horizontal prismatic

joint allows to reach horizontal spaces, but the accuracy decreases toward the arm ends. They are

used mainly to move large objects.

3. Polar or spherical = 2R-1P = R-R-P

The shoulder has two revolute joints (one vertical, one

horizontal axis) followed by a prismatic joints (with axis

orthogonal to the last one).Each DOM corresponds to one polar

coordinate. The task space is a spherical sector that may include part of the floor, to allow the

manipulation of objects there.. The structure is less rigid than the previous ones, and the accuracy

decreases with the elongation of the prismatic arm. The RRP can reach all the spherical point.

4. SCARA = 2R-1P = R-R-P

The shoulder has two revolute joints followed by one

prismatic joints (all with parallel/vertical axes). The

correspondence between DOM and cartesian coordinates

is true only for the vertical component. The effect of gravity is compensated by the structure itself.

The structure is rigid in the vertical component and compliant in the horizontal components. This

robot is mainly used for small components manipulation and vertical soldering or assembly tasks

(e.g., in electronic boards assembly)

5. Articulated or Anthropomorphic = 3R = R-R-R

The shoulder has three revolute joints: the first one is

vertical, the other two are horizontal and parallel. The

structure is similar to the human body, with trunk, arm

and forearm, with a final wrist. No correspondence

between joint and cartesian coordinates. Task space is a sort of sphere sector. It is one of the most

common structures in industry, since it provides the best dexterity. Its accuracy is not constant

inside the task space. 5 ROBOTICS STRUCTURE AND ACTUATION SYSTEMS

Robot performance – Robots are not only classified according to the joint but are kinematically

classified also according to the following performance:

- Workspace: is composed by all points that can be reached by any position and orientation of the end-

effector.

- Accuracy: is the ability of a robot to position its end-effector at a preprogrammed location in the

workspace. Accuracy is a function of the precision of the arm kinematic model (which is only simplified

rigid-body kinematic configuration) the precision of the world, tool, and fixture models. Thus,

manipulator accuracy becomes a matter of matching the robot geometry to the robot solution in use by

precisely measuring and calibrating link lengths, joint angles, and mounting positions.

- Repeatability: represents the ability of the manipulator to return repeatedly to the same location. It

depends on the method of teaching or programming the manipulator. Most manufacturers intend this

figure to indicate the radius of a sphere enclosing the set of locations to which the arm returns when

sent from the same origin by the same program with the same load and setup conditions. Repeatability

is important when performing repetitive tasks such as blind assembly or machine loading. Depends on

the precision of the robot with respect to the model.

- Resolution: represents the smallest incremental motion that can be produced by the manipulator.

In an ideal situation workspace, accuracy and repeatability has be as greater as possible, wile the

resolution should be very small for very precise movement, in reality it is not possible, we have to

find a tradeoff between time, costs and performances.

ROBOT DESIGN – Robots are now often designed for specific applications and to perform limited

sets of tasks. Robot design focuses on the number of joints, physical size, payload capacity, and the

movement requirements of the end-effector. Payload capacity example: Amazon drone cannot

delivery all kind of objects because of payload capacity that can delivery only that weighing less than

5 kg. If we have a reliable control system, we will be able to achieve with the same situation higher

accuracy.The configuration of the movable skeleton and the overall size of the robot are

determined by task requirements for reach, workspace, and reorientation ability. These features

affect the precision of end-effector path control needed for applications. They also define the

absolute positioning capability necessary for small part assembly, the repeatability needed for

material and package handling, and the fine resolution that allows precise, real- time sensor-based

motions. A critical concept in robotic system design is the range of tasks the robot is expected to

perform. The robot should be designed to have the flexibility it needs to perform the range of tasks

for which it is intended. This determines the topology of the robot mechanism and the actuator

system. The choices of geometry, material, sensors, and cable routing follow from these basic

decisions.

ACTUATORS – The electric, hydraulic and pneumatic motors and other elements that cause the

links of the mechanism to move are called actuators. Actuators can be built in many different ways,

most prominently: electrical motors, pneumatics and valves. In this course we will only deal with

electrical motors due to the fact that are widespread and more used also thanks to the rater

availability of electricity with respect to fuels. A servomotor is a rotary actuator or linear

actuator that allows for precise control of angular or linear position, velocity and acceleration. We

can find:

- pneumatic: characterized by pneumatic energy (compressor), pistons or chambers mechanical energy is

difficult to control accurately (change of fluid-compressibility) no trajectory control. It is commonly used

for opening/closing grippers (some part of the robot such as hands) or as artificial muscles (McKibben

actuators) 6 ROBOTICS STRUCTURE AND ACTUATION SYSTEMS

- hydraulic: provide hydraulic energy (accumulation tank) and is regulated by pumps/valves mechanical

energy. Advantages: no static overheating, self-lubricated, inherently safe (no sparks), excellent power-

to-weight ratio, large torques at low velocity (w/o reduction),; disadvantages: needs hydraulic supply,

large size, linear motion only, low power conversion efficiency, high cost, increased maintenance (oil

leaking)

- electrical: are widely used in robotics, and are the only one that we are going to study. Advantages:

power supply available everywhere, low cost, large variety of products, high power conversion efficiency,

easy maintenance, no pollution in working environment; disadvantages: overheating in static conditions

(in the