Elbow Design, Servo Motor Selection and Control Implementation of the Agile Parallel Kinematic Manipulator

Abstract

The purpose of this work was to contribute to the creation of a prototype of a new type
of robot called Agile Parallel Kinematic Manipulator or AgilePKM for short. Designing,
building and controlling a new type of robot is a task which goes beyond the scope of any
master thesis project, but there are subtasks which are more suitable to handle within
the available time frame. A principle design will be handled, meaning no actual hardware
will be available or constructed. There are three subtasks that are addressed as described
below.
The AgilePKM consists of many joints and links where one of the most crucial joints
is the elbow joint. The first subtask was to construct this joint to efficiently meet the
requirements and advantages of the new robot structure. The elbow joint is a very exposed
joint as it exclusively actuates vertical movements, carrying not only the payload but the
forearm structure as well. This was to be done while facilitating sufficient movement
for the extensive working space and minimizing weight and cost for the proof-of-concept
prototype. To ensure that these targets were fulfilled an adapted needs identification
process was carried out followed by benchmarking and concept generation. From this
the most promising concept was chosen and further developed to reach the most suitable
design.
The second subtask was to select servo motors. This task involved making a lot of
assumptions and asking for specifications which was far from final due to the lack of a
mechanical design. This was the reason to present a general recipe on how to select a servo
motor, in addition to presenting the final selection for the prototype. The specifications
as well as the motor and driver are subject to change when building a second prototype.
This thesis will then form a good base of knowledge for such changes.
The third task was to implement kinematic transformations for the robot, and to
integrate those in an available automation software platform. The selected platform was
TwinCAT3 by Beckhoff using the integrated CNC kernel provided by ISG Stuttgart. The
kinematics was already developed in Python for easy prototyping and debugging. It was
manually translated to C++ for performance and then imported to TwinCAT via a C++
module. The ISG kernel takes care of the trajectory planning. It was experienced that
after several hours with the tool one notices the restrictions of the tools, such as the
inability to dynamically limit the acceleration of each axis.
The report forms a good basis for the design of many types of robots. In the case
of the AgilePKM, the selections and implementations made in this thesis will be directly
built upon and the prototype will be realized in the year following this thesis work. As
mentioned, no hardware is available at the time of writing.

Popular Abstract

Design, servo motor selection and control implementation of
the Agile Parallel Kinematic Manipulator
The industry of today is yearning for faster, cheaper and more agile solutions for
production and packaging. A lot of the concepts of today’s robots were developed
in the 80’s. Since then, a lot has happened both software and hardware wise within
robotics, but the base concepts have mostly been the same. The AgilePKM aims to
disrupt the market with its innovative design.
There are three basic types of robots
commonly used in the industry: articulated,
Selective Compliance Assembly Robot Arm
(SCARA) and delta.

The articulated robot has many similarities
with a human arm. It has its motors working
in series and its main advantage is having a
large working space. They mostly have six
axes making it possible for the robot to
reach any point in any direction within the
working space. One downside is that this
robot carries not only the external loads,
but also its own motors. This makes the
robot’s moment of inertia unnecessarily
high.
The SCARA robot is a specific type of
articulated robot with usually three axes. It
is mostly used in assembly lines for tasks

such as simple assembly or simple pick-and-
place operations. It also carries its own

motors causing a high moment of inertia.
The delta robot does not have its motors in
series but in parallel. This makes it possible
to place all motors in the base of the robot.
Since the robot will have to move very little

of its own mass, it is very quick and
particularly suited for fast pick-and-place
tasks. Some of the downsides are that is has
to be mounted in the ceiling taking up a lot
of space and it has a limited working space.
The Agile Parallel Kinematic Manipulator
(AgilePKM) aims to take the pros of each of
these types and minimize the cons. The
goal of the design is to place all motors in
the base in order to keep the moving mass
low and also keep the joints serially
connected in order to increase the working
space.

When designing the first prototype of the
AgilePKM there are many aspects to
consider and they are all affecting each
other. Acceleration of up to 15 g is needed
to compete with the fastest SCARA and
delta robots. The high acceleration sets
high demands on the mechanical
components and servo motors as well as
the programmer. The tasks handled in this
CAD-model of the AgilePKM in its early stages

Examples of articulated, SCARA and delta robot.

Felix Lundblad Lunds Tekniska Högskola - IEA
Hampus Farmängen Lunds Tekniska Högskola - IProd
article are the elbow, servo motors and the
automation platform.

The “elbow” is a crucial component of the
AgilePKM as it actuates both vertical and
horizontal movements. Its implementation
is particularly interesting mechanically
since the joint axle is rotated in vertical
movement which is uncommon for single
degree of freedom joints in robots. The task
of withstanding the loads from the high
accelerations of the test cycle, and at the
same time incorporating a compact and
lightweight design, is a challenge. An
efficient design is reached by benchmarking
solutions for similar hinge joints and
generating new concepts adapted for the
AgilePKM. The final design consists of a
custom front and base designed for
manufacturability, an adjusted bearing
arrangement with two standard angular
contact ball bearings and a precision pivot
pin paired with a lock nut. The identified
needs of the elbow are fulfilled but the final
prototype is not physically manufactured
since the AgilePKM is not fully designed yet.
Instead it is verified by a finite element
analysis simulating the worst case loads
from the test cycle.
Servo motors are chosen in such a way that
a standard test cycle can be completed
incredibly quickly. This implies high torque
on the servo motors. Should the test cycle
be run at maximum speed multiple times in
rapid succession, the motors will break due

to overheating. Hence the test cycle is only
a measurement of the maximum
performance. The chosen motor, MS2N04-
D0BQ from Bosch Rexroth, will able to
complete the test cycle within competitive
time frame.
When programming a new type of robot
there are many things to consider. In the
thesis the kinematics of the robot is
calculated and explained. Kinematics is the
description of motion for system of bodies,
using equations to determine required joint
angles to reach a certain position. Since the
robot incorporates both serial and parallel
kinematics, suitable kinematics does not
exist and has had to be developed. The
robot also has proper restrictions
implemented in order to avoid moving
outside of the mechanically restricted
working space. In addition to this,
restrictions for the acceleration of the end
effector is also needed in order to stay
within reasonable forces for the
mechanical construction. There have been
some problems in limiting the acceleration
dynamically, which is proposed as future
work.
In conclusion the project has been quite
successful. The design of the elbow and the
selection of servo motor have met the
demands and suits current design of the
AgilePKM well. The automation platform
has been implemented and is able to
control the AgilePKM once the prototype is
built. Future work includes a redesign of the
elbow and reselection of servo motors
based on new loads from simulations of the
test cycle. This, along the implementation
of dynamic restrictions in the automation
platform, will further optimize the
prototype and ensure competitiveness with
the exciting robots on the market.

• author: Farmängen, Hampus and Lundblad, Felix
• supervisor: Hans Walter
• organization: Production and Materials Engineering
• course: MMTM01 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• language: English
• id: 8997583
• date added to LUP: 2019-12-09 13:20:24
• date last changed: 2019-12-09 13:20:24