2019-08-26 Motor Block Design


Content
1OrangeBot Power Train Design 1
1.1Design Priorities 2
1.2Specifications 3
2Components 4
2.1Layout 5
2.2Wheel 7
2.3Type of motor 7
2.4Motor 8
2.5Gearbox 9
3Design 10
3.1Layout 10
3.2Size Limitation 10
3.3Vendor 10
3.4Wheel 11
3.5DC Motor with Gearbox and Encoder 12
3.6Hub and Mounting 12
3.7Motor block components: 13
4Conclusions 14

1OrangeBot Power Train Design

For the Pi Wars 2020 a new robotic platform is going to be designed based on the specifications and the challenges.
This document focuses on the design of the power train of the robot.



1.1Design Priorities

A brief preliminary analysis of the challenges was conducted to estimate what the key design principles of the platform should be for the power train.
In decreasing order off importance, the design priorities are:
  1. Precision: The robot must be able to achieve the attitude set by the pilot/auto-pilot with low error. The robot must be stable and recover quickly from disturbances.
  2. Latency: The robot must be able to achieve the attitude set by the pilot/auto-pilot quickly.
  3. Power: The robot must be able to climb steep ramps, clear bumps and accelerate/turn quickly.
  4. Speed: The robot must have a good top speed and acceleration.
  5. Clearance: The robot must be able to clear big obstacles and not get into unrecoverable attitudes easily.

1.2Specifications

OrangeBot Specifications:
  • Maximum Width: 225 [mm]
  • Maximum Length: 300 [mm]
  • Maximum Height: 370 [mm]
  • Maximum Weight: Infinite [Kg]
  • Minimum Top Speed: 15 [Km/h]
  • Minimum Pushing power: 1 [Kg]
  • Minimum Battery duration: 1.0 [h]

2Components

The power train is composed of several elements:
  • Layout
  • Wheels
  • Motors
  • Suspensions

2.1Layout

The most important decision for the robot is the layout of the platform.
Wheels vs. Threads
First decision is how to transmit torque to the ground.
A wheel based design is mechanically simpler and achieve better precision, efficiency and top speed.
A thread based design has inherently better clearance and power.
OrangeBot is going to use wheels as precision is a key design focus.
Steering vs. Fixed Wheels vs Omnidirectional Wheels
A steering design employ big engines for torque and small engines for steering. It achieves the best top speed.
A fixed wheel design is mechanically simpler and achieve a zero steering radious and best precision.
An omnidirectional wheel design achieves the best manoeuvrability while sacrificing clearance, power and top speed.
Again, as precision is the first factor of the design, a fixed wheel design is to be used.
Number of independent/slave axis
As the robot must navigate an XY space, a minimum of two independent axis are required.
Power, clearance, precision and control increase as the number of axis increases.
Increasing the number of axis decreases the maximum top speed.
Number of independent axis should be maximized.
Possible layouts:
  • Two master wheel with a small slave passive pivot wheel
  • Two master wheels and two slave passive wheels.
  • Two master wheels and two slave active wheels.
  • Four master wheels
As precision is the design focus, a four independent master wheel design is going to be used.


2.2Wheel

There are several design levers with the wheels:
  • Tyre material and pattern improve grip and slippage which improve power and control.
  • Increasing diameter increases grip and decrease slippage improving control.
  • Increasing diameter increases clearance.
  • Increasing diameter increases top speed.
  • Decreasing diameter decreases the sensitivity of the controls and improve precision.
  • Increasing tyre width increase grip and decrease slippage
  • Decreasing tyre width increase efficiency and top speed
Overall, the wheel diameter should be maximized and a good tyre material and pattern should be chosen.

2.3Type of motor

There are several types of motors that can be used in a battery powered robot. Most relevant for the design are:
  • Stepper motors: Good holding torque, easy control
  • Brushless motors: Best power density, encoderless sensing possible, smaller
  • DC Motors: Cheap, best overall compromise of complexity and power density
“OrangeBot” is going to use DC motors with encoders as they offer the best compromise in terms of speed, torque, cost, complexity of the controls and ruggedness

2.4Motor

Motors have many parameters to them. Most important are:
  • No load speed
  • Stall torque
  • Efficiency
  • Operating voltage
  • Encoder
The combination of motor parameter influences key design metrics
Encoders:
With an encoder, a closed loop control can be deployed, vastly improving precision and control.
Motors with encoders should be chosen as precision is the most important factor of this platform.
Top Speed:
The top speed is decided by the combination of operating voltage, motor and wheel diameter. This put a tight constraint on the value of the No Load Speed@operating voltage
Efficiency:
Better efficiency decreases waste heat and increase battery life.
Better efficiency decreases noise.
Motors have a peak efficiency operating point where they achieve their peak efficiency.
Efficiency should be enough so that the motors do not destroy themselves.
A swappable battery almost negates the efficiency problem for battery life.
Overall, for this platform, efficiency of the motors is not a major concern.
Operating Voltage:
Motors can be undervolted or overvolted.
Operating voltage is not a concern at this stage of the design as it just put constraints on the design of the electrical stage of the robot.
Stall Torque:
The stall torque directly influences the acceleration rate and pushing power of the robot.
No Load Current & Stall Current:
Motor currents are not a parameters that influence the design at this stage.
The currents and voltages will be a specification for the later design of the electrical stage and battery. An eye has to be kept not to take unreasonable values (es. 600V motor or 100A motor)

2.5Gearbox

Bare motors are too fast and weak to be used directly. Some kind of transmission to decrease speed and increase torque is required.
Quick calculations shows that the gearbox must have a ration of about 5:1 to 25:1 to achieve target speed with meaningful combinations of DC Motors/Wheels.
There are several choices for the transmission design:
  • C gears: Used in RC cars. Cheapest option. Allow motor to be used sideways or out of axis, increasing maximum motor length from about 80[mm] to about 180 [mm]
  • Belt/Chain: Allow motor to be out of axis with the wheels, increasing maximum motor length from about 80[mm] to about 180 [mm]
  • Planetary Gearbox: Cheap. Rugged. Easy/trivial mounting. Often provided with the motor.
  • Harmonic Drive: Rugged. Flat. Silent. Efficient. Absolute best performance by a huge margin. Easy/trivial mounting.
  • Spur Gearbox: Cheap. Easy/trivial mounting. Often provided with the motor.
“OrangeBot” is going to use Planetary Gearboxes. They offer the best compromise in performance.
Had budget/time been infinite, a BLCD with harmonic drive combo would have been the best performing option allowing to push the power density of the platform to ludicrous levels.

3Design

3.1Layout

The chosen layout is four independent assemblies of:
  • Encoder
  • DC motor
  • Planetary gearbox
  • Wheel with rubber tyre

3.2Size Limitation

The size limitation of the platform imposes hard limits on the power train.
The width specification limits the length of the assembly to about:
225 [mm] /2 -margin = 100 [mm]
The length specification limits the maximum diameter of the wheels:
300 [mm] /2 -margin = 130 [mm]

3.3Vendor

Components have to come from a vendor, and choice is limited from their offering. Several options have been evaluated, RobotShop has been chosen to provide the components as it has everything needed but no custom duties and reasonable lead time.

3.4Wheel

Design starts from the choice of wheels that have to transmits torque to the ground.
Specification is to maximize diameter, chose an high grip tyre.
Leading candidates were:
Overall, this 127mm wheel won the day. An additional hub has been brought to replace the bulky hub that comes with the wheel.
As an added bonus, the C shape of the wheel allow to reduce the size of the motor block assembly.


3.5DC Motor with Gearbox and Encoder

Some quick calculations done with this online tool shows that the motor block must be able to output at least 700 [RPM] to achieve the target speed of 15 [Km/h].
Motor size minus shaft is limited to about 80 [mm], limiting the torque that can be achieved.
A large number of options have been evaluated using this tool to help extrapolating the motor parameters over the full characteristics and taking into account dissipated power and overvolting.
Amongst the options evaluated:
After a deep and through search, the choice landed on https://www.robotshop.com/eu/en/cytron-12v-1140rpm-6-37oz-gear-motor-encoder.html with the following theoretical performance.
This motor promises a top speed of about 30 [Km/h] well in excess of the desired specification while still providing 240 [mNm] and 1.89 [N] of pushing power per wheel.
A control loop is going to be included in the electronics to control and limit speed and power and achieve high precision with the controls.

3.6Hub and Mounting

An hub is needed to interface the wheel with the motor. This hub has been selected and will be modified to fit.
Mounting brackets have been bought as well to make life easier during the mechanical design and assembly of the frame.

3.7Motor block components:

Components as they arrived from the supplier.


4Conclusions

This document has outlined the specifications, design focus and component selection process for the motor block of “OrangeBot” to comply with the specifications of the Pi Wars 2020











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