Outdoor+Robot+Platform

=Project Outdoor Robot Platform=

A project named Outdoor Robot Platform is currently performed in the research group of Mechatronics. The goal of this project is to design, make and test an outdoor robot platform. A concept design has to be made that is supported by calculations. The parts of the outdoor robot platform need to be ordered and assembled to realize the outdoor robot platform in the next step. After realization the outdoor robot platform needs to be tested. The Outdoor Robot Platform will function as a base for upcoming outdoor projects in the future.

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Mechanical design
The mechanical design is divided in several parts that the outdoor robot platform consists of.
 * ** Transmission: ** By using a transmission, the rotation of the motor can be transferred to the part that causes the outdoor robot platform to move.
 * ** Moving ** (driving)**:** Moving will be possible when the transmission rotates the already purchased wheels that have been chosen to drive with.
 * ** Driving transmission: ** The transmission will be driven by a motor that has been chosen and purchased.
 * ** Sealing rooms: ** The outdoor robot platform needs to be waterproof to prevent water damage to components within the housing.
 * ** Running software: ** The software is the heart of the outdoor robot platform and makes the outdoor robot platform operational. The software is going to be executed on a hardware device that needs to be stored on the outdoor robot platform in a waterproof shielded room to prevent damage.
 * ** Providing power ** (battery pack) ** : ** The battery pack supplies the motors and the load of the outdoor robot platform of current. Because of the weight that a battery has, the decision was made to use two battery packs that can be switched halfway the operation time.
 * ** Driving motor drivers: ** The motor drivers need to be controlled by a microcontroller to drive with the desired acceleration and speed.
 * ** Driving motors: ** The speed of the motors needs to be handled by motor drivers that supply the motors with a regulated voltage. The value from a microcontroller will be converted to a corresponding voltage by the motor driver which will cause the motor to turn with the desired speed.
 * ** Equipping different hardware: ** To make the outdoor robot platform universal and useable for different goals it is important to have the ability to equip different hardware.The hardware that should be mountable on the outdoor robot platform can be sensors, vision (webcam) and a module with a function like a gripper.

Transmission
The transmission consists out of multiple parts that have to work together to transfer the rotations made by the motors to the wheels. The choice of parts for the transmission will be described below.

The choice of bearings, gears and the diameter of the shaft that drives the wheel all influence each other. The bearings and gears have to be mounted on the shaft. Therefor all of the parts need to be chosen together, because not every part is available in all dimensions.
 * __Bearings, gears and shaft diameter wheel __**

__//Wheel shaft: //__ After indexing the available diameters for bearings and gears, the choice is made to use a shaft made of steel that has different diameters, respectively 15 and 12 millimeters and M10 thread. Figure 1 shows the shaft with a flattened end to make it possible to tighten the nut on the thread with a wrench during the assembly. //__Selecting__ __gears:__// The gear that is chosen to mount on the shaft is shown in Figure 2 and is easily mountable on various shaft positions because of the slidable chuck. The decision is made to make it possible to create a driving ratio of 1:1 and 2:1 with gears. The 2:1 gear ratio delivers more torque to the wheels then the 1:1 gear ratio but the maximum speed will lower. Therefor the 2:1 gear ratio is for driving over rough terrain with steeper slopes and the 1:1 gear ratio for driving over flat surfaces with more speed. The size of the gears is determined by the distance that needs to be bridged from the motor shaft to the wheel shaft and the ratio of the gears. Figure 3 displays the maximum dimensions of the motor and bearing. The gears need to bridge at least (44,6 + 56) / 2 = 50.3 millimeters. The space in between the components is marked as ‘X’ and can be calculated after the gear size is chosen. [|Calculations gears] Figure 4 displays the dimensions of the gear combination that is chosen for a gear ratio of 2:1. The dimensions in Figure 4 apply when the gears are aligned. The distance between the motor shaft and wheel shaft can be calculated with that dimensions. The distance between the shafts is (36 + 72) / 2 = 54 millimeters. Now the value for ‘X’ in Figure 3 can be calculated to determine the space between the motor and the bearing. 'X' = 54 - 50.3 = 3.7 millimeters. Because the specifications given by the manufacturer of plastic gears are around or even below the torque that needs to be transferred from the motor torque to the wheel shafts, it is recommended to use steel gears with a width of 15 millimeters (module 1.5) to have the strength needed to transmit the torque from the motor shaft to the wheel shaft. //__Selecting bearings:__// The outdoor robot platform should be built robust as it is going to be a base for projects in the future. A bearing or other part may not break by a collision with a wall when driving 1,62 m/s which is 5,87 km/h (maximum speed with gear ratio 1:1). A bearing with housing is ideal because it is strong and easy to mount. The maximum force that can arise by a collision with a wall when driving 1,62 m/s is approximately 4600 Newton. Calculations about collision with a wall have been made to determine the maximum force that the bearings need to be able to handle and can be found in the pdf "Force on a bearing by collision". Figure 5 shows a stress analysis of the concept for the outdoor robot platform where a force of 4600 Newton is applied on one shaft that is mounted in two bearings. The result displayed in Figure 5 is exaggerated to show what effects the applied force causes. The axle will move 0.4 millimeters over a length of 450 millimeters by a collision with a force of 4600 Newton on the axle which is within margins.

The same bearings as used in the simulation above are chosen to use for the outdoor robot platform.This will include four square-flanged bearings of type HDF15 with a shaft diameter of 15 millimeter (Figure 6) and four compact diamond-flanged bearings of type HDHCP12 with a shaft diameter of 12 millimeter (Figure 7). [|Information bearings] 

//__Connecting the wheel on the shaft:__// The purchased wheels came in a package that included a steel plate with shaft to connect to the wheel. Unfortunately the plate with shaft cannot be used because the shaft is too short. That is why a new shaft and mounting plate have to be designed and made to make a connection to the wheel. Figure 8 is showing a steel mounting plate where a wheel can be bolted on to. Then the steel shaft in Figure 9 can be turned into the mounting plate until the contact surface of the shaft and the mounting plate are touching. The mounting plate will be secured with a locknut on the other side of the mounting plate to prevent it from turning of the shaft. The thread in the mounting plate should keep the mounting plate from slipping over the shaft when the shaft is turning. 

The bearings and motor will be mounted as displayed in Figure 10 and consists out of aluminum profiles where steel and aluminum plate material with a bearing or motor can be mounted on. The advantage of using these profiles is the possibility to slide the plate material on the profile until it is in the position it should be in. Then it can be tightened against the profiles to create a perfect alignment between the gears or bearings. Figure 11 shows a stress analysis of the aluminum profiles mounted in a frame. The force on the shaft is 4600 Newton and causes a displacement in the aluminum profiles of approximately 0,1 millimeter over the width. Only the blue side in top of Figure 11 was fixed during the simulation which means that the displacement is likely going to be less than 0,1 millimeter because everything will be bolted together. With these results it can be concluded that this mounting method will easily hold the components in place.  Another advantage of this mounting method is the addition to support the motor shaft by using a bearing in line with it. The supporting bearing is mounted on the yellow plate material in Figure 10 and the exploded view of the components used for this part of the assembly can be viewed in Figure 12. In Figure 12 the motor shaft will be connected to the supported gear shaft with a grub screw. The gear and shaft will be connected together by three bolts and will be supported by a bearing mounted with four bolts on plate material. This construction secures the place of the motor and prevents the motor from pushing the gear on the motor shaft away on the gear mounted on the wheel shaft. A stress analysis is made to visualize the displacement of the gear when a force of 100 Newton is exerted on it and is displayed in Figure 13. The gear would only move about 0,006 millimeter in this case and that confirms that the motor shaft is well supported with this construction. For a prototype it is an advantage to have versatile mounting positions to adjust alignment to perfection and to have the possibility to make changes easy on a strong construction. That is why this method is chosen to use for the outdoor robot platform.
 * Mounting bearings and motors**

Moving
The maximum speed (1,5 m/s) and acceleration (1 m/s²) are determined in the requirements. The purchased wheels (Figure 14) and motors (Figure 15) are able to drive the outdoor robot platform with 1,5 m/s with a gear ratio of 1:1. The speed and acceleration that the purchased motors can reach is calculated for different situations. A report of these calculations can be found in the PDF-file "Calculations different driving situations".

__The following calculations and results can be found in the report:__ - Nominal speed and acceleration on flat ground are 1,62 m/s and 1,1 m/s² - Acceleration with two driving wheels on flat ground is 0,306 m/s² - The maximum slope at nominal load is 6.45°, steeper slopes will cause the motors on the outdoor robot platform to draw more current to deliver more force on the wheels. This will also lower the speed of the outdoor robot platform when driving up the slope. - It is not possible to drive over objects form standstill in front of them.

A few suggestions to lower the needed torque to drive up a slope are made in the pdf file "Driving up a slope" because the torque of the ordered motors might be too low. The best suggestions are keeping the weight of the outdoor robot platform low, making it possible to mount different gear ratios and letting the motors run under nominal speed/torque to create more torque to drive up slopes.

**Driving transmission**  The outdoor robot platform will be driven by four already purchased 24 Volt DC-motors with a gearbox of 49:1 and an encoder mounted on the back (one on each wheel). The motor is displayed in Figure 15. Datasheets of the motor, gearbox and encoder can be viewed below.

Sealing rooms
The outdoor robot platform needs to be sealed to prevent damage to the components by rain or water and dirt that splash up from the wheels while driving. The robot will be sealed by using sealing rubbers, adhesive pieces of rubber and glue. A sealing rubber will be used to seal the top watertight on the outdoor robot platform. Rubber has the property to be flexible and therefor it can still guarantee to seal uneven surfaces. The profile in Figure 16 is made out of a sliding rubber part that can be slid over the edge of a steel plate and a rubber bulb filled with air. The rubber bulb guarantees a watertight closure because it works as a spring that fills areas further away from the edge it is slid on and it is pressed by areas close to the edge it is slid on. The adhesive rubber in Figure 17 can be stuck on smaller metal plates with dimensions of 200×120 millimeters to seal a hole. The rubber acts like a sponge and will close gaps with different distances within a few millimeters.

Running software
The outdoor robot platform needs to be equipped with a screen, keyboard and mouse to make adjustments in the executing software program possible on outdoor locations. The outdoor robot platform should not be dependent of a Wi-Fi connection to make adjustments in the software program because Wi-Fi can malfunction and the signal can be lost. That is why is why the outdoor robot platform needs to be physically equipped with a screen, keyboard and mouse or a laptop.

Because a screen needs to be powered by the power source that is present on the outdoor robot platform the total operation time will decrease. There is also a possibility that the voltage for the screen needs to be regulated by a voltage regulator to offer the correct voltage to the screen. A laptop has its own battery that can power the laptop for several hours and a CPU (Central Processing Unit) with a lot of computing power. A laptop is also capable of storing and/or sending real-time data to another computer while driving. Because the usage of a laptop has many advantages compared to an external screen, a laptop will be used on the outdoor robot platform. In Figure 18 a laptop is stored in a room directly under the housing of the outdoor robot platform. The laptop can be accessed by opening a hatch or removing a plate in the housing of the outdoor robot platform. Because the hatch hinges open, it limits the space on the top of the robot that can be used to place hardware. Only fast dismountable hardware can be placed on the hatch to gain fast access to the laptop. A bracket can be mounted on the front of the housing where sensors and other equipment can be attached to permanently. **Providing power (battery pack)** The hardware of the outdoor robot platform needs to be provided with current to be operative. The current can be provided by a battery that is placed within the outdoor robot platform. The battery needs to deliver 24 volts or two batteries of 12 volt need to be connected in series to create a 24 Volt source because the voltage that the motors use at maximum power is 24 Volts. //__Selecting a battery:__// A battery has a specific amount of ampere/hour (Ah) available as capacity. The outdoor robot platform has to be able to operate at least two hours with one charged battery. The needed capacity can be determined by, multiplying the minimal operation time with the amount of ampere that the users (electric connected components) use on the battery. Four driving motors will be connected to the battery which will pull a nominal current of 2,3 ampere per motor. Next to the 9,2 ampere that the driving motors will use there is 5,8 ampere reserved to provide other equipment on the outdoor robot platform of current. Adding 9,2 ampere and 5,8 ampere together makes 15 ampere, which is the needed ampere per hour to let the outdoor robot platform be operative for one hour. When determining the capacity of the battery attention should be paid to what percentage the battery can be discharged. The following formula is u sed to calculate the batteries ampere/hour: The actual percentage capacity in the calculation shown above displays how many percent of the battery capacity is really used. The formula calculates how much ampere/hour a battery should have when it can be 80% discharged. The battery has to be charged when reaching that percentage. The manufacturer of the battery will provide a table with numbers that indicate how many times a battery can be charged and discharged for a specific percentage of discharge per cycle. For this application a battery of 37,5 Ah is needed for two hours operation time. Because the weight of a 37,5 Ah LifePO4 battery is almost the maximum weight for the platform, the decision is made to use two LifePO4 batteries of 20 Ah. The outdoor robot platform will operate on one battery until it is empty. Then the empty battery has to be easily replaceable by a full battery. By this solution the operation time of two hours can be achieved. A smaller battery with less capacity will be used in the period that the outdoor robot platform is going to be tested. A battery with more capacity will be purchased when the outdoor robot platform is actually going to be used outside to reach the operation time. //__Position of the battery:__// The battery pack will be placed in the middle of the outdoor robot platform to divide the weight equally over the wheels. The length of the outdoor robot platform is 628 millimeters according to the design. Figure 19 displays the amount of room to change when the battery packs through the side of the outdoor robot platform. 'X' = 628 - 2 × 254 = 120 mm. That means the maximum width of the battery pack is 120 - 2 × 10 (sealing hole) = 100 mm. Figure 20 shows a plate that is bolted against the side of the outdoor robot platform. The advantage of bolting a plate to the side is that the hole is closed with almost the same pressure on each side which means it will seal the hole.



Driving motor drivers
The motor drivers need to be driven by a microcontroller. The microcontroller will be programmed with code to execute one or more functions. The program will send set points to the motor controllers that are made within the code that is being executed on the microcontroller. The propeller proto board displayed in Figure 21 has been chosen to use for the outdoor robot platform because of the ability to run parallel processes. More information about the propeller proto board can be found in the datasheet "32212-32812-PropellerProtoBoard-v1.3". The programming language of the propellor proto board is designed by parallax and is called Spin. More information about the programming language spin can be found in the pdf "Spin language manual ".

Driving motors
The motors will be controlled by motor controllers developped by pololu (Figure 22). The motor drivers can be connected to a power source that delivers 5.5 to 40 Volts and are able to deliver a continious current of 12 ampere what is enough for the motors on the outdoor robot platform. More information about connecting the motor driver or other futures can be found in the pdf "simple_motor_controller".

Equipping different hardware
Each task requires specific hardware. Therefor it needs to be posssibele to mount different hardware on the outdoor robot platform. Room is kept free on the frontside of the outdoor robot platform as displayed in Figure 17 to mount hardware. The hardware can be mounted by tightening the hardware or mount of the hardware to the 3 millimeters thick aluminum plate.

Purchasing and producing parts
The outdoor robot platform needs to be mechanically realized and tested before other expensive parts like a laptop and high capacity LifePO4 battery will be purchased. Those parts will be purchased when the outdoor robot platform works as expected. The parts that have been purchased and produced to assembly the mechanical parts of the outdoor robot platform will be described here.

Outsourcing production parts
<span style="font-family: Arial,Helvetica,sans-serif;">The two steel shafts which need be made four times each are needed to make the drive unit for the outdoor robot platform and require some specific diameters and operations. Production drawings of the shafts displayed in Figure 23 can be found in the pdf files "Part 1 Shaft wheel" and "Part 2 Shaft gear motor". It was decided that it is best to outsource the production of the shafts because it would be a time consuming process to make the parts in the workshop ourselves. <span style="font-family: Arial,Helvetica,sans-serif;">

Watercutting parts
<span style="font-family: Arial,Helvetica,sans-serif;">Drawings need to be saved and delivered as .dxf file to watercut parts. Different drawings need to be made for each thickness of a material but can contain multiple parts and have to be made with scale 1:1. <span style="font-family: Arial,Helvetica,sans-serif;">The operator of the watercutter will cut out the parts which are in the drawings for each material and thickness. Figure 24 displays a .dfx file of a part that the watercutter is cutting in Figure 25. The parts could be bend into their shape after the parts were made with the watercutter. The parts that have been watercutted can be viewed in the pdf files below. <span style="font-family: Arial,Helvetica,sans-serif; line-height: 0px; overflow: hidden;">

<span style="font-family: Arial,Helvetica,sans-serif;">Purchasing parts
<span style="font-family: Arial,Helvetica,sans-serif;">The bearings, gears, rubber seals and aluminum profiles that were already shown in the mechanical design have been purchased from suppliers. The drawings with measurements for the aluminum profiles that are needed to make the construction in Figure 26 can be found in the pdf files below. <span style="font-family: Arial,Helvetica,sans-serif; line-height: 0px; overflow: hidden;">

Realization
The assembly of the outdoor robot started after fabrication, delivery and collecting of all the materials needed to build the outdoor robot platform. Preparations were made during this processes for assembling all the parts. The outdoor robot platform needs to be assembled by adding sub-assemblies together. These sub-assemblies consist out of two drive units and the frame. The assembly process of those sub-assemblies will be described below.

Assembling the drive unit
Two drive units have to be made for the outdoor robot platform. A drive unit consists of different parts which can be found in the pdf "assembly drive unit with part list". The framework on the left side in Figure 27 is drawn in Inventor and will be made first and will function as the base of the drive unit. This resulted in the framework displayed on the right side in Figure 27. <span style="font-family: 'Calibri','sans-serif'; font-size: 14.6667px;">Adding the mounting plates, bearings, gears, motors and shafts is the next step. The complete drive unit drawn in Inventor is displayed on the left side in Figure 28 and the assembled drive unit with all components is shown in on the right side in Figure 28. The bolts of the mounting plates are not tightened but are loose to align the bearings of the drive unit with the bearings that are mounted on the frame.

Assembling the frame
The frame consists out of two bended aluminum plates that function as sides and a bended aluminum plate that functions as front, bottom and back. It is made from aluminum because it is a lightweight metal and has enough strength when it is bended into its shape. The frame drawn in Inventor is displayed on the left side in Figure 29 and the frame made of aluminum on the right side in Figure 29. The frame is not bolted together because the bearings of the drive units need to be alligned with the bearings that are mounted on the sides of the frame first. To properly allign the bearings it is needed that the frame can be disassembled during the allignment. The frame is bolted to the drive units as shown in 30. The bolts in the mounting plates of the motor and bearings will then be tigthened slightly when the bearings of the drive units are aligned with the bearings of the frame. Then the gears can be alligned by sliding the motor with the support bearing in the correct position and the bolts of the mounting plates can be tightened slightly. All bolts can be tightened further after carefully removing the frame of the drive units. The frame, drive units and mudguards can then be assembled.

Realization of the outdoor robot platform
Mounting the wheels is the next step. The wheels can be mounted by turning the mounting plate of a wheel on the wheel shaft and then locked with a lock nut against the wheel shaft. An 8 millimeter wrench can be put on the other end of the shaft to hold it while tightening the lock nut on the wheel shaft. The last few components (sealing rubbers, hole cover plates and the top covers) can now be put and bolted on the frame to complete the mechanical assembly of the outdoor robot platform. Figure 31 displays the outdoor robot platform drawn in Inventor on the left side and the outdoor robot platform after realization with the purchased and produced parts on the right side. The total assembly of the outdoor robot platform can also be viewed in the pdf "Total assembly".

__Test run Outdoor Robot Platform:__ media type="custom" key="23447130"