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ME102: Mechatronics Design. Partners: Mark Theis, Elizabeth Gammariello
ME102: Mechatronics Design. Partners: Mark Theis, Elizabeth Gammariello
Objective: Ask any tennis player, and they will likely share one of the few dislikes of the game is picking up tennis balls during practice or a match. These 66 mm diameter bright yellow objectives roll away easily and take time to bend down and grab. This led us to our project’s opportunity: How can we make picking up tennis balls off of a tennis court easier for the player using mechatronics design? We designed Roll-On-You-Bot, a wheel driven robot that can maneuver on a tennis court, scoop up tennis balls, and store them so that they may be collected by the player.
Objective: Ask any tennis player, and they will likely share one of the few dislikes of the game is picking up tennis balls during practice or a match. These 66 mm diameter bright yellow objectives roll away easily and take time to bend down and grab. This led us to our project’s opportunity: How can we make picking up tennis balls off of a tennis court easier for the player using mechatronics design? We designed Roll-On-You-Bot, a wheel driven robot that can maneuver on a tennis court, scoop up tennis balls, and store them so that they may be collected by the player.
High Level Strategy:
High Level Strategy:
The Roll-On-You-Bot (or ROY-Bot) drives around tennis courts or concrete surfaces. The ROY-Bot begins in an idle state. Once the button is pressed, the robot enters the collecting state during which it moves forward at full speed for 1 second and then comes to a gradual stop. If the robot scoops up a ball, it will detect the added weight in the front bucket and enter the full state. At this point, the robot waits for the user to push a button so it can empty the ball into the chassis. If the robot did not scoop up a ball, it will wait for the player to push the button so it can move forward again. If, while the robot is moving, it comes too close to an obstacle, such as a wall, net, or player, it will stop moving and wait for the user to physically reset the robot and move it to a different location.
Initially, ROY-Bot was going to detect the position of nearby balls, drive to them, and collect them. Our team quickly realized, however, that this was unreasonable to implement in our prototype due to the cost of sensors, such as a lidar. Moreover, the software challenges, while an interesting feat, were beyond the scope of the course. This is a potent area of investigation for future iterations of ROY-Bot. Another change was to the ball emptying mechanism. Originally, we planned to use a rack and pinion to lift up the collector bucket and dump balls into the chassis. However, based on advice from mentors and team discussion, we modified this to an arm that a motor rotates to lift the bucket.
Robot Design
Robot Design
At the front of the robot, the white 3D printed bucket scoops up a tennis ball. With the ball inside, a force sensor under the bucket notifies the microcontroller. There is also an HC-SR04 ultrasonic distance sensor that measures frontal clearance. The white bucket lifts up due to actuation from the motor and arm, depositing the ball into the chassis. Sometimes, the ball goes above the top of the robot, so a small backboard helps guide it into the storage compartment. At the rear of the robot are three status LEDs (red, yellow and green) and a push button for control. Power cables protrude from the rear which supply the motors and microcontroller. Inside, a breadboard routes the sensors and motor controllers to the Arduino Uno microcontroller and the power supplies. The two drive motors attach to 38 mm gears for the 4:1 transmission to the wheels. The corresponding 152 mm output gears attach to the wheel shafts. The wheels themselves are secured on the exterior of the chassis. One enhancement that we expect could improve the performance of the drivetrain would be the addition of locating features to the motor mounts. This would align them optimally on both sides of the robot to their respective transmissions.
Function-Critical Decisions:
Function-Critical Decisions:
When designing the drive train, we separated the motors from the wheels by means of a gear box transmission. This provided two advantages: firstly, it increased the gear ratio by 4:1, giving us more torque to drive our chassis, and secondly, it rested the weight of the robot on larger shaft and bearing pairs that are rated for higher radial loads. However, there was still the force from the meshing gears to consider. We determined the maximum radial force on the motor shaft as follows:
When designing the drive train, we separated the motors from the wheels by means of a gear box transmission. This provided two advantages: firstly, it increased the gear ratio by 4:1, giving us more torque to drive our chassis, and secondly, it rested the weight of the robot on larger shaft and bearing pairs that are rated for higher radial loads. However, there was still the force from the meshing gears to consider. We determined the maximum radial force on the motor shaft as follows:
𝜏max= 0.63 Nm, Fmax =𝜏maxR = 33.0 N, Fmax, rad = Fmaxsin(20°)=11.29 N
𝜏max= 0.63 Nm, Fmax =𝜏maxR = 33.0 N, Fmax, rad = Fmaxsin(20°)=11.29 N
The input gear has a 38 mm diameter, and with the maximum torque available from the motors in our configuration, the highest radial force was 11.29 N, equivalent to 1.15 kg. We contacted the manufacturer and they specified they rate their motors for 2kg of radial load. With this advice, we found separating the motor from the input gear by a second shaft and flexible shaft coupler was unnecessary. Moreover, this led us to a similar approach with the front motor connected to the ball scooper bucket, so we connected the scooper arm directly to the motor in this setup.
The input gear has a 38 mm diameter, and with the maximum torque available from the motors in our configuration, the highest radial force was 11.29 N, equivalent to 1.15 kg. We contacted the manufacturer and they specified they rate their motors for 2kg of radial load. With this advice, we found separating the motor from the input gear by a second shaft and flexible shaft coupler was unnecessary. Moreover, this led us to a similar approach with the front motor connected to the ball scooper bucket, so we connected the scooper arm directly to the motor in this setup.
CAD
CAD
Electrical Diagram
Electrical Diagram
We used an Arduino Uno since it nearly had enough GPIO for our purposes. The motors that we used had an integrated encoder, and we used an MD13s motor driver to run it. Unfortunately, the Arduino Uno only has two interrupt pins, so we were only able to use the encoders on the drive motors for PID speed control. We had three LEDs to show the various states, and a button for user input. Finally, we also used the HCSR04 ultrasonic sensor and a force sensor that increased its resistance with force, making it a good use in a voltage divider circuit.
We used an Arduino Uno since it nearly had enough GPIO for our purposes. The motors that we used had an integrated encoder, and we used an MD13s motor driver to run it. Unfortunately, the Arduino Uno only has two interrupt pins, so we were only able to use the encoders on the drive motors for PID speed control. We had three LEDs to show the various states, and a button for user input. Finally, we also used the HCSR04 ultrasonic sensor and a force sensor that increased its resistance with force, making it a good use in a voltage divider circuit.
State Machine
State Machine
Routine 1 (enter collecting):
LED: Green
Drivetrain motor: on
Ball emptying linkage motor: off
Routine 2 (enter error):
LED: Red (flashing)
Drivetrain motor: off
Ball emptying linkage motor: off
Routine 3 (enter full):
LED: Red
Drivetrain motor: off
Ball emptying linkage motor: off
Routine 4 (enter idle):
LED: Yellow
Drivetrain motor: off
Ball emptying linkage motor: off
Routine 5 (enter emptying):
LED: Yellow (flashing)
Drivetrain motor: off
Ball emptying linkage motor: on
The above state machine diagram represents the final version of our state machine used during the showcase. All states have remained the same during the project. There were some changes to the transitions. These include: removing front distance checking during the emptying state, and checking for button presses during the drive state to change to the idle state if pressed.
The above state machine diagram represents the final version of our state machine used during the showcase. All states have remained the same during the project. There were some changes to the transitions. These include: removing front distance checking during the emptying state, and checking for button presses during the drive state to change to the idle state if pressed.
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