[CNIT 581-SDR / Spring 2019] Week 16 – Final

These days, many people use smart devices. As a result, many people easily paint with electronic devices. For the convenience of our users, we enabled robots to draw pictures through electronic device applications. We designed a new robot, developed an application, and controlled the robot by following the coordinates obtained from the drawings drawn in the application.A. System ArchitectureTo implement an interactive robot drawing system, we setup a mobile robot system, which consists of two parts. The first one is that possible user draws a line or a shape in a mobile application, selecting the size of a drawing. The information of the drawn trajectory saved into a tuple list and will be saved into a JSON format file so that the mobile application and robot system can share the user’s drawing.Considering a robotic system that can draw some picture anywhere, Hamster robot platform is selected. The physical robot system consisting of two mini robots calculates a new path on physical spaces to let the robot draws by reading the data. The mobile robot system reads calculated trace throughBluetooth connections so that robots can move. We also use a simple socket communication to access data created on the mobile device beyond the restricted area of iOS application.

Design of RobotWe designed our robot by moving two robots and attaching the pen on the center between two robots. One robot can be used for our project instead of using two robots if the pen can be hold on the center of robot’s wheels. The robot can leave the trajectory while it is turning right or left if location of penis not on the center of wheels.Both mini mobile robots with a pen attach in between the wheels can sketch anything user drawn on the mobile devices platform with ease. Instead of integrate servo motor to control a pen, we used a 3D printed bracket to hold the pen for simplicity in the project. The complication from utilize mini servo motor with the Hamster mobile robot are due tot he power needed to activate servo motor are 4.8v but the battery of the Hamster robot supply 3.7v. There are only two ports supported by the mobile robot where the mini servomotor used three ports. To control the Pulse Width Modulation(PWM) through Hamster mobile robot with the limitation of ports available are a difficult task in tuning to get the correct angle for servo motor movement.The initial stage of bracket design are loose and affected the movement of the robot wheels and the pen drawing direction.Different 3D brackets are designed for product testing to hold both Hamster robot with the consideration of tilted outer wheels and the correct hole size to hold the one size pen without material deformation from the Polylactic Acid (PLA)bracket.

We built a simple mobile iOS application to provide user to draw lines on the display. There is a white space with five buttons as Fig 3 shows. The three buttons below the white drawing area are for giving information about picture size. If the user touches the mobile display to draw something, black lines appear following users’ gestures. After the application finishes, and clicks ’send’ button, the stacked pixel coordinates become one of the components of the JSON file. The reason that the JSON structure is chosen is that the robot drawing system requires sizing and coordinate information together.This file structure allows the programmer to easily access the desired data using variable names.We have found that there is a significant difference between our expectation and real results of coordinate structure. This is because the pixel system does not have all the series of coordinates for the given input of the user. Instead of having all the coordinates, they have a gap between each coordinate as the set of Bezier path segments. This path has a start point and a motion point for each sub-path, which is also affected by the angle of the path the user has drawn. The greater the change in the curve angle, the narrower the gap between the subpaths become. Since our robot system calculates turning angles between the current original coordinate and new target position, a large burden can be generated when sub path shaving a narrow gap between them are given. Therefore, while cleaning the coordinates marker such as ’move to’ and ’quad to’ flags from bezier path, we have been working to simplify coordinate values at regular intervals.

There was an issue of accessible data to transmit theJSON data to our robot system. We could not get created the structured file from another device because the iOS environment does not allow each application to access other system resources outside of the application area by using the concept of a sandbox. As an alternative, there were two possible solutions: socket communication and the creation of a representational state transfer application program interface(RESTful API). We chose the socket communication because our project has been done in a small area, and we are using our local laptop for Bluetooth communication, thus creating RESTful API through HTTP requests was redundant. Therefore, we utilized localWi-Fi environment to transmit the JSON file from the tablet to a local device for python-based robot system, opening a port to use Transmission Control Protocol. (TCP)

The movement of robot is divided into two methods, changing its heading position and moving forward following coordinates. The control system reads the series of coordinates of drawing stored in JSON file sent from the application. With current coordinates and target coordinates, the robot can get the heading angle and distance to move. After calculating the heading angle and distance, the robot turns its heading angle coordinate and moves a given distance towards the target coordinates.

θ= arctan(y′−y, x′−x)

The heading angle can be calculated by using arctan. The coordinate (x’, y’) is target position and the coordinate (x, y) is the current position. The turning angle the robot should move is difference between current heading angle and new heading angle. The set of robots turn in place with turning angle which is converted to degrees from radians. After calculating the turning angle, the robot decides the turning direction. The robots turn right if the angle is in [0, 180] while it turns left if the angle is in [-180,0].dI:dO=vrI:vrO


The direction of robots should be opposite and outer most wheels should faster than inner wheels. The outermost wheels were farther than inner wheels 3.8 times from the center, so the velocity of outermost wheels were 3.8times faster than the velocity of inner wheels. dI is the distance between center and inner wheel and dO is the distance between center and outer wheel. vrI and vlI mean the velocity of each robot’s inner wheel, and the velocity of each robot’s outer wheel.If the robot turns forward to the target position, the robot is ready to move. The PID controller [10] is needed to make the robot move precisely. The robot cannot go straight because two motors could have different winding resistances such as different drive currents and torque and the floor might have varying surface friction. The robot keeps going towards to the right side, so we applied the compensation to right wheel to make it move straight. We calculated the compensation for the right wheel by using PD controller to reduce the acceleration.We tuned the movement by PD controller instead of PID controller, because using PD control has an advantage that the robot can controlled smooth and stable.V c=e∗Kp+ (e−e′)∗Kd(4)The difference between target acceleration and current acceleration to make robot move straight. e means the difference between current acceleration and target acceleration and multi-plied by the constant Kp, called the proportional gain constant.e’ means the previous acceleration error. The compensation of velocity is calculated by adding the differential of error multiplied by Kd, which is the derivative gain constant, to e* Kp. The constants Kd and Kp were set by the experiments.The robots go straight for certain time in proportion to distance between two coordinates. The size of drawing can be changed by changing the ratio between moving time and distance.

Demo Video: https://www.youtube.com/watch?v=TtvmhD8yqoQ&feature=youtu.be

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