Robotically Assisted Manufacturing Workcell

The Vandalbot Senior Design team was tasked with developing proof of concept for a robotically assisted work cell that could assemble various rivet and nut plate assemblies. The purpose is to create a automated and safe work cell that would free workers from the highly repetitive task of putting these assemblies together. Thorough documentation of the project will be helpful to future students and design teams and provide students with practical experience in industrial automation.

Problem Statement
Hundreds of nut plate assemblies are built every day using human labor. This work is monotonous and physically taxing for the workers involved, often resulting in injury due to repetitive motions. In addition to causing workplace injury, the repetitive nature of these tasks results in an accumulation of process errors. The goal of this project is to create an automated work cell centered on an industrial robot which can tirelessly and consistently perform these assembly tasks which are ill-suited for human workers.

Design Goals

 * Safety
 * Robot enclosure protocols
 * Removing humans from the manufacturing task
 * Providing students with practical experience in industrial automation
 * Creating thorough documentation for future students and design teams
 * Creation of a proof of concept for a process that can be used for production
 * 1st semester goals
 * Understand current assembly process
 * Establish baseline of mock parts
 * Develop a proposal for an automated process using one industrial robot
 * Develop a concept for tooling and part orientation, including prototype tools and fixturing
 * 2nd semester goals
 * Programming of the robotic process
 * Final design and construction of fixturing and tooling required for process
 * Presentation and exit (feeding/removing parts in work cell)
 * As a stretch goal: multiple, collaborative robots (will require additional safety features)

Detailed Specifications
1. Quick and Efficient Process
 * At least as fast as a human operator
 * Robot should be able to operate with minimum human input
 * Robot can be adaptedto various nut plate assembly tasks with minimal setup time

2. Correct Rivet Placement
 * Rivet depth must be withing the tolerance requirements, -0/+0.010 inches
 * Rivets must be placed at a minimum distance of two rivet head diameters away from the edge of the plate and from other rivets

3. Correct Nut Plate Placement
 * The clocking of the nut place is positioned within 2 degrees from the drawing specifications
 * The nut plate is positioned within 0.030 inches from the drawing specifications

4. Minimal Human Interaction
 * Assembly requires at most a worker to place parts into the work cell and remove completed assemblies

Current Manufacturing Process
This is the current manufacturing process for a nut plate assembly as built by a human worker. Note that some assemblies have multiple types of rivets and nut plates.
 * 1) Receive 'shoebox' full of parts
 * 2) Follow specifications on work instruction
 * 3) Gather materials
 * 4) Use a test plate to check Winslow drill depth
 * 5) Drill countersinks and rivet holes with Winslow drill
 * 6) Load rivets and tape down
 * 7) Flip part over and place nut plates into position
 * 8) C-squeeze to fasten rivets
 * 9) Place completed assemblies back into ‘shoebox’

Robotic Cell Improvements/Problem Solving
1. Encoder Error
 * Traced back to dead encoder batteries
 * Replacement batteries were ordered and issue was resolved

2. Calibration
 * Joints J1, J2, J3, and J5 can be calibrated because they have built in mechanical stops
 * Joint J4 requires a CALSET screw in order to set a mechanical stop for calibration
 * Joint J6 requires a CALSET jig to calibrate
 * CALSET screw for J4 located inside J3 access panel
 * Contacted Denso support regarding CALSET jig, which is sold separately from the robot
 * J6 CALSET jig ordered and received
 * Robot fully calibrated and ready for operations

3. Enable Auto Mode Error
 * Error appeared when attempting to switch robot to auto mode in order ro run programs
 * According to the manual, there is a circuit that needs to be shorted in order to enable this mode
 * Robot assembly includes a toggle switch to short this circuit
 * Tested switch with DMM and found it functioning properly
 * Checked fuses in robot controller and found that none were blown
 * Tried a different robot controller and problem continued
 * Checked wiring and found a lose wire, which was soldered back together
 * Robot now able to switch into auto mode and run programs

Project Design
Our concept for the solution of the project is to use a two robot work cell. The part will be introduced into the work cell and fastened into place. On one side of the plate one robot (robot one) will have a nut plate drill end effector and will go to each location and drill the holes and countersinks for the rivets to go into. The robot will then go through a tool change switching out the drill for a rivet feeder/bucking bar combination end effector. On the other side of the plate the other robot (robot two) will have an impact hammer with a nut plate grabber attached to the end. Robot one will go to the first location and, using a pneumatic system, feed rivets into both holes. Then robot one will move slightly and act as a bucking bar for the robot two. Robot two will go to a nut plate dispenser and grab a nut plate then bring it to the first location and place the nut plate over the rivets. It will then use the impact hammer to push down the rivets. The robots will then repeat this process for each nut plate location.

Reducing the Scope of the Project
After working on some preliminary designs and talking with our client we decided to narrow the scope of the project. We had decided that the most important aspect of the project was creating a way to position the nut plates and rivets and squeezing the rivets. Since using a robot to locate and drill holes is a well understood operation we decided to remove them from the scope of the project as well as removing the need to do a tool change. Since we currently only have a one robot work cell we decided to replace robot one with a fixed rivet feeding system.

Our proof of concept will include the robot with the impact hammer and nut plate grabber end effector. Each test part will already have the rivet holes and countersinks drilled into them. The part will be introduced into the work cell and fastened into place. The rivet feeder will shoot the two rivets into place and then a pneumatic actuator will slide a plate on the face of the rivet feeder to act as a bucking bar. The robot will move to the nut plate dispenser and grab a nut plate then place the nut plate onto the rivets. The impact hammer will then squeeze the rivets into place. The part will then be manually moved to the next location.

Rivet Feed Anvil
Rivets are pneumatically fed through the two slots. An actuator slides the plate to act as a bucking bar for the impact hammer. Since the thickness of the plate is equal to the length of one rivet the plate is able to move without the rivets blocking the way since one rivet will slide within the plate.

Robot End Effector
The impact hammer has a duel impact head so it can hammer both rivets at the same time. Testing will allow us to determine what force from the hammer will be necessary to get the desired results.

Nut Plate Grabber
The nut plate grabber is a 3D printed part so it will have just enough spring in it to be able to let release the nut plate once placed. The wedged ends help in grabbing the nut plate and the nut plate will be pressed against the back profile to ensure accurate positioning.

Nut Plate Dispenser
The nut plate dispenser is tilted at an angle for the nut plates to overcome static friction. Rails within the dispenser prevent nut plates from over lapping. Dispenser allows nut plate grabber to precisely seat each nut plate by pushing it against the back wall.

Part Frame
Operator will manually clamp each part into place. The frame is able to adjust the angle of the part according to part specifications.