Integrated System Build & Test
Table of Contents
Team Vision for Integrated System Build & Test PhaseDuring this phase, we had originally planned to begin construction of the Z axis. However due to several challenges (workshop bottlenecks, worldwide pandemic, pending orders) the 3rd axis was unable to be integrated.
The team successfully built and tested the Y axis while integrating it with the previous systems. The phase was also highlighted by the transition to working on the project remotely due to COVID-19.
Adding the Second Dimension
The StructureA temporary structure was constructed out of wood and 3D printed brackets to allow for quick testing of the 2nd axis while waiting for the 3rd axis components to arrive. The core function of the temporary structure was to align the Y axes in parallel, and to provide overall structural members to maintain rigidity and reject bending, twisting, warping, or other undesirable forces. Since the Z-axis parts did not all get delivered during the active part of this semester, the wooden members served adequately for the duration of the integrated testing phase.
In OperationThe structure above was operated with the Duet2 Wifi successfully. The procedure for building and aligning the Y-axes with the X axis required some iteration and improvement due to issues that will be discussed in the Test Results section. The gif below demonstrates a circle generated from an STL model by Cura. Note that the video demonstrates motion in both the positive and negative directions for the X and Y axes.
Test Results Summary
The test plan for the motion and structural systems consisted of several methods designed to confirm the ability of the motors, ballscrews, structural members, and electrical system to integrate and perform at a level that satisfies the relevant engineering requirements. The concept and scope as well as results from the tests for the individual systems will be described and then performance of the integrated system will be summarized.
Motion SystemThe first aspect tested was the robustness of the motors. The most demanding conditions that the motors would have to operate under are high phase current, high velocity, high acceleration, frequent stopping or changes in direction, all while under the normal operating load. The Duet 2 software enables the operator to force the motors into these conditions. Assessment of their robustness was conducted by monitoring the ability of the motor to not miss steps (by using calipers to measure the dimensional accuracy of the performance in comparison to the command), the ability of the motor to continuously overcome the torque and speed obstacles presented under the demanding operating conditions, and the temperature of the motor block over time. Throughput of the motion system can be interpreted by comparing the obtainable velocities and acceleration profiles of the axis motors with the obtainable flow rate of the extrusion system as demonstrated in the previous phase.
The following values are the stepper driver configurations we found to be most optimal given the motor testing described above. The rows crossed out were not in use at this point.
We failed to develop an analytical model to predict flow rates attainable by our extruder and auger geometries as a function of the mixture characteristics. We also failed to test the extrusion system to its limits in order to validate the attainable flow rates. Although, in comparison to the motion and structural systems performances, preliminary calculations indicated that the extrusion system would be the limiting factor in determining the operating velocities and acceleration profiles for the system as a whole.
Videos taken during the motion system testing can be accessed through the following links:
Structural SystemThe second aspect tested was the performance of the structural system. This test was performed by aligning the Y axes and the X axis using a wooden rig, and operating the system through the full range of motion allowed by the operating area of the X-Y plane. Assessment of the system's performance was conducted by monitoring whether the print-head could cover the entire operating area without inducing binding, constraint, twisting, warping, or bending to the structural members. Throughput of the combined motion and structural systems was interpreted by comparing the results of the two tests to determine where the operating regions of speed, acceleration, and mobility overlap, and then comparing these abilities with the obtainable flow rate of the extrusion system as demonstrated in the previous phase.
The tests revealed that there were a few components on the structure that had fabrication issues or defects that impacted performance. The slight misplacement of several bolt receiving holes on the mounting brackets that join the X and Y axes caused over-constraint. The tests also suggested that the build procedure was an important factor in the resulting system performance. The build must be performed on a level surface or twisting and bending can result in the structural members. The ballscrews must be well lubricated before every use to avoid the possibility of binding due to frictional forces. Perhaps the most important lesson from the structural system testing is that alignment of the Y axes must be performed carefully and exactly in order for the axes (separated by the X axis and supports) to maintain parallelism. The most difficult challenge to overcome from this test was the binding induced by a lack of perpendicularity between the X and Y axes. The issue caused the motors to continue driving the X axis out of its slight misalignment and twist the entire structure several centimeters until the motors could no longer support the torque required to drive the X axis down the ballscrew. We believe that revising the build procedure to accommodate a more thorough and exact alignment procedure would resolve these issues.
The revised alignment procedure relies on the principal that if several important fastening points are not tightened until after their respective members are aligned, then there is significantly less strain on the structural members as a result of the building procedure. Using the X axis as the alignment device, the structural members were fastened into place only when the X axis was forcing the Y axes to be parallel at that point. This procedure would be verified using physical measurements to ensure perpendicularity between the X and Y axes, and parallelism between the Y axes. Using a continuous improvement methodology, the cycle can be completed as many times as necessary. The resulting performance is full operation ability at any point on the X-Y plane, without binding, bending, or other forms of constraint.
Combined Motion and Structural SystemsBy confirming the abilities and limitations of the subsystems independently, the combined subsystems could be tested for their ability to meet some of the engineering requirements. Two evaluations were designed to facilitate this testing under the limited resource and remote access conditions resulting from the COVID-19 event. The first test attempts to measure the performance of the printer's ability to trace a path generated as a simulated layer. The test mimics a style of digital art called light painting. By suspending a camera above the printer, with a remote shutter device, and a programmable blinking LED on the center of the print head the path can be traced as a long exposure photo of the print job is taken. Using the actual path recorded in the photograph and theoretical path generated by the slicer, the performance of the printer to realize a path could be measured as a function of its actual variation from the theoretical path. There was not enough time to actually perform the variation analysis. The second test measures the performance of the motion and structural subsystems in achieving dimensional accuracy. Similarly to the concept of the light painting test, this test uses a marker attached to the center of the print head to create a visual representation of the actual path on a whiteboard serving as the print bed. While the dimensional accuracy of the line being drawn is a function of the marker tip itself, the dimensional accuracy of the print head to maintain the path can be extracted from comparison with the marker drawing representation of the actual path and the theoretical path generated by the slicer. The difference between the light painting and the marker tests is that the resolution and scope of the marker test is greater than that of the light painting. The light painting was used to assess preliminary feasibility, and the marker drawing serves as the final test.
The results of analyzing the performance of the marker test conclude that variation between actual and theoretical positioning is very low. In fact, the marker drawing did not provide enough resolution to realize an observable variation, so for the example provided below the deviation was 0mm. It is possible that the experiment could be redesigned to generate a physical "read-out" with greater resolution than the marker. However for the scope of this project, sub-millimetre accuracy is not necessary to meet the engineering requirements. To extract the results from the test the theoretical positions for a movement (from the G-code) are compared to actual measurements of the position as recorded by the marker. For example, layers 31 and 32 of the pyramid path should be 30mm and 15mm squares according to the G-code file (segments below.) Confirming the physical dimensions of those layers, the error is 0mm with a ruler.
G0 X425 Y335 Z320
G1 F2400 X395 Y335 E159.15494
G1 X395 Y305 E159.15494
G1 X425 Y305 E159.15494
G1 X425 Y335 E159.15494
G0 X417.5 Y327.5 Z330
G1 F1399 X402.5 Y327.5 E79.57747
G1 X402.5 Y312.5 E79.57747
G1 X417.5 Y312.5 E79.57747
G1 X417.5 Y327.5 E79.57747
We did observe some shortcomings to this test besides low resolution. There is enough vibration in the system to induce some "wobble" to the tip, capturing the, albeit small, but noticeable vibrations in a measurable form. With a higher resolution capturing mechanism, this would increase the scope of the test, however the large tip dry-erase marker does not provide that. Additionally, it was difficult to minimize error in the difference between the levelness of the print head, marker tip, and print bed. While the test is limited by it's resolution, it confirms the feasibility of the visual-reference style accuracy tests, it provides proof that the printer is meeting engineering requirements related to positional accuracy and dimensional variation, and serves as a baseline test to improve reliability and scope.