Final Design

Description of Final Design:

In the final design, the sample was clamped between two aluminum surfaces covered in rubber. These grips used to clamp the sample were simple rectangular bars of aluminum, fastened down with socket head screws. A linear potentiometer provided a way of measuring the distance between the grips, serving as the feedback sensor for closed loop strain control. The linear potentiometer’s output was sent to an Arduino Uno, which converted this signal to an input to the linear actuator. The linear actuator pulled one grip, while the other remained in place.

Table 1. Comparison of Actuators

Justification of Final Design Decision

Initially, the ball screw and stepper motor combination was found to have the most flexibility, since the parts could be ordered separately to meet the project’s specific needs. However, the integration of separate components into the load frame design was expensive and complex. The captive linear actuators offered by Haydon Kerk met all the functional requirements in one self-contained component. Although the size 8 double-stack actuator fit all the functional requirements, an applications engineer at Haydon Kerk recommended using the size 11 single-stack actuator, since the load frame would be operating very near the size 8 actuator’s recommended force limit.

Annotated isometric view of micro load-frame CAD model, showing the key components

Key Components

Linear Actuator

The linear actuator provided a means of pulling samples apart in tension. It was a pre-packaged unit containing a stepper motor and other mechanical systems that converted rotary motion into linear motion.

Functional Requirements

- Travel 25 μm (at most) per motor step
- Apply constant axial forces up to 50 N
- Actuator body less than 29 mm high

Comparison of Actuators

Haydon Kerk Size 11 Captive Linear Actuator

Grips

The grips held the samples while they were being stressed in tension. 0.02” thick 90A polyurethane was applied on the surface of the grips to electrically insulate the samples.

Functional Requirements

- Hold samples in tension, up to 50 N
- Minimal (<1 μm) lateral deflection of rubber surface
- Accommodate samples up to 2 mm thick
- Electrically insulate samples

Comparison of Grip Designs

Table 2. Comparison of Grip Designs

Justification of Final Design Decision

An initial search for commercially available grips was conducted. All of the commercial grips found were either too large, or too expensive. This prompted a decision to machine custom grips. A preliminary design involved the use of a cam-lever system. While easy to use, the cam style grip would have been too hard to fabricate in the time frame of this project, and too hard to electrically insulate. Simple, plate style grips were selected in the end. Instead of directly tapping aluminum, helicoil inserts were used to avoid relatively soft aluminum threads.

Table 3. Comparison of Displacement Sensors

Justification of Final Design Decision

A laser was an initially attractive option because of its resolution and non-mechanical operation. After further investigation however, commercially available lasers were either too large, or too expensive. A linear potentiometer was chosen because of its simple operation, resolution, and ability to easily interface with an Arduino. The decision between the round and square potentiometers was influenced by price, and ease of mounting on the load frame. While the square potentiometer required more machining than the round option, the cost savings justified the added machining.

CAD rendering of final grip design (top) and grips used in the actual prototype (bottom)

Displacement Sensor

The displacement sensor provided feedback for closed loop control of the distance between the grips.

Functional Requirements

- Measure distance between grips
- Accommodate samples up to 2 mm thick
- Fit within load frame constraints

Comparison of Displacement Sensors

ETI Systems Linear Potentiometer

Controller

The controller is what carried out the closed loop control of the linear actuator. It received an input from the displacement sensor and drove the linear actuator to the desired set point, checking the position between motor steps..

Functional Requirements

- Interface with software and motor driver to run motor operations
- Interface with potentiometer and use data for closed-loop motor control

Comparison of Controllers

Table 4. Comparison of Controllers

Justification of Final Design Decision

BeagleBone was briefly considered because it offered a powerful solution. The students in this project were not as familiar with BeagleBone however, and its capabilities were beyond the scope of this project. A LabVIEW card was also considered because it would allow easy control of the load frame, and an attractive user interface. The LabVIEW card was not selected because of its high cost, and the lab in which the load frame was used did not have LabVIEW. Arduino was chosen because of its extensive online support and its simplicity.

Arduino Uno Microprocessor, ADC shield and Motor Driver shield

3D-printd housing for Arduiono and connected shields

Motor Driver

The motor driver took inputs from the controller and provided power to the linear actuator.

Functional Requirements

- Operate motor at manufacturer’s recommended voltage and current
- Able to interface with Arduino controller

Comparison of Drivers

Table 5. Comparison of Drivers

Justification of Final Design Decision

After testing the motor and potentiometer setup with several drivers, an Adafruit Arduino shield was chosen for the final design. The primary sources of noise in the load frame circuitry came from breadboard usage, as well as unshielded cables. Ultimately, the Adafruit Arduino shield was selected because it would eliminate the need for external circuitry and reduce the noise caused by this circuitry.

Blocks

The blocks were the three main structural components of the load frame. The blocks consisted of the motor block which held the linear actuator in place, the moving block which was pulled along the rails by the linear actuator, and the stationary block.

Functional Requirements

- Mount individual components, including potentiometer, grips, actuator, rails, and bearings
- <3 μm deflection at applied load of 50 N
- Fit within dimensional constraints of load frame

Justification of Final Design Decisions

6061-T6 aluminum was chosen because it was readily available, met the design requirements, and was easily machinable. The blocks were designed with an overall goal of minimizing the amount of space taken up. To this end, a pocket was machined in the stationary block so that the potentiometer could be housed inside. In addition, holes were machined in the moving block to allow the bearing and bushing to be mounted semi-internally in the moving block.

Moving Block (Middle Block)

Stationary Block (Potentiometer Mount Block)

Motor Mount Block

Cross-section view of load frame, showing how linear actuator (right) and potentiometer (left) connect to the moving (middle) block

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