Student Projects
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Contact Information:
Country: England
Year Submitted: 2018
University: University of Leicester
List of Team Members (with year of graduation): Ross Anderson (2018), Ross Cameron (2018), Jake Fairweather (2018), Josh Hough (2018), Dan Whitear (2018)
Faculty Advisers: Ioannis Kyriakopoulos
Main Contact Email Address: dmw31@student.le.ac.uk
Project Information:
Title: Design and Manufacture of a Modular Wear Testing Rig
Description: Tribological processes account for around 23% of the World's energy consumption, inferring that comprehensive research into these processes has the potential to reduce global greenhouse emissions by a great deal. The focus of this project was to design and build a modular wear testing rig, capable of simulating tribological contacts, such that the data obtained may be used to better understand the mechanisms at play.
Products: NI myDAQ (part number 781326-01), NI LabVIEW 2017 , modules, and toolkits used in project.
WEG CFW300 IP20 0.75kW 230V 1ph to 3ph AC Inverter Drive, Sensor Techniques Limited Model STC ‘S’ Beam Load Cell, Analog Devices AD8057 ARZ op-amp, Honeywell SS49E Hall Effect sensor
The Challenge:
Tribological processes account for around 23% of the World's energy consumption, inferring that comprehensive research into these processes has the potential to reduce global greenhouse emissions by a great deal. The focus of this project was to design and build a modular wear testing rig, capable of simulating tribological contacts, such that the data obtained may be used to better understand the mechanisms at play. The Engineering Department at the University of Leicester has one wear testing rig, capable of running ‘pin-on-disc’ tests. Due to shortcomings of the current rig, the decision was made to design and manufacture a modular wear test rig capable of performing multiple types of wear tests. These types of wear test are:
- ‘pin-on disc’, where a load is applied to a test sample and the sample is rotated at a variable speed
- a reciprocating wear test, where a load is applied to a test sample and that sample reciprocates at a variable sliding distance and speed
The rig was designed with accuracy, reliability and maintenance/longevity in mind, as these are the biggest faults of the existing test rig. Basic use of LabVIEW was specified within the requirements document for the project, however it was the team's decision to integrate automatic control within the new test rig, to provide a more streamlined solution. The team focused on the 'reciprocating' wear test, due to budgeting and timescale constraints, but ensured that no other tests would be precluded by the design, in order to meet the objective of a 'modular' wear testing rig.
The Solution:
Analysis of the requirements specification document provided to the project team yielded two main areas in which the use of National Instruments hardware, alongside LabVIEW, would prove beneficial. These areas are as follows:
- Control of motor speed
- Accurate measurement of tangential (or friction) forces
The hardware chosen to achieve these points was a NI myDAQ, frequency inverter, Hall Effect sensor and a load cell. By their nature, wear tests are liable to exhibit increasing friction throughout the test duration, due to wear debris and the tribological mechanisms at play. For this reason, the use of feedback control to maintain motor speed was chosen as the best solution. This required the use of a Hall Effect sensor and a magnet to measure RPM, and the use of a frequency inverter with digital inputs for automatic control. Before the required hardware was received, prototype LabVIEW vi's were produced in order to demonstrate the functionality required for automatic control. The functionality required from the prototypes is summarised as follows:
- RPM measurement
- RPM setting, monitoring and tracking
- Failsafe for drastic RPM changes
- Test duration setting and cutoff
With functionality required from the LabVIEW VI specified, prototyping of the LabVIEW VIs was undertaken, with the goal of producing VIs that would demonstrate feasibility of the control system and that would require minimal alteration upon implementation of electrical hardware. The prototypes built are summarised as follows:
RPM Sensor
The prototype VI for the Hall Effect sensor (RPM counter) is shown in Fig. 1 below. The user can vary the ‘Reading From HES’ control to either 1 or 0, representing a change in the signal received from the Hall Effect sensor. The VI then counts the number of ‘Changes’ in the signal in the top ‘while loop’ (each ‘Change’ indicating a pass of the magnet) and converts this to an RPM using the number of iterations of the second ‘while loop’, which is timed to run every second.
Fig. 1
RPM Setting, Monitoring, Tracking and Failsafe
Fig. 2 below shows the front panel controls and the default states in the state machine structures for the RPM setting, monitoring, tracking and failsafe prototype vi. The user can alter the ‘Desired RPM’, and the VI changes the ‘Actual RPM’ to equal the desired value. The top state machine compares the actual and desired RPMs. If the two are equal, it will continue to monitor them, if not it will switch to the ‘Change RPM’ state (see Fig. 3). The bottom state machine waits for a certain amount of time (to allow the RPM to reach the desired value – i.e. to let the motor get up to speed) and then checks for sudden changes in RPM (see Fig. 6 and Fig. 7).
Fig. 2
Fig. 3 shows the ‘Change RPM’ state in the top state machine. The state decides whether to increase or decrease the RPM by comparing the actual and desired RPMs.
Fig. 3
Fig. 4 shows the ‘Increase RPM’ state in the top state machine. This state increases the ‘Actual RPM’ in increments of one.
Fig. 4
Fig. 5 shows the ‘Decrease RPM’ state in the top state machine. This state decreases the ‘Actual RPM’ in increments of one.
Fig. 5
Fig. 6 shows the ‘Monitor RPM’ state in the bottom state machine. After the bottom state machine has waited for the RPM to reach the desired value, it begins to check for sudden, large changes in RPM, so that if there was a fault, the system could be shut off. If the difference between the actual and desired RPMs exceeds 25, it will transition to the ‘RPM sudden change’ state (see Fig. 7)
Fig. 6
Fig. 7 shows the ‘RPM sudden change’ state in the bottom state machine. This state simply stops the VI.
Fig. 7
Test Duration
The prototype VI for setting the test duration can be seen in Fig. 8 and Fig. 9. The user can vary the desired and actual RPM values, so that the timer will only start when the RPM is at the desired value. The user can also set the desired test duration in minutes, and the VI will stop when this time has been reached.
Fig. 8
Fig. 9
Hardware Implementation, Testing and Redesign
Following initial design and prototyping, the DAQ, Hall Effect sensor, inverter, motor, and load cell were all acquired, allowing for implementation and testing of the electrical system.
RPM Sensor
The RPM sensor was set up and tested first. A vi was produced to count the number of voltage peaks observed by the sensor as the magnet passed it. This vi can be seen in Fig. 10, below.
Fig.10
Inverter Setup/RPM control
With the RPM sensor working, the next task was the setup and programming of the inverter to control motor speed via digital inputs. The inverter was wired up and tested using its control panel, verifying that it could successfully run the motor. The control panel was then used to program in the relevant motor parameters to prevent overloading. Upon programming the inverter to respond to digital inputs and setting up a prototype circuit and corresponding VI to activate these inputs, it was discovered that a grounding issue inherent to the DAQ/inverter combination caused the DAQ to shut off when the required circuit was made. Due to time constraints, resolution of this issue was not possible. Unfortunately, this issue entirely precluded the use of automatic control within the project, without significant further work being undertaken.
Load Cell
The final piece of hardware to be set up was the load cell. The output at rated load for the load cell obtained was 1.858mV/V, which at a supply voltage of 10V would result in a voltage reading of 18.58mV at the maximum rated load of 10kg. In order to obtain a good resolution of the output signal, an amplifier was used. The analogue inputs of the DAQ are able to measure ±10V (maximum working voltage ±10.5V), requiring that the signal be amplified by a factor of approximately 538 in order to achieve the best possible signal resolution. An AD8057ARZ amplifier with a gain of 55dB (gain factor of approximately 562) was chosen as the closest available option to the required gain, with a slight reduction in supply voltage allowing for maximum signal resolution. As the DAQ supply voltages available were ±5V or ±15V, a voltage divider circuit was used to supply a suitable voltage. Use of a 1kΩ and a 5.6kΩ resistor and the +15V supply resulted in an output of approximately 9.6V. So, the output voltage from the load cell at rated load, with a 9.6V supply, after amplification is:
1.858 * 10^-3 * 9.6 * 562 = 10.02V
This value falls within the working voltage for the DAQ, and provides good signal resolution.
Final System
Despite setbacks regarding feedback control for the system, the design process resulted in a system capable of measuring and recording RPM, average interfacial speed and frictional force data. The final vi can be seen in Fig. 11:
Fig. 11
