Turntable
!!! DO NOT REMOVE THE LOAD CELL ADAPTER PLATE FROM THE LOAD CELL !!!
The LTWT turntable is, in essence, a giant stepper motor with a load cell attached directly to its output shaft, and an adapter plate on the load cell to facilitate the easy mounting of multiple different experiments. Pictured below is the assembly, with AE2610's wing experiment mounted. Some details on this installation:
An ATI Gamma 6DOF load cell with SI-130-100 calibration is currently integrated. An ATI IFPS is used to power the load cell and amplify its strain gage outputs, with a National Instruments DAQ capturing the 6 IFPS voltage outputs.
The load cell is thermally isolated from the stepper motor via nylon standoffs; this creates an air gap and a non-conductive path for the stepper heat to flow into the load cell.
The height of the assembly is set such that the top face of the load cell adapter plate is flush with the floor of the tunnel.
There are no bearings preventing the stepper from experiencing side loads and moments caused by the load. Instead, it is assumed that the internal bearings, one top and one bottom spanning the length of the motor, provide sufficient isolation. This may not be true for all scenarios.
The stepper motor is driven by a closed-loop stepper driver which has an encoder to detect and rectify any un-commanded rotation (provided it has enough torque to do so). Stepper motors work by driving the coil a number of steps, which are the smallest unit rotation. Our stepper is set to 10,000 steps/revolution.
An ESP32 microcontroller (essentially a better Arduino) interprets serial commands given by the user and converts this to the requisite pulse commands which are then sent to the stepper driver. This code is essentially a wrapper to the FastAccelStepper library, but with a convenient API for commanding different forms of motion. The primary forms of motion include:
Move by a certain angular displacement with or without velocity or acceleration limits
Move at a constant angular velocity with or without acceleration limits
Sweep between two angles at a certain velocity with or without acceleration limits
Execute a pre-defined motion (requires hard-coding)
There is no active measuring of angular position. Instead, the ESP32 keeps track of how much rotation it has commanded (aka how many steps) and sends this value back over the serial connection. This means missed steps or slipping motion that occur due to lack of torque, for example, get missed. The stepper motor is massively over-specced and a closed-loop driver was chosen for this reason.
Mounting to the Turntable
!!! DO NOT REMOVE THE LOAD CELL ADAPTER PLATE FROM THE LOAD CELL !!!
The load cell is fitted with an adapter plate that enables quick installation of different models. This 1/2" thick aluminum adapter plate, the bottom plate pictured below, incorporates counterbored mounting holes for 3 different ATI load cells; Deltas and Gammas (shown in yellow), and Axia80s (shown in red). 1/4" dowel pins, shown in green, are pressed into the adapter plate to provide for accurate location of the model mounting plate (the top plate pictured in its blank form, with corresponding dowel holes shown in green). 1/4-20 shoulder bolts then pass up from the underside to pin the model mounting plate to the load cell adapter plate.
CAD for these parts can be downloaded below:
Highly Precise Mounting
!!! DO NOT REMOVE THE LOAD CELL ADAPTER PLATE FROM THE LOAD CELL !!!
If you need to align your model to the load cell with highest precision, you should locate it off of the dowel pins. To do this, design your experiment or an adapter plate for your experiment, with dowel location holes and tapped holes at the locations in the "key hole locations" image above. The dowel holes should be machined slightly undersized, then reamed to 1/4". These locations should be assured via the use of a mill, CNC, or other similarly precise machine... hand drilling or waterjet will not be in the correct location. The dowels protrude roughly 1/4" so be sure to provide them enough depth clearance.
Decently Precise Mounting
!!! DO NOT REMOVE THE LOAD CELL ADAPTER PLATE FROM THE LOAD CELL !!!
If you can tolerate some misalignment, or you can calibrate this out via an experiment or in post-processing, you can simplify manufacturing to enable use of a waterjet, laser cutter, and potentially even hand-fabrication. To do this, use the aforementioned "key hole locations" in your design, but once you've cut the initial holes, progressively open up the dowel pin holes as required until the plates mate properly. Then, instead of using the shoulder bolts with a 5/16" shoulder diameter, use normal 1/4-20 bolts and washers. You could also use clearance holes instead of tapped holes, with nuts on the top or bottom sides instead.
Rough Mounting
!!! DO NOT REMOVE THE LOAD CELL ADAPTER PLATE FROM THE LOAD CELL !!!
If you are just mounting to the turntable and don't need accurate alignment with the load cell, you can follow the steps above for "decently precise mounting" but machine oversized dowel pin clearance holes from the outset.
Alternative/Improved Mounting Methods
!!! DO NOT REMOVE THE LOAD CELL ADAPTER PLATE FROM THE LOAD CELL !!!
The current load cell adapter is mounted semi-permanently to the load cell, which is delicate and needs proper treatment. However, this can be removed and replaced if absolutely required, such as for replacement for improved mounting methods or temporarily for certain experiments if other users can accommodate it. Please reach out to Lee with suggestions/requests on this.
Using the Turntable
Description of Stepper System
The stepper system includes the ESP32, the motor controller/driver, the stepper motor, and the power supply. The user communicates with the ESP32 via serial commands from a PC. The ESP32 interprets the user commands and generates direction and pulse signals. These signals form the backbone of the stepper control and are sent to the motor controller. Here, these signals are amplified and directed to the various terminals/coils of the stepper motor. The motor controller is essentially a bridge between the low voltage, simple interface of the ESP32 and the high voltage, more complicated wiring of the stepper motor. Power is supplied to the stepper motor using a standard power supply that connects to normal 120VAC wall power. The wiring diagram for the system is below.
Motion Limits
There are multiple types of limits built into the turntable to prevent damage to the load cell equipment, the stand, any models, and the wind tunnel.
There are software limits that dictate the motion of the turntable. If any command to the turntable goes over these limits (without the correct password), nothing will happen. This limits are defined in the ESP32 code and are:
Position: +- 2500 steps (90 degrees) from straight back
Velocity: 100 steps/s
Acceleration: 100 steps/s^2
There are also hardstop limits built into the stand. These are around roughly +-135 degrees from zero. If these are hit, motion will stop and the stepper will start to skip.
In the future, there may be Hall effect sensor limits.
Serial Commands
An ESP32 microcontroller interprets serial commands sent by the user and decodes information from these commands to drive the motor. These commands include parameters for:
Desired position [in steps]
Desired velocity [in steps/s]
Desired acceleration [in steps/s^2]
Current steps per revolution
This is a physical setting on the motor controller. This should not be changed by a user without consulting lab management.
Desired number of bounces/special code
This field includes a desired number of bounces (ie, sweep from +X steps to -X steps) OR a special command code (ie, if = 30006, reset to zero steps position). X is determined in the position field.
Password
This field includes a password entered by the user. If this is validated, the user can input values for position, velocity and acceleration that are outside of the limits implemented in software (but NOT physical limits).
The syntax for this command is: <acceleration,position,velocity,# of bounces,steps/rev,password>. So an example would be <10,28,10,0,10000,1234>. This has an acceleration value of 10 steps/s^2, a position value 28 steps, a velocity value of 10 step/s, 0 bounces, 10,000 steps/rev, and a password entered of 1234 (this is not the real password).
This command can be sent to the ESP32 in various ways:
Serial window
Arduino serial window, RealTerm, or another simple serial comms package. You will likely be fine using LabVIEW, depending on your application.
LabVIEW
A LabVIEW interface has been developed that allows smooth communication with the stepper via serial and logs data from the ATI load cell. This is discussed in detail in a section below.
LabVIEW Virtual Instrument (VI)
A LabVIEW interface was created to interact with the turntable. This was largely for AE lab classes (such as AE2610), but it is generally useful. It is also able to be copied and customized for different research purposes. The image below shows the interface.
This LabVIEW program can control the position of the turntable, handle data collection (force, turntable position, pressure), and readout various measurements onto the screen in real time. The position of the turntable, the ESP32 status and runtime, pressure/density/airspeed, and force data are all displayed.
When the LabVIEW VI is opened, the user must first select a log file. This will be the file that the desired data is written to for later processing. Next, the user needs to select the COM port for the ESP32. The best way to determine the correct port upon first startup, is to try the ports that are listed in the dropdown until one reads out status and runtime. Note: you will have to soft restart the VI (icon in the top left of the VI) for the COM port change to take effect. Once you have selected the correct COM port, you are good to go. However, if the USB between the ESP32 and the PC is disconnected, you must hard restart LabVIEW by closing the VI completely.
Once booted up, force data will display on the two graphs. These must be biased (zeroed, tared) using the bias button on the display.
In the air density section, pressure, temperature, and R must be entered to calculate air density. This is used to calculate airspeed along with pressure from other sensors (such as the Baratron, which must also be tared).
Data is logged using the log data button on the right side of the display.
To move the turntable, press the button to enter jog mode. You then can jog the turntable ~1 degree or ~0.1 degree CW or CCW to the desired position. You can also return the turntable to home using the go home button and zero it at its current position using the zero button.
The LabVIEW VI is kept in the AE labs SharedData drive in D:\AE2610\SubsonicWing.
Current Capabilities
These capabilities are currently able to be deployed for lab classes and research:
These actions can be done in the current version of the LabVIEW VI
Go to any angle position by jogging
Go home
Zero current position
Log force data
Log pressure data
These actions can be done over a serial terminal to the ESP32
Go to any angle by commanding its position
Go home
Zero current position
Bounce from +X to -X
Possible Near-Term Future Capabilities
There are a number of capabilities that are not quite ready for deployment either because they don't fully work yet or it hasn't been decided whether they are necessary:
Hard-coded traverse functionality
Code into the ESP32 a traverse with X stops at stations that are Y degrees apart and wait for Z time.
Programmable traverse functionality
User can input positions for turntable to go to and wait and log data. Possibly done with an Excel or CSV input.
Hall effect sensor limits
Sensors that detect when the turntable is out of limits.
Operating the turntable
Using the AE2610 Wing.VI on the LTWT PC (current as of 10/24/22)
Note the following:
Alpha (angle of attack) measurement - The alpha reading is derived solely from the ESP32 keeping track of the number of steps it has commanded the stepper to take; there is no angle position feedback. Therefore, if you hit the E-STOP at any point during motion, actual alpha will become de-synchronized from expected alpha. Furthermore, if the load on your test article is large enough, the stepper may slip and also become de-synchronized (this is very unlikely!). Either way, the remedy in both cases is to re-home the turntable.
Load cell orientation - The ATI Gamma is orientated such that, when homed, the positive-y axis is oriented directly into the flow (aka measures drag directly), the positive-z axis is directly down towards the center of the earth (aka measures weight of the test article directly), and the positive-x axis is orthogonal to both y- and z- axes, pointing away from the control room (aka measures lift directly).
Logging - This VI only facilitates manual jogging (no automated sweeps), and is configured to log only dynamic pressure, alpha, untransformed axial and normal forces, and pitching moment (as per AE2610 requirements). A software update is needed to add airspeed, direct transformed lift/drag, and all 6 loads logging.
Debugging connectivity: the nuclear option - if you are having issues with the ESP32 connecting, motion not occurring as you want, or some other connectivity issue, the surefire way to get something working (assuming there is no larger mechanical/electrical issue) is to do as follows:
Power everything down and according to the shutdown procedure below
Restart the PC
Re-follow the steps below
Check that the turntable cables are plugged in correctly:
DAQ and ESP32 should be plugged into the USB hub
Stepper driver should be plugged into the E-STOPPED power strip
DAQ and USB hub should be plugged into the PERIPHERALS power strip
Ensure the load cell cable is routed so that it can't snag
Ensure the Baratron pressure transducer is connected to the pitot-static (P- goes to static port, P0 goes to stagnation port), and the voltage output is connected to the DAQ
Verify stepper motor to load cell connection - occasionally, the set screw holding the load cell assembly onto the stepper motor shaft can loosen, causing unsynchronized motion and increased vibration. To check this, lightly wiggle the load cell in yaw and gently pull up simultaneously; if you feel some backlash or the load cell lifting, the set screw will need to be tightened. DO NOT ATTEMPT TO DO THIS ON YOUR OWN... Call Lee Whitcher who will guide you through it or come and do it ASAP.
Install your test article (unless you are geometrically homing the turntable, in which case you should skip this step and return once homed)
At this point the turntable isn't energized - this is so the stepper motor doesn't resist you during installation, reducing forces/moments on the load cell to help protect it from damage
If you are using the provided shoulder bolts and Nord lock washers, make sure the coarser teeth of the two washer faces are facing in at each other, with the finer teeth facing outwards. The bolts, washers, and red allen wrench to install them, should always be kept in the plastic tub in the drawer of the control room PC desk.
Energize the turntable in the control room:
Plug in the orange PERIPHERALS cable
Plug in the black E-STOPPED cable
Hit the green button on the E-STOP box to de-activate the emergency stop
Plug in the USB HUB cable to a free port on the PC
Prep the LabView software
Open LabView 2017 (2021 is also installed on the machine which you SHOULD NOT use)
Navigate to "D:\AE2610\<Latest Semester Folder>" and open Wing.VI
Select a save file location (anywhere except this folder is fine, to keep it uncluttered - make a desktop folder maybe) and give the file a name. The VI will save your data points line-by-line as comma separated variables in a generic file with no type; you needn't give a file name extension as you can open it in Notepad++, but if you give it a .csv extension it will automatically format as a table to be opened in Excel.
Before running the VI, select the correct COM port in the dropdown (as of 10/24/22 this is COM4 but check Device Manager if it has somehow changed)
Start the VI by hitting the triangle Play button on the menu bar
Check successful ESP32 comms by looking for a word in "Motion Status" and an incrementing "Controller Time"
If you don't have successful comms, check the COM port and that the cables are properly connected. If this looks good, power cycle the ESP32 by hitting the reset button on the device itself, or by unplugging the USB cable AND orange PERIPHERALS cable before re-connecting. You may also need to fully restart LabView by closing the VI AND the menu window that comes up after the VI closes, then re-opening LabView 2017
Input the ambient conditions from the FlowKinetics unit to enable air speed measurement
Zero (aka tare/bias) the instruments
Click "Bias Load Cell" to zero the load cell
Click "Tare Baratron" to zero the pressure transducer
Set the traverse home position - see the Homing section below
Verify correct turntable operation
Ensure E-STOP is not enabled (aka the stepper is energized) and check you are seeing motion by enabling JOG MODE and jogging the turntable whilst observing the test article
Ensure load cell is reading by applying gentle loads with your hand to the test article and observing the subsequent loads displayed in the VI
Ensure air speed readout is correct by running the wind tunnel and comparing the calculated speed with the FlowKinetics
Commence your experiment
Shutdown procedure - once you have finished your experiments, follow these steps:
Follow the LTWT shutdown procedure to make the tunnel safe
Unplug all three control room cables (USB HUB, PERIPHERALS, E-STOP)
Uninstall your test article
Return the shoulder bolts, washers, and red allen wrench to the plastic tub in the control room PC desk
Homing the Turntable
There are multiple methods to home the turntable (aka set the zero position to be parallel with the air flow).
Geometrical method
Ensure your test article is not installed
Obtain a large t-square
With the turntable de-energized, roughly align the turntable by eye such that the dowel pins are perpendicular to the tunnel walls (due to the magnets of the stepper, which naturally has 200 steps per revolution, the motor will naturally find a position in 1.8 degree increments)
Energize the turntable
Start the VI and enable JOG MODE
Loosely butt the t-square up against the dowel pins of the turntable mount plate
Jog the turntable until you can accurately align the t-square with the walls of the tunnel and the dowel pins (you will only have to move the turntable a small amount since you roughly aligned it earlier)
Click "Set Home" in the VI
Aerodynamic method
Install a test article that is symmetric about the wind axis (such as a vertical symmetric wing or flat plate, that will produce no side forces on the load cell when at zero alpha)
With the turntable de-energized, roughly align the turntable by eye such that the dowel pins are perpendicular to the tunnel walls (due to the magnets of the stepper, which naturally has 200 steps per revolution, the motor will naturally find a position in 1.8 degree increments)
Energize the turntable
Start the VI, bias the load cell, and enable JOG MODE
Power up the wind tunnel according to the LTWT procedures (no particular speed is required but the higher the better to increase visibility of loads - just don't overload your symmetrical test article)
Jog the turntable until you observe a pure drag (y-axis force only, no side forces in the x-axis)
Click "Set Home" in the VI
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