Mechanical Installation
There are three steps to mounting the Theseus Micro VPS on your aircraft:
Figure out Sensor Mounting
Calculate Mounting Calibration (very important!)
Finally, Compute Mounting
Sensor Mounting
For any question about your mounting solution, contact us and we will assist.
The sensor modules comes with three 1/4"-20 mounting holes. The sensor module can withstand light rain and environmental wear.
The sensor can be mounted under the wings or on the belly of your aircraft. It is fine if your mounting solution partially obstructs some of the cameras of the sensor module.
For example, if you are mounting under a wing on a fixed-wing aircraft, it is acceptable for one of the side cameras to be obstructed up to 70%.
Add vibration isolation to your mounting solution if you are mounting near a source of vibration or on a rigid surface.
We recommend using the top mounting (1&2) holes instead of the GoPro mount (3) to avoid calibration issues.
Example mounting solutions
Once you've settled on your mounting solution, proceed to measuring the orientation and translation of the sensor relative to your flight controller.
Mounting Calibration
This step is crucial to ensure nominal performance of the Micro VPS.
Note: If your compute module and sensor are separate units, the orientation and position of the compute module is irrelevant here, we're only interested in the sensor and the flight controller. If the compute and the sensor are integrated you can ignore this note.
Why does this matter?
Part of the Micro VPS stack involves inertial measurements, which tracks the movement of the vehicle in a local frame of reference (e.g., the point where the system initializes is the origin, and displacement is measured relative to that point). The Micro VPS communicates with the autopilot to transform this inertial frame into a North-East-Down frame using the attitude data from the autopilot (onboard IMU and compass). Proper calibration ensures this transformation is optimal and reduces projection error.
Orientation
The system needs to know which way the sensor is mounted relative to the flight controller (FC). Take your right hand, index finger pointing forward, and align it with the FC, as shown. Usually there will be an arrow on the FC that indicates the forward direction. If your FC is not flat, you'll have to rotate around your index finger to match the orientation of the FC. See the image for reference. This is called the flight controller reference frame.
Note on reference frames
This exercise is done using NWU (XYZ => North, West, Up) frames to make it as intuitive as possible. If you're used to the flight controller/Ardupilot reference frame being NED (XYZ ⇒ North, West, Down), don't worry, we will account for this in a later step.
Now we will do the same thing to find the sensor reference frame. Align your right hand index finger pointing directly out of the front camera, your middle finger pointing left, and your thumb pointing up at right-angles to the top surface of the sensor. See image for reference. This is the sensor reference frame.
Now we need to find the sequence of rotations around each of the 3 axes (i.e. index finger, middle finger and thumb) that rotate the sensor reference frame to the flight controller reference frame.
We will label these axes:
X: index finger
Y: middle finger
Z: thumb
If the two reference frames are already aligned, then you have nothing to worry about. If not follow on.
A positive rotation means going counter-clockwise around the axis. If you take your right hand thumb and align it with the axis, curling your fingers represents this positive, counter-clockwise direction.
Let's run through an example. The left image shows the sensor reference frame, and the right shows the FC reference frame. This is a simple scenario - we only have to tilt our hand upwards a bit to get from sensor to FC reference frame, i.e. a rotation around the Y axis.
Now, is the rotation negative or positive? If we align our thumb with the Y axis and curl our thumb, a positive rotation (in the direction of our curled fingers) means rotating downwards. So, here we have the opposite, a negative rotation.
How much are we rotating by? We know the mounting for the sensor here is angled by 20 degrees.
So our final rotations, also known as euler angles, are: {X: 0, Y: -20, Z: 0}.
For vehicles with more than one axis of rotation, the order matters. Make sure you keep track of the order of rotations as well.
Euler angles to rotation matrix
Use Theseus Micro VPS Dashboard to enter the euler angles under and generate the vehicle configuration. In the main dashboard, select "Generate Config" under the Vehicle Parameters widget.
Enter the euler angles from the previous step into the fields in the Vehicle Configuration Generator window.
You can save the config to your computer to upload later, or upload it right away to the drone. Make sure to select the newly created from the configuration files dropdown menu after you upload it.
Compute Mounting
The compute module offers flat surfaces for adhesive mounting.
Sensor and compute modules are mounted on your vehicle and you completed the calibration procedure.
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