Heli Hacker's blog archive for July, 2010 - RC Groups
Heli Hacker's blog View Details
Archive for July, 2010

# Update: DCM for better piro stability

Posted by Heli Hacker | Jul 31, 2010 @ 03:33 PM | 6,531 Views
With the tail rate mode and HH mode both working well on the PhuBar, I turned my attention to improving piro stability. I had noticed some instability if I do a simple piro when the heli is not absolutely level.

I believe this may be partly because the roll, pitch, and yaw data are processed independently of each other. This works fine until you have a fast yaw rate. Ideally the system needs to rotate the pitch axis and roll axis when yawing occurs. If the heli yaws 90 degrees, then pitch becomes roll and roll becomes pitch, with respect to a fixed earth-based coordinate system.

What is needed is an integrated representation of the attitude of the heli with respect to the earth frame of reference, where motion on one axis can affect the other two. In other words, pitch and roll need to be "yaw aware".

It seemed that a good way of doing this was to implement a Direction Cosine Matrix , as outlined here:

http://diydrones.com/profiles/blogs/...ry-first-draft

I spent the last week implementing this in Spin Code on the PhuBar3. I am in the process of tuning the gyro gain factor to get the matrix to update accurately, but so far it looks good. It rotates the pitch and roll axes appropriately when the unit is rotated around the yaw axis. The nice thing about DCM is that roll and pitch correction values can be plucked out of two of the elements of the matrix - they just need to be scaled before they can be fed into the swash mixing equations.

I am finally using some of the matrix math they tried to teach me a long time ago in Linear Algebra class !

# Heat-Shrink covering

Posted by Heli Hacker | Jul 31, 2010 @ 03:15 PM | 6,644 Views
With clear heat-shrink wrap, the PhuBar3 is about 7.5 grams. The heat-shrink stuff works best if I cut it larger than needed, then trim it around the connector after shrinking it. This one has an 8-position header on it, instead of having cables soldered directly to it.

Heat-shrink tubing is the clear 54mm material from this ebay seller:

http://stores.ebay.com/TUBING-EXPRESS
.
.

# Vibration Comparison

Posted by Heli Hacker | Jul 13, 2010 @ 10:33 PM | 6,814 Views
I took a vibration measurement today on the PhuBar3, similar to the one I did earlier on the PhuBar2. The amplitude of the vibration is somewhat higher on the PhuBar3, most likely because it is mounted differently, and not as isolated from the frame as the PhuBar 2 was.

This was done with the ITG-3200's built-in low-pass filtering set to 5hz. This is a much lower cutoff frequency than the software filter in the PhuBar2 which is set to 47hz. This is comparable to the software filter in the PhuBar2, which is set to 5.3hz

Just comparing the two visually, it looks like the impact on the servo control signal is comparable between the two devices.