Next interesting step: the autothrottle mechanics.
As mentioned in an earlier post I was looking at the servo based solution as seen on the Buildaboeing YouTube channel.
In his last video Peter explained that one of the problems is that the mechanism must be mirrored for the two throttle handles, but that this means that one servo will run in the opposite direction.
Even if you would be able to swap the direction in the controller I could imagine that this may give trouble keeping both handles running nicely synchronized.
There’s another problem: the servos are too high to fit in the space between plates 6, 7 and 8.
So you cannot mount them at exactly the same position on plate 7 and 8.
As I’m not an expert in mechanical constructions I did some searching on the internet and found an interesting freeware Windows tool called Linkage to simulate mechanical links.
I used it to model and simulate the throttle handle, servo, slider potentiometer and the links between them.
The idea was to play around with it to see if it would be possible to find a solution so that the servos can run in the same direction, be placed at different locations below the MDF plates but in such a way that the handles move synchronized.
While struggling with this I realized that I have a CNC router: I still need to get used to the idea that the possibilities are almost endless with that: I might as well be able to make my own gears.
With gears the solution for the problem is easy: the servos can be placed anywhere around the gear attached to the throttle handles and move the handles exactly synchronized.
In the past I found a plugin for generating gears in Sketchup. So I wondered if there would be something similar for Inkscape and started searching the internet.
Already the first hit revealed that Inkscape has a built-in gears extension and it suits my needs. In the latest release of Inkscape this gears extension has been enhanced, but as I encountered some problems with that release of Inkscape I’m still working with release 0.48.
For a robust connection of the gear to the servo I made the gears so large that they can be bolted to the aluminum servo arms.
An additional advantage is that the larger the gears are, the lower the servos can placed, further away from the busy area between the MDF plates.
The only thing I’m still in doubt about is the gears ratio: the handles rotate 90 degrees, the servos can do 180 degrees, so for more accurate motion I could use a ratio of 2:1 servo to handle.
But in manual mode the handles must move the servos. This requires quite some force and using a ratio 2:1 will make that even worse and put quite some stress on the mechanics. So I decided to start with a 1:1 ratio for the gears.
In a fixed connection between handle and servo it won’t be possible to move the handles manually as long as the servos are powered and even if they’re unpowered some force needs to be applied to the handles to overcome the large gear ratio of the servos.
I kept thinking about constructions that use a clutch as a solution for this problem. While searching Aliexpress for clutches I found micro electromagnetic clutches for an affordable price. These connect or disconnect a D-shape axis to an ABS gear by means of electromagnetic force (driven by 24 volt).
That will lower the stress on the mechanics in manual mode (clutch disconnected) and may even make it possible to override the autothrottle, although I need to test that as soon as the clutches have arrived.
Some changes in the construction will be needed for this: as there is no 1-to-1 correspondence between servo position and handle position anymore I need servos that can rotate continously (standard ones can be modified for this) and the software will need to use the potmeter values to position the handles correctly in autothrottle mode. Also it now needs to make sure that the servos stop when the handles are in their end position. The microswitches that are already present can be used for this.An advantage of using a clutch is that I can use a larger ratio between handle and servo for smoother positioning.
Although I’ve bought some spare clutches for my throttle quadrant I noticed that apparently the type of clutch that I have used was from some kind of rest party because it’s not sold anymore already for a long period. But it looks like there is a new type available now that could replace it: it’s a bit smaller and thinner than the ones that I used so should fit in the same space and has a gear with exact the same diameter, number of teeth and axis sizes. It has a bit lower maximum slip torque, but as I can use mine at 12V it should be sufficient for the application.
Now that I can make my own gears I’m also considering using ‘normal’ potentiometers instead of slider ones too because I expect that wil give better linearity.