Setting 100% servo throw onto half an axis

mastermalpass

Elite member
At least, that's the best I can describe it. All the results for what I put into Google are people asking for help with getting their servos to behave conventionally. I am wondering if it's possible to make servos behave UNconventionally and I hope these crude vector drawings I made on my phone will communicate what I'm getting at.

So normally, you have a servo on an axis and though you can adjust how much throw or expo the servo has in response to that axis, the servo is always centred on that axis. The servo arm essentially follows the stick.

Screenshot_20230125_214620_vector-ink.jpg


I am figuring out concepts for in-duct shutters that divert a ducted airflow through smaller ducts and what could really help is if I could stick 100% of a servo's throw to one half of an axis, like this:

Screenshot_20230125_214633_vector-ink.jpg


Notice how it ignores the stick when it goes one way, but uses 100% of its throw going the other way. I would then assign the other half of the axis to another servo.

Has anyone done anything like this before? If not, I can always design a kind of control horn that allows the push rod that moves the surface on one half of the axis but lets the pushrod slip through on the other half. However, that not only wastes half the throw but more importantly; imperfections will bring the risk of things binding and the surface being pulled 'out of sync', if you will. So if what I'm thinking of is possible, it would be very much appreciated!

PS: I have a Spektrum DX6i - only 6 ports on the Rx. I wouldn't be surprised if the answer is; 'this is doable, but you need more channels', but here's hoping I can still pull it off on 6.
 

telnar1236

Elite member
At least, that's the best I can describe it. All the results for what I put into Google are people asking for help with getting their servos to behave conventionally. I am wondering if it's possible to make servos behave UNconventionally and I hope these crude vector drawings I made on my phone will communicate what I'm getting at.

So normally, you have a servo on an axis and though you can adjust how much throw or expo the servo has in response to that axis, the servo is always centred on that axis. The servo arm essentially follows the stick.

View attachment 234116

I am figuring out concepts for in-duct shutters that divert a ducted airflow through smaller ducts and what could really help is if I could stick 100% of a servo's throw to one half of an axis, like this:

View attachment 234117

Notice how it ignores the stick when it goes one way, but uses 100% of its throw going the other way. I would then assign the other half of the axis to another servo.

Has anyone done anything like this before? If not, I can always design a kind of control horn that allows the push rod that moves the surface on one half of the axis but lets the pushrod slip through on the other half. However, that not only wastes half the throw but more importantly; imperfections will bring the risk of things binding and the surface being pulled 'out of sync', if you will. So if what I'm thinking of is possible, it would be very much appreciated!

PS: I have a Spektrum DX6i - only 6 ports on the Rx. I wouldn't be surprised if the answer is; 'this is doable, but you need more channels', but here's hoping I can still pull it off on 6.
In Spektrum you can program travel for the servo. So, you would put one end to zero and the other to the maximum, whatever it is, and then make a mix with zero in that direction and the maximum in the opposite. It would take 2 channels with a servo on each channel.
 

Merv

Site Moderator
Staff member
...I am wondering if it's possible to make servos behave UNconventionally ...
Yes, it's been done.
I used the way back time machine and found this episode that does exactly what you are proposing.
Josh was a lot younger back then.