Electornic Servo Limiter

[Atomic_Sheep]

Hello,

I'm building a RC model and I'm trying to put an electronic limiter on the servos to prevent them from moving past certain points.

The battery pack that I'm using on the model is 1.2V x 4 (4.V 600mAh). I am not sure how much of this voltage actually goes through to the servos, the battery pack is connected to the receiver which is then connected to the servos. Perhaps there is a bus inside the receiver which provides power for the receiver and the servos in parallel but I have no idea. I don't have an oscillascope either, so please bare this in mind.

Obviously, we would be inserting some sort of electronics circuit between the receiver and the servos given what I described above.

For those who are unfamiliar with RC servos, the servo position/angle is dictated by pulse width. I'm not sure, but I have a feeling the period of the pulses doesn't change, just the pulse width although I'm sure this isn't universal. Anyway, lets take 1.5ms as the pulse width for the middle position. Anything for example over 2ms I want to prevent i.e. I want to force the voltage to 0 when signals go above 2ms in pulse width and below 1ms I want to 'boost' the signal for an additional period of 1.0-x.

Baring in mind that only the original battery pack can be used as a source of power for this circuit, can anyone suggest anything to solve this problem?

 

[alec_t]

below 1ms I want to 'boost' the signal for an additional period of 1.0-x.

I'm not clear what you mean? Is that increasing or decreasing the pulse width?
My understanding of standard RC servos is that they will move to one limit position when the pulse width is 1ms or anything less than 1ms. I don't see how 'boosting' the signal will affect that.

 

[audioguru]

My RC transmitter can be programmed to two different maximum amount of movement for each servo. Each servo has its own switch to select high or low.
I switch to high which is programmed to 100% for stunts and switch to low (70%) for gently flying around.

 

[cowboybob]

Short of AG's situation (programmable limits), why don't you just alter the attachment point(s) on the servo(s)?

From your description, that would limit the arc length to within your specs.

Although, not exactly do-able "on-the-fly".

 

[Atomic_Sheep]

By boosting I mean... when the pulse of say 0.8ms ends, I need to be effectively sending the voltage to the servo for another 0.2ms to fool the servo into thinking that the signal was 1ms, so by boosting I mean boosting a 0 voltage for the 0.2ms duration of 1ms-x (x being the actual wavelength dictated by the signal).

Unfortunately, I don't have a digital RC transmitter that is capable of doing this, so trying to make this alternative.

It's not the deflection of the control surfaces that I'm worried about, it's the airframe limits that are the problem. The limited space I have means that I literally can't fit the servo in with it's maximum range of motion without the servo horns/arms hitting bits of the airframe past certain points. The servo that I have purchased is able to rotate 180 degrees in either direction which is way too much anyway. The easy way of fixing this would be to glue a physical limiter to my transmitter to prevent the sticks from phisically being able to be moved to these extremes, but I'd rather a more elegant solution.

 

[alec_t]

This should do the trick. Output pulse width is limited to be within the range 1ms-2ms, regardless of the input pulse width from the Rx. Both limits are adjustable using trimmers.
The circuit consists essentially of two monostables whose outputs are logically combined with the input pulse. It is powered by the Rx 4.8V supply. Decoupling capacitors (100n, not shown) should be connected across the supply pins of each IC.

Edit: You would need one of these circuits for each movement-limited servo.

 

[cowboybob]

How about shortening the horn arm (thereby avoiding the airframe) as well as the corresponding arm on the control surface? You might also gain some torque, although I suspect it'd even out.

I had to do this on a Corsair to which I added ailerons that weren't part of the original design. Also had to use reverse motion levers to squeeze it all into the wing.

 

[Atomic_Sheep]

Wow thanks Alec, will have to review a few times before I can figure out what's what, but many thanks, much appreciated.