270 or 360 Degree non-continuous servo or stepper for gauge

[AirMach]

Hi,

I am in the process of planning for the build of a F104G cockpit.

For some of the gauges we will use an LCD at the back, but for others have to create the gauges.
This means stepper motor or servo motors. An example gauge is Oil Pressure. It starts from 0 at 6 o'clock and ends with 100 at 5 o'clock, which is around a 340 degree travel.

Alternatives found are:

• Servo motors
  

I already bought some sg90 servo motors, and I found out (after they were received) that they can travel only 0 - 180 degrees.
There are a lot of instructables for modifying a servo motor for continuous rotation, but not that much information for fixed controllable 360 degree rotation (or less).

Of course I saw some stepper motors eg. here: http://www.hobbyking.co.uk/hobbyking...48sec_13g.html

Would the MG90D be a good candidate ?

• Stepper motors

With the stepper motors, there is a problem with the home zero (0) recognition.

Questions:
Do you know any cheap servo motors with a 360 degree or similar travel?
What is your suggestion, based eventually on your experience?
What will there be the need for a Stepper motor based gauge ?

I am planning to use the gauge through either Link2FS or Mobiflight. Not sure which for now, I am inclined towards mobiflight as I already have the FSUIPC addon.

[hyamesto]

AirMach:
Part of my indicators (analogue gauges of a Cessna 182) are made with simple 180º servo, but the addition of a gear system 2: 1 for 360 degree rotation.
Regarding the instructables for modifying servos, they do not say you loose the feedback, and can not establish the same position each time (only controls the speed and direction of rotation).
The MG90D may be a good candidate if you can buy one and test. (it´s the first time i see this kind of servo).

But you can try another solution:
Try to see some solution to increase the angle range, specially in point 3:

http://svglobe.com/arduino/output_servo.html

Using internal gear wheel axis, and depending of the internal gears, you can achieve rotation until 580º. (maybe this is the way adopted by the MG90D!!!)
Any doubts, let me know how i can help you.

Regards.
Horacio.

[AirMach]

AirMach:
Part of my indicators (analogue gauges of a Cessna 182) are made with simple 180º servo, but the addition of a gear system 2: 1 for 360 degree rotation.

Hi Horacio, Where did you get the gear system ?
I am actually reading a lot the baron58 website since I'm amazed by his work.

I'll try the mod tonight on the SG90

[hyamesto]

AirMach:


The gear system is made in Cad software (inventor, solidworks, etc) and laser-cut in acrylic. In this way, you can make the best gear relation to meet your needs.
Some photographs of laser-cut spur gears: (the red, yellow and brass are auto-slot scalextric type to compare).



Some Cad drawing of altimeter gear system:

You can use a servo replace gears kit too, but it is difficult to find a paired gears to achieve 360º.


Regards.
Horacio.

[AirMach]

Thanks for the Pics. Laser cutting would be expensive here so I finally went for the change of AXES option. A 1.0 mm paper pin was the donor. I measured the original axe and it's 1.1mm.

One word of advice is that when you open the servo, be careful with the grease inside it. I wasn't and so the first attempt was a bust.

Thanks again for the svglobe website tip. Basically the steps are:
1. Unscrew the 4 screws of the servo and remove the cap carefully
2. Take the sprockets out again taking care of the grease
3. Cut the original pin so that the right upper sprocket will contain both the bottom pin and the upper pin which I superglued to the sprocket
4. Cut the opening in the cap.

One thing I noticed is that even though I was very careful, I still have a bit of a wobble in the pin, due to the very slight angle. I'll have to live with that for now.

After doing this change you can still use the 0 to 180 degree rotation control in Arduino but the 180 degree will correspond to the new axis rotation (+-620 degrees). This means that in order to have the needle actually turn approx 180 degrees now, you will give the arduino:

myservo.write(52);