Simple Altitude-Based Deployment
Posted: Thu Jun 13, 2013 12:27 pm
Here's something I started a long time ago and only recently finished up and tested out. This is a highly effective and dead simple electronic deployment system. The only downside is cost, as it requires an altimeter with deployment capabilities. If you are already looking for a logging altimeter, though, it's not that bad. I use the Perfectflite StratoLogger, which you can buy online for $72.
The StratoLogger's ejection ports give a one second connection to it's power supply (4-12V, IIRC) at ejection. One is set to fire at apogee and the other fires on descent at a user-defined altitude. These are meant to fire e-matches in a pyro rocket, but I have them set up to turn a servo motor. I took the servo apart, removed the control circuitry, and soldered on some extension leads to the motor's existing leads. It's been a while since I did this, and there might have also been a mechanical stop which prevented the servo horn from over-rotating. If there was, I removed it. So it's not really a servo anymore, just a geared down DC motor. I run the altimeter on a 2 cell Lipo, giving about 8V at full charge, and I'll probably recharge it when it hits 7.3V. That's a little high for this 4.8-6V servo, so I put a 20 ohm resistor in series with the servo motor. During testing the servo was drawing around 100mA when unloaded at about 5V. This resistor gives a voltage drop of about 2V, so the motor only receives about 5.3-6V. When the altimeter fires, the servo turns through several complete rotations, which is more than enough to release a rubber band.
Mechanically, the deployment system is similar to the tomy timer release I used earlier- http://www.wra2.org/forum/viewtopic.php ... ease#p8207, but with the tomy timer replaced by the servo motor.
On this side of the payload you can see the altimeter, lipo battery, power switch, and my MD80.
Here you can see the axle and hook I've added to the servo horn. This a fairly important feature, which I'll discuss after the last picture.
Since the axle is supported right near the copper "hook", very little of the pull from the elastic is actually exerted on the servo. Since the rubber band goes around the hook so close to the axis of rotation, it exerts very little torque on the motor. This way there is enough friction in the un-powered servo motor to retain a fairly strong pull from the rubber band. The motor requires no holding current. It only draws current for the 1 second that the altimeter fires. When it does turn, it's not fighting the rubber band, just releasing it, so you don't have to worry much about how much torque the motor can deliver. This setup allows for a small servo, and a small battery. This lipo should last hundreds of launches before it needs recharging (unless perfectflite has understated the current that the altimeter draws when it's beeping out the locating signal). It might actually be better just to power this with some small button or coin cell batteries.
I put this through three test launches the other day, and the release mechanism worked flawlessly. This system has several advantages over time based deployment. First of all, the altimeter detects launch for you, so you don't need to do that manually to start a timer. Secondly, if you have it set to fire at apogee, you don't need to predict flight time. Just turn it on and launch. Thirdly, it can easily be adapted to dual deployment or a high speed low altitude deployment (one of my current goals).
Let me know what you think.
Alex
The StratoLogger's ejection ports give a one second connection to it's power supply (4-12V, IIRC) at ejection. One is set to fire at apogee and the other fires on descent at a user-defined altitude. These are meant to fire e-matches in a pyro rocket, but I have them set up to turn a servo motor. I took the servo apart, removed the control circuitry, and soldered on some extension leads to the motor's existing leads. It's been a while since I did this, and there might have also been a mechanical stop which prevented the servo horn from over-rotating. If there was, I removed it. So it's not really a servo anymore, just a geared down DC motor. I run the altimeter on a 2 cell Lipo, giving about 8V at full charge, and I'll probably recharge it when it hits 7.3V. That's a little high for this 4.8-6V servo, so I put a 20 ohm resistor in series with the servo motor. During testing the servo was drawing around 100mA when unloaded at about 5V. This resistor gives a voltage drop of about 2V, so the motor only receives about 5.3-6V. When the altimeter fires, the servo turns through several complete rotations, which is more than enough to release a rubber band.
Mechanically, the deployment system is similar to the tomy timer release I used earlier- http://www.wra2.org/forum/viewtopic.php ... ease#p8207, but with the tomy timer replaced by the servo motor.
- DSCF2887 (563x640).jpg (248.95 KiB) Viewed 152 times
- DSCF2888 (587x640).jpg (247.46 KiB) Viewed 152 times
- DSCF2889 (480x640).jpg (213.48 KiB) Viewed 152 times
- DSCF2890 (386x640).jpg (192.17 KiB) Viewed 152 times
I put this through three test launches the other day, and the release mechanism worked flawlessly. This system has several advantages over time based deployment. First of all, the altimeter detects launch for you, so you don't need to do that manually to start a timer. Secondly, if you have it set to fire at apogee, you don't need to predict flight time. Just turn it on and launch. Thirdly, it can easily be adapted to dual deployment or a high speed low altitude deployment (one of my current goals).
Let me know what you think.
Alex