Remote controlled servo deployment   

Introduction
We have all experienced the frustration and agony when one of our rockets ,that took several hours to build, lawndarted and broke up. Not to mention the risk of hitting a human target with a falling rocket.
After the successful design of the high-voltage pin puller, I wanted to have remote control instead of a timer to deploy the parachute.
There are several different methods to detect rocket apogee, but none of them can beat the human eye/brain combination. Even with a timer on board, you are always anticipating the moment at which the parachute should deploy theoretically. With the remote control at your fingertips, you are the only one to decide when and if the parachute should deploy.
Ideally, in a very large open field, you can wait until after apogee to deploy the parachute. Depending on wind conditions, you can decide yourself at what height the parachute has to open. This way, you are able to determine for yourself how many calories you want to burn chasing and retrieving the drifting rocket. Even when there is only one single tree in sight,we all know by experience that trees are rocket magnets and that your rocket will end up dangling in that tree top. When you see the rocket heading towards the tree upon descent, you may decide to NOT deploy the parachute by forcing away your finger tip from the remote control button.
The disadvantage of this remote control project is the cost: about 20 Euros for the servo and another 20 Euros for the remote control electronics.

The easiest way to implement remote control is to use a commercially available transmitter / receiver combination. This may be the standard remote control from a plane or car if you are a RC enthousiast already. However, when using the standard hobby RC equipment, keep in mind that the range between your transmitter on the ground and the rocket may be well over 100 meters. One big advantage is that you should normally have a direct view path to the receiver in the rocket. Unless you are launching somewhere in the middle of the city center, which is not the best place.
When you are not in the RC hobby yet (as in my case) and want to build a remote control deployment for your rocket anyway, this could be the solution for you. All you need are basic electronic skills like soldering and maybe measuring a voltage. The electronics are kept as simple as possible with no exotic components involved. If you are still interested, get yourself a bottle of your favourite regional beer and settle down.

For the remote control transmitter / receiver pair, I chose a 433MHz solution from a company here in Belgium called Velleman. Click on the image to get a link to some more info on the receiver and transmitter.

433MHz Transmitter 433MHz Receiver
The dimensions of the transmitter and receiver are very small. With direct sight conditions as we typically have with launching water rockets, range should be more than sufficient ( 200 meters is enough for me ).
Their original target application is remote control for garage door openers, etc. The transmitter transmits a digital code at low baud rate. At the receiver output, a microcontroller decodes this digital signal and when the code is right, the gate / lock is opened.
Have no fear. In my deployment design, I am not using a microcontroller, just ordinary off-the-shelf components.

Remote control transmitter
  • Transmitter schematic diagram.
    One of my primary requirements was that the transmitter should be small enough to fit into your pocket. After all: you only need a single button = eject! Another requirement: simplicity !
    The transmitter is powered by a small 12V battery.
    The heart of the circuit is a good old CMOS quad Schmitt-trigger NAND gate: the 4093. The NAND gates U1..U4 are the 4 gates inside this single chip. The chip is powered directly from the battery voltage, no extra 5V or 8V stabilizer. This allows us to use the battery until it really drops below acceptable range. Also, with 12V the transmitter range is bigger than with 5V. The disadvantage of this simple power supply is that the pulse width and frequency generated by the 4093 is supply voltage dependent. In practice this is not so critical.
    U1 forms a basic oscillator with a frequency of about 60Hz. The square wave at the output is differentiated by 2 x 100nF + the 2 x 100K potmeters. The positive edges of the oscillator result in a negative puls at the output of U2 and U3 with an adjustable pulse width. The pulse width should be adjusted between 1 ms and 2 ms. In practice: 1 pulse width is the position of the servo horn in its standby state, the other pulse width is the position of the servo when activated. With the dual-pole pushbutton it is possible to select between one pulse width or the other going to inverter U4. U4 simply reverses the polarity of the pulses and at the output we have positive pulses : this is the way they should go to the servo.
    So far, this simple circuit may be used as a simple standalone servo tester. The pulses coming out of U4 pin 10 may be applied directly to the servo control input. However, this is only valid if the circuit would be powered at 5V. Do not apply 12V power supply or pulses to your precious servo motor directly! The servo control pulses are applied at the data input of the 433MHz transmitter together with a filtered power supply.
    If you have some spare contacts on the pushbutton, a nice feature is a beeper that is activated when you push THE pushbutton. The beep will be the evidence to yourself and (especially) to a critical audience that you did not forget to push the button while your beautiful rocket is heading face down for your lawn (aka lawn darting).

    The first picture shows the transmitter housing with extendable rod antenna (62 cm = full wavelength at 433 MHz) , on/off switch and 2 potmeters for the pulse width settings: one setting is the normal position (servo does not pull the deployment trigger) and the other pulse widht is the deployment position (servo pulls on the deployment trigger). The second picture shows the inside: pushbutton, beeper, 12V battery, 2 potmeters, CMOS chip and transmitter module.

    Transmitter housing Transmitter inside

    Remote control receiver
  • Receiver schematic diagram.
    The receiver electronics are extremely simple. The power supply for the 433Mhz receiver and the servo motor is split up by using 2 separate 5V stabilizers. Do not try to use just a single stabilizer. Servo motors genereate spikes and other noise on their power supply lines. In order to make sure that the receiver is not influenced by power supply pollution, use a separate 5V stabilizer. The battery I used is a normal 9V battery block. This is quite heavy, but lasts for a very long time. Note that a rechargeable 4.8V RC battery pack may be used here instead of the 9V battery and the stabilizers. However, the 433MHz receiver is quite picky: the data sheet requirements for the power supply are 4.9V...5.1V. The 9V battery is quite heavy. The very light 12V battery that is used in the transmitter is useless here because of the high internal impedance of the 12V battery that limits its output current too much for servo operation.

    Mechanical setup
    The remote control electronics is one thing. Translating electronic signals into mechanical motion is another. That's where the digital servo motor comes into the picture. Digital servo motors as used in RC planes / boats / cars / robots.. are relatively complex devices internally but they are easy to control. Basically, all you have to apply to a servo motor to make it work is the power supply = 5 Volt and ground and a control signal that determines the angle of rotation. This control signal is a pulse width modulated signal with a frequency of about 50Hz. Pulse width is somewhere between 1ms and 2ms. Anything in between will position the servo motor shaft differently. If you are eager to learn more about servo operation, check out e.g. this link:
  • Servo basics Very important: there are different sizes and shapes of servos. The ones that are used in cars, boats,.. are usually quite large and heavy but cheap. There are also very light, but more expensive servos that are suitable for airborne applications, like the one in the picture: a Futaba S3107:


    I started off with a large and cheap servo, like the ones that are used in model cars.
    Click on the picture to see the resulting setup:

    BIG servo in the loaded position BIG servo in the rest position

    Another very useful feature of servo motors is that they try to keep the shaft in the same position even when an external mechanical force is applied to the shaft or servo horn. In other words: as long as there is no mechanical force trying to move the servo horn, power supply current is very low but as soon as an external force is applied, current increases to keep the motor shaft in the same position. In my design, the servo horn is used to pull a trigger only. In rest, current consumption is very low. It's only during the trigger pulling action that the servo experiences some external force. This is only for a very short time when the remote control button is pushed and the servo moves to its activated position.
    The actual deployment mechanics are based on older designs published on the net called VDTT (Vertical Deployment by Tomy Timer). The mechanical timer is replaced here by the servo motor. One of my personal requirements was that it had to be very easy to prepare the rocket again for a next launch. All I have to do is push down the plunger of the vertical ejection mechanism until it hooks into the trigger mechanism. One important issue here is to keep all forces perpendicular with respect to each other to make sure that they do not "see" and influence each other. I'm far from a mechanical genius, so probably somebody will come up with a better mechanism.
    This is the same setup with the smaller Futaba servo mounted in the hole for the big one. Notice the difference...

    Using a small servo

    The next two pictures give a good impression of the completed deployment unit. The wooden plunger has a diameter of about 8 cm. The inner diameter of the bottles I use is about 10 cm. Plunger throw is about 5 cm. All the electronics and a rechargeable battery are mounted on the back side of the deployment unit. Note that there is a two way on/off switch: in the OFF position, the battery + pole is connected to a 3 mm jack at the side of the unit that is used to recharge the 9 V battery. The servo horn is connected to the trigger with a small length of nylon fishing line. This way it is possible to position the horn in the normal rest position so that it does not pull the string and there is still some slack. This ensures that current comnsumption is minimal. Only during the actual pulling action will the servo consume some more current. In rest, there is no mechanical force on the servo horn.

    Back of the completed servo unit Front of the completed servo unit

    The green wire is the antenna. It will be stuck to the outside of the deployment housing with duct tape. Length of the antenna = 15.5 cm or 1/4 wavelength at 433 MHz. Another alternative and smaller in size is the helical antenna.
    At long last, 6 months after finishing the prototype, we finally had the chance and the time to launch it.
    IT WAS A BIG SUCCESS!!!
    It deployed 7 times out of 7 launches without any problem. It was a very good feeling to watch the rocket launch, wait 3 to 4 seconds and then, at apogee, press the button and see the chute being ejected instantaneously!
    Click on the thumbnails to see the bigger pictures.

    Maiden launches!!!
    Lauch crew ready Rocket on the lauch pad Countdown Post recovery inspection Deployment! Radio controlled separation of chute and nose cone Soft landing in a field


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