Vertical Trajectory System (with some 3D printed parts)

OverTheTop · Postby **OverTheTop** » Mon Dec 07, 2015 4:02 pm

Vertical Trajectory System (with some 3D printed parts)
I have hinted at this project in a couple of other threads. As they say, “Show us the photos or it didn’t happen.” So here we go!

I have been thinking about a VTS for quite a while now. After thinking about what I want to achieve with this project I have defined the goal. It is to make an effective Vertical Trajectory System using off-the-shelf purchased electronics. I don’t particularly want to go down the track to produce a fancy, all-singing, all-dancing, high-performance unit with custom firmware. I really want to make it happen with electronics destined normally for the RC aircraft world.

Along the way I found out Cryoscum and Strud were also working on similar systems (with fancy gain scheduling and all sorts of wonderful features), so I had a bit of a chat with them as well. Studying the aerodynamics is non-trivial and took up a lot of time also. It also convinced me to go for something that was relatively tame to improve my chance of success. My control system theory also told me the same thing. I will also endeavor to keep it simple to hopefully add to the success of the project.

Lots to think about: Control moments, flight envelope, servo torques, bending moment diagrams, material properties, drag, lift, aerodynamic center movement… The list goes on.

I was originally going to go with levers to multiply the torque from the servos, but due to complexity (and backlash which is a killer in this application) I went for direct connect to the servo splines. The excursion of the fins will be mechanically limited to keep the torque required within what the servos are capable of providing.

ON WITH THE PROJECT!
I have an autopilot that I used in my fixed-wing training model. Lots of different stabilisation modes, so lots of possibilities. After thinking though how all the modes work I figured that there was a mode that was workable in a rocket. So I whipped up a quick proof-of-concept with some cheap servos and an RC receiver for a pulse generator.

: VTSProofOfConcept.jpg (224.39 KiB) Viewed 7260 times

Once connected up it showed that the operation mode was as I expected and all actuators behaved as I expected. I should be able to extract roll, yaw and pitch control with the autopilot and three servo channels.

So onto SolidWorks and produced a mount for four servos, and a fin design.

The fin design (double wedge) is suited for supersonic flight and has quite a predictable aerodynamic center in the supersonic part of the envelope where servo torques become more important.

: Fin1.png (101.72 KiB) Viewed 7260 times

Mechanical stops are built into the fin hubs to prevent any sudden control system excursion ripping a fin or three off.

: VTSServoMount.JPG (216.66 KiB) Viewed 7260 times

A mount for the servos was printed (ABS) then tapped for the servo mount screws and airframe screws. Plugs were printed to go into the cavities in the mount. These provide pilot holes to guide a holesaw at a later stage.

The fins were printed horizontally to give better strength along the plane of the fin. These fins get screwed to hubs that attach to the servo splines.

I had considered using a long (hand-made) bolt through each fin to hold the fin to the servo. You can see the hole in the top of the fin for that provision. I have gone away from this idea as the thread (for going into the servo) would be cut with a die, rather than rolled. This results in a stress raiser at the root of the thread, and with a long lever arm would be a likely failure mode. It would have been nice to use these bolts as the fin would be more in compression radially, adding to the strength of the unit.

The mount was fitted to the airframe and illuminated from within to locate the mounting holes and approximate the locations of the fins. 10mm holes were drilled at the approximate location of the fin hub centers.

The assembly was fitted (along with the temporary guiding plugs) into the airframe and the hub holes drilled to 35mm using a Sutton holesaw with ¼” centering drill guided by the pilot hole in the temporary plugs. Care had to be taken to ensure the filament-wound fiberglass did not splinter badly when the saw broke through inside.

: VTSAirframeReadyForHole.JPG (110.69 KiB) Viewed 7260 times

: VTSHoleSaw.JPG (122.2 KiB) Viewed 7260 times

Once the holes were cut the edges were given a quick coating of Loctite 401 CA glue to keep the edges from fraying. It was then sanded and painted (signal red over white undercoat).

Hubs were fitted in preparation for the fins to go on.

Each fin was attached using M2.5 capscrews (qty4). My printed NC (off my Nike Apache) was then put on just to get an idea what the finished product will look like.

Next step is to design and print the housing for the rest of the electronics.

SpaceManMat · Postby **SpaceManMat** » Mon Dec 07, 2015 5:00 pm

Looks pretty nice to me. How fast are planning on pushing it?

Lister · Postby **Lister** » Mon Dec 07, 2015 5:13 pm

Very cool.. looks like we are going to have some cool rockets next launch season

So when is this going into production

CATO · Postby **CATO** » Mon Dec 07, 2015 8:13 pm

Very nice indeed...

High Impulse · Postby **High Impulse** » Mon Dec 07, 2015 8:59 pm

Very cool. On a side note, what is solid works like to use?

OverTheTop · Postby **OverTheTop** » Tue Dec 08, 2015 8:58 am

Some Electronics
Ok. I said I wanted to use an entirely plug-and-play system from the initial concept, but unfortunately I need to make up a couple of small interface boards to help with the systems integration.

If you click on the images they are a bit clearer to read.

Control Interface
First up is a circuit that lets me take a signal, say from an altimeter or a G-switch, and use it to control whether the VTS system is active or not.
Here is the schematic:

This uses some nifty devices from Analog Devices that produce a PWM stream of appropriate duty cycle based on an analog voltage input. The FY31AP autopilot normally takes its commands from an RC receiver, so we need to use the same type of control signals to select the correct modes when needed.

Logging Interface
The next circuit takes the servo signals that the control system is sending to the servos and converts them directly to a voltage. I can then feed these voltages into the extra pyro channels on the TeleMega. This gives me the ability to log, at a high data rate, the actuator commands during flight with results completely integrated with the rest of the logging system. Keep that trick up your sleeves when you need to measure anything else during flight

It is very easy to implement.

My circuit uses more clever devices from Analog Devices. They take the PWM stream that the servos see and convert them to an analog voltage. The advantage of this system is response time. The traditional approach of using filters suffers badly from lag and resolution issues. This device works on a single frame response time to output the appropriate voltage. No use logging the data if it is so slow and mushy that it is useless! Amplifiers then provide some gain and buffering before being fed into the altimeter:

I have managed to get time to have this one laid out as well.

: PWM-VoltsPCA.png (62.67 KiB) Viewed 7486 times

I still need to get the other board laid out and then the PCBs fabricated.

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How fast are planning on pushing it?

I am looking at just subsonic for the first couple of flights. Nothing too aggressive. After that we will see what happens!

On a side note, what is solid works like to use?

As Atari’s Nolan Bushnell quoted back in 1971, “All the best games are easy to learn and difficult to master.” That’s how it is with SolidWorks. It is quite intuitive to get going and do some serious modelling on. I have done 2D CAD with AutoCAD for many years. So far I have only done five drawings with SW, so I am very much in the learning phase, and probably driving it somewhat like a Neanderthal. Having said that I am very happy with my results. I just need to work on adding knowledge and finesse to my handling of the package each time I use it. That way I will evolve and refine my technique, but I need to keep working on it.

OverTheTop · Postby **OverTheTop** » Thu Dec 31, 2015 3:53 pm

Interwiring Diagram

I am still considering how to make all the electronics for this hang together mechanically, and the complexity is getting slightly out of hand. So, time to draw an interwiring diagram. Here is how I see the whole system hooking up together.

Click for a clearer image

There is a bit of a gremlin in the image down in the lower right corner. The graphic conversion program conveniently disconnected the power line from my logging interface

The 9ch Rx is a standard RC part and it has been chosen because of one important feature. Failsafe. This means that when contact with the Tx is lost the outputs go to a defined pulse width (servo position). This lets me adjust the final positions of the servos with the Tx linked, then just switch it off and the predefined pulses are sent from the Rx. This has the advantage that I don’t need to mechanically centre the fins, or use digital servo adjustments to get my fins centred.

The autopilot is originally designed for a fixed-wing RC model. It interposes between the Rx and the servos, applying the necessary corrections as commanded. The trick to getting this one to work is selecting the correct mode. It is set as a V-tail model, in order to get both the ailerons and elevators mixed onto the two servos correctly for pitch and roll control out the same actuators. That leaves the rudder channel for yaw control. So roll control using this setup is only achieved using two of the four servos.

The two yaw servos take the same command from the autopilot. Since they are on opposite sides of the airframe the signal to one needs to be reversed so they both operate in the same direction. A small signal reverser is added into the line of one of them.

Servos are Turnigy TGY-4409MD digital servos. Quite grunty (9.45kg/cm at 0.11s) and relatively cheap. At maximum draw with all servos running flat out the supply current will be about 12A total.

To keep the battery current down a little I have chosen a 3S 500mAh LiPo to run the servos. This goes through a switch-mode battery eliminator to regulate the voltage correctly.

The rest of the avionics is powered by a 2S 500mAh LiPo through a universal battery eliminator. Keeping this supply separate from the servos means if the servos load the supply the rest of the system keeps operating as intended.

Power will be switched by the usual RBF pin system.

Control of the autopilot is accomplished by a circuit that converts an incoming switch closure or digital signal to a pulse width. There are two of these channels (SW1 and SW2 into the AP) which determine which mode the autopilot is in. I intend only to have it off or on. Pulse widths are selected appropriately for the required operation.

There is another interface board that takes the PWM commands to the servos and converts them directly to a voltage. There are four of those channels. The voltages are then fed to unused pyro channels on my altimeter or telemetry. This provides real-time logging of the actual servo commands, tied in nicely with the regular avionics logging and telemetry such as rotation rate, verticality, altitude, speed, accel etc…

It became apparent during my drawing of this diagram that I needed to make an interface PCA to tie everything together. Much neater than a bundle of wires.

Now to figure out the best way to make all this hang together.

OverTheTop · Postby **OverTheTop** » Fri Apr 29, 2016 7:24 pm

Time for a Printed Circuit Board

Just got some PCBs this week from a supplier in China

: DSC04813resize.JPG (235.73 KiB) Viewed 7237 times

Notice that there is so little room on the top that the designators had to be printed on the bottom.

After not much work the first board was assembled. Yet to test it.

Details on this thread for the manufacturer if you need to use their services.
viewtopic.php?f=52&t=5480

SpaceManMat · Postby **SpaceManMat** » Fri Apr 29, 2016 9:53 pm

These are servo driver boards?

OverTheTop · Postby **OverTheTop** » Fri Apr 29, 2016 9:56 pm

They turn digital inputs (high/low) to a PWM signal. It will be used to switch the COTS autopilot between the appropriate modes using switch closures (altimeter outputs) to control the functionality.

They could also be used to set servos between two positions based on the digital input. Two channels per PCA, of course.

Refer earlier in the thread to the schematics.