Selfbuild an USB PC <-> PPM PIC inteface

[mute]

Hi all, i'm working on a USB interface that will be connected to the PPM output of the most common RC. Your PC will see it as a common Joystick and so you can use it with FMS or also to drive a car or to play PACMAN
Actually i'm testing it on my Hitec Focus4 and only two axes are working (i started this project 5 days ago). By the way the final circuit will theorically allow transmisson of up to 14 axes and 12 buttons. All RC compatible with my one will be supported... (i hope ).
You can find more detail about this project (FMSUSB) on my website:
Is someone interested in it? If so i will propose more details in my pages... otherwise i will only write a brief description.

Bye,

Walter

[Scott Mark]

Yes, I'm interested. But will building a PIC cable cost less than buying one? I don't have a PIC compiler, a PIC burner, etc, etc.

Scott

[mute]

I don't know if for you it will cost less than buying one, but for me the answer is yes
I have my own PIC programmer and may be that you have a friend that owns one. More or less the entire amount for the circuit will be less than 10 euro. If you need a programmer you can build my Walpic for a few euro (the scheme is also on my site).

A good news about the project, today i successfully tested the six channel version!

Now i can fly with all my RC axes! )

In a few days i will write the pages about the interface.

Walter

[mute]

Now 12 channels are working!
The work is in progress!!!

P.S.: I added a channel mapping function to my device...

[phildc]

Can you tell us the "resolution" you achieved with this setting:
PPM -> PIC -> USB joystick
I mean how many stick positions on each side of the neutral for a 45° or 100% signal , or how many in total?

Other question: will you make your design available for others?

Thanks,
Philippe

[mute]

[phildc]

Resolution is important to the behaviour of the plane in the simulator. Depending on the type of plane, you can get noticeable "flutter". Also trimming (e.g. for an horizontal flight) can be difficult or impossible.
In fact you can loose the quality of your transmitter with a limited resolution.
Typical resolution for audio input is around 23 steps on both sides, which is too little in most cases.
For PIC's going to a COM-type input (this can also end on USB, just a port conversion), you typically get over 80 on both sides. This is equal or exceeds the physical resolution that the pilot can achieve on the sticks and trims.
Your resolution of 50 on each side is medium and will probably do the job. To be tested...
My question: I did not look into the application of joystick in this particular case (PPM .... USB as a joystick), but I wonder why you can't achieve a little bit higher resolution?

Anyway, go on with your good job.

Philippe.

[mute]

[phildc]

[Eric Brouwer]

With the USB Joystick, the resolution is set to vary between 0 and 100%, with 50% defined as the centre. This is more than adequate. I have been using this resolution with the Serial PIC interfaces, Joystick interfaces as well as the USB Joystick interfaces.

In fact, when trying to get the resolution too high (say 255), one will notice that the transmitter output pulse varies as well, and with the 1% accuracy of the PIC's 4MHz internal oscillator, the signal will vary between a couple of numbers, say for centre stick, the PIC's data will vary say between 125 and 130.

Also, depending on the method of PIC programming, a delay of typical 6us is needed to sample the PPM signal. This already results in more than 0.5% error on the original pulse length.

One must also remember to cater for special features on different transmitters. Some transmitters can be programmed to give pulses varying between 0.75 and 2.25ms. This will obviously have an effect on the final resolution.

I have been using the FMS Serial PIC, joystick as well as USB joysticks on FMS, RealFlight, RealFlight G2 and several Microsoft games, without any problem on glithcing, or unstable surfaces. In fact, with the Raptor 60 in RealFlight G2, the chopper will stay in a hover on the same spot (wind disabled) when using my USB interface.

Most of the new generation digital joysticks that I tested, make use of the standard USB paramaters, which will detect only 100 points throughout an axis movement. Don't be too critical on the resolution. I doubt it if one will notice the difference between a resolution of 80 or 150 points. Above 150 points, the inaccuracy of the PIC software, PIC power supply, or even the transmitter becomes noticable.

By

[phildc]

You are right, Eric. You point out the limits for the PIC. From my experiences with my flight recording soft, some erratic movements may start with a resolution of around 160 points for the total 200%. This same resolution covers also the -125% to +125% with smaller and higher numbers.

Philippe
Brussels

[mute]

[mute]

I published the shcematic, PCB and a pre-release of the firmware for the device:



Bye,

mute

[Isaac]

[mute]