Re: IIO for soft realtime control?

[James Peverill <arsenix@gmail.com>]

On 01/06/2013 06:32 AM, Jonathan Cameron wrote:
> On 01/06/2013 04:31 AM, James Peverill wrote:
>> I have been working on realtime robotics under Linux, and just learned about
>> iio. It seems like many great hardware drivers are being written (some of which
>> I had started working on in parallel), and it would be great if I could
>> integrate what I am working on with iio. It seems mostly targeted at streaming
>> data acquisition and output, but also includes triggering and event support. Has
>> there been any thought to running realtime control loops using the iio
>> subsystem? For this application I would be reading from sensors (i2c interface
>> accelerometers etc) in the 10's of hz (ideally at least 50) and then updating
>> actuator outputs.
>>
>> Any thoughts on the feasibility? I would probably accomplish this by writing a
>> kernel driver to glue the input iio devices to the output. I'm not sure what
>> type of latency and jitter would result from this configuration, but this could
>> limit the applications.
>>
> I don't know of anyone else doing this with IIO at the moment (though someone may
> well shout out in the near future!)
>
> So based on 5 mins of thinking about it the situation at the moment is...
>
> We handle streaming inputs reasonably well and the just merged in kernel 'buffer'
> interfaces (which are really just hooks to get the stream of data from a device)
> should allow a kernel module to get hold of this data fairly efficiently (certainly
> 50Hz shouldn't be a problem!).  We don't currently have interfaces for getting hold
> of events in kernel though (threshold etc) but that should be fairly trivial to
> add.
>
> Unfortunately streaming support on the output side is not currently there.  Right now
> you can set individual DAC outputs from other kernel drivers (though some nice
> but trivial util functions might be missing) but buffered writing support isn't
> there yet.  There is a lot of interest in adding this, but from what I recall there
> are some 'interesting' issues with actually supporting precisely timed writes to
> DAC chips (usually there is some dma involved).
> Some spi implementations are capable of doing this I believe but there isn't any
> kernel support for it as far as I know.
> I'm guessing some DAC chips have hardware fifos (right now I can only find a TI
> technote on a separate fifo for this purpose).
>
> I've cc'd Lars and Michael because I know this sort of support is of interest to them
> and they will doubtlessly be able to give some more informed comments!
>
> If the buffered output were present then using IIO for real time control should
> be straight forward.
>  From the level of interest I've seen in this functionality support will definitely
> occur at some point though this will happen faster obviously if someone
> (no hints at all :) takes on implementing such support!
>
> In the meantime, using IIO for your input drivers may make sense even if they
> more involved actuator side comes later.  Or possibliy the slow current dac
> interface will be 'good enough' timing wise for your application?
>
> If you are able to, I'd love to hear more about your project, both in terms
> of what you are doing (google gives me a fair idea as there aren't
> many James Peverill's in the world ;) and in terms of the
> control you are doing in kernel.
>
> Jonathan
My interest is in flying robots, as your google searches turned up! The 
ability to run control loops from within a Linux system though has a lot 
of varied uses. Most flying vehicles and other real time robotics uses 
small microcontrollers for the fast control, but I am of the opinion 
that there is no reason you can't run those on a Linux based system. 
I've played around with Xenomai and other "hard" realtime Linux 
enhancements, and with these it is certainly possible to meet the timing 
requirements to control most mechanical systems. Adding the realtime 
layers though adds quite a bit of complexity and requires rewriting 
drivers to get the benefits. Now I'm investigating whether a standard 
kernel driver will suffice. Currently my target hardware platform is the 
Raspberry Pi (700mhz Arm11), although I like to keep things as hardware 
independent as possible.

In terms of IIO, what would be really cool is if there was a way to glue 
together the IIO blocks into a control chain. Ie you could take an ADC 
device and chain it with a PID driver and an output device. Inside the 
ADC device driver it would pass the data to the next chain element after 
a new value was read. This way you could chain together devices in a 
user mode sysfs context and end up with a complete control loop running 
inside the kernel. For most of the small flying vehicles I deal with 
this type of input->PID->output would be adequate as long as it could 
run at 50hz with no more than a few ms worst case latency between sample 
acquisition and output change. For hovering vehicles 100hz or 150hz can 
be beneficial but those are the highest rates of interest for my 
applications.

One critical difference for this type of application though is that 
"new" data is always preferred over older data, and stale samples are 
discarded if there is some type of delay in the system. For data 
acquisition applications you don't ever want to lose any samples so the 
focus is on high speed buffering.