1
$\begingroup$

I would like to know what type of microcontroller should I buy for this control loop task.

Scenario:

I have a digital signal coming from the following equipment:

$\boxed{\text{Ultrasonic}\\\text{transducer}} \longrightarrow\boxed{\text{ADC}}\longrightarrow\boxed{\text{DSP Board}\\\text{(some high and low pass filter)}}\longrightarrow\boxed{\text{USB 2.0}\\\text{(40MBPS) output}}$

I want to be able to use this digital signal coming trough USB as input in my controller and calculate the TOF (time-of-flight) of the echo and react with my controller every $1\textrm{ ms}$ or less using a PID control law.

  • How should I deal with this digital signal output in my microcontroller, to be able to detect the echo in generate a reaction in less than $1\textrm{ ms}$?

    Schema

  • Any suggestion of microcontroller that I could use for this task?

  • And how can I deal with such high speed input?
|improve this question
$\endgroup$
  • $\begingroup$ where in your system do you have the 100MS/s you claim in the title? $\endgroup$ – Marcus Müller Jun 19 '16 at 20:40
  • $\begingroup$ I highly doubt you'd benefit much from such an extraordinarily high sample rate for ultrasound, by the way. $\endgroup$ – Marcus Müller Jun 19 '16 at 20:40
  • $\begingroup$ and 1ms is enormously long compared to that sampling rate $\endgroup$ – Marcus Müller Jun 19 '16 at 20:42
  • $\begingroup$ what's the sample bit depth? $\endgroup$ – Marcus Müller Jun 19 '16 at 20:43
  • $\begingroup$ If you have a bit of cash to spend: ni.com/rseries $\endgroup$ – Arnfinn Jun 20 '16 at 13:01
1
$\begingroup$

No Microprocessor is able to deal with such extreme rates as 100MS/s. That is clearly the domain of custom ICs and FPGAs, bringing the data to a very capable CPU, not something small and embedded.

Regarding USB:

I don't know your sampling bit width, but 40MB/s is not very much if you need to transport 100MS/s – you get 12bit per sample, in the ideal, zero-overhead case (which doesn't exist).

As a matter of fact, I know a series of devices very well that can deliver 100MS/s and more to a PC – these work through expensive interface such as a proprietary external PCIexpress interface, or better even through 10Gbit Ethernet (which you'd normally only find in data center / ISP equipment).

With a single 100MS/s stream you can easily flatten a single CPU core of a pretty capable x86_64 computer – really, that MUCH MUCH more data than what "microcontrollers" handle.

This might really be a case of insufficiently discussed system design – Nyquist and my understanding of the term "Ultrasound" say that you'd never need more than twice the highest signal frequency as sampling rate. I wouldn't expect ultrasound sonar to have frequencies higher than maybe 4 MHz, but I might be wrong.

|improve this answer
$\endgroup$
  • $\begingroup$ The overhead on any bus like PCIe, USB, and Ethernet is way too large to allow for real-time closed-loop control in the MHz range. So even if the streaming rate and processing rate is high enough, the time needed to read and write is a severe limitation. The fastest I have ever been able to turn-around on a PCIe bus is 150 kHz. An FPGA is definitely the way to go. $\endgroup$ – Arnfinn Jun 19 '16 at 22:08
  • $\begingroup$ @Arnfinn couldn't agree more. However, for the 1ms latency OP seems (?) to ask about, going through a PC is feasible. $\endgroup$ – Marcus Müller Jun 20 '16 at 9:59
  • $\begingroup$ Sounds reasonable to me -- 100 MS/s seems unnecessarily high. If the sampling time for control law is 1 ms, a PC would do the job. $\endgroup$ – Arnfinn Jun 20 '16 at 13:00
  • $\begingroup$ @Arnfinn since you're already "advertising" NI equipment in your comment to OP's question: I work for Ettus Research, a daughter of NI; the USRP series generally has a ADC/DAC<->FPGA<->computer peripheral bus architecture. There's USB2, USB3, Gigabit Ethernet, 10Gigabit Ethernet/PCIe and devices with a direct link to an embedded CPU, and yes, you can do high rates with that to a PC. $\endgroup$ – Marcus Müller Jun 21 '16 at 18:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.