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lba_821103 · Joined: 18 Oct 2007 Posts: 10

Firstly, Thanking you for your helping last time.
Could you explain the mean of obtain the current_velocity with the below method? You'd better detail explain the math meaning with the method. thank you!

#define FILTER_SHIFT 1
static int32_t filter_reg = 0;
static int16_t filter_update(int16_t input)
{
// Update the filter with the current input.
filter_reg = filter_reg - (filter_reg >> FILTER_SHIFT) + input;

// Scale output for unity gain.
return (int16_t) (filter_reg >> FILTER_SHIFT);
}
filtered_position = filter_update(current_position);

// Use the filtered position to determine velocity.
current_velocity = filtered_position - previous_position;
previous_position = filtered_position;

ginge · Posted: Wed Nov 28, 2007 9:24 am Post subject:

Hi lba,

What is happening here is a simple digital filter. The article listed in the source code explains in detail how the filter portion works

http://www.edn.com/index.asp?layout=article&articleid=CA6335310

In mathematical terms it is

y(n)=(1-2^–k)*y(n–1)+x(n), where x is the input, y is the output, and n is the sample index. Parameter k is the shifting value, which in our case is FILTER_SHIFT. ( equation borrowed directly from the article) This determines the bandwidth of the sampling. With FILTER_SHIFT == 1 we get a bandwidth of 0.1197 over 3 samples.

As for your question about the current_velocity...

Firstly we take a running average of the position using filter_update then we subtract the last position from the current average position. This will tell us the amount of positional units moved each sample.

An example of this might be:-

Sample 1: filtered_position = 400
Sample 2: filtered_position = 410 previous position = Sample (n)-1 or (400)
current_velocity = filtered_position - previous position
current_velocity = 410 - 400
current_velocity = 10

So in the example above, we have a speed of 10 units per sample, where each sample is 10ms. So the current velocity is 10 units/ms.

Let me know if you need clarification of the filtering equations, the article is clearer than I could ever be.

Barry
_________________
http://www.headfuzz.co.uk/
http://www.robotfuzz.co.uk/

lba_821103 · Joined: 18 Oct 2007 Posts: 10

Thank you for your helping.
I still don't know filter_position-previous_position that is the value of current_velocity. From what you provide the article ,I don't find the principium of calculate the current_velocity.
You can explain the mean of current_velocity=filter_position-previous_position.

lba_821103 · Joined: 18 Oct 2007 Posts: 10

Thank you for your helping!
My classmate have read the article and explain the meaning of method for you. I also clear about the method. Thank you again!

ginge · Posted: Fri Nov 30, 2007 9:03 am Post subject:

Hi lba,

Velocity is a measure of distance over time, so filter_position-previous_position is how much the servo has moved since the last measurement. Each measurement is taken 10ms apart, so time = 10ms.

In Math terms...

speed = distance/time

Factoring in OpenServo terms...

current_velocity = (filter_position-previous_position)/time

current_velocity = (filter_position-previous_position)/10ms

If we use the example I posted above...

current_velocity = (410-400)/10ms

current_velocity = 10/10ms

current_velocity = 1 unit per ms

I hope this clarifies it a little.

Barry
_________________
http://www.headfuzz.co.uk/
http://www.robotfuzz.co.uk/