Saving Encoder's Velocity and Position Estimates for Analysis

[mohdyaser]

I want to save the velocity and position estimations from the encoder into a list to do extra analysis (and get the acceleration) and plotting. I simply tried to append the .encoder.vel_estimate to a list after a position changing command like below:

velocity = []
pos = []
while True:
    odrv0.axis1.controller.input_pos = 10
 
    velocity.append(odrv0.axis1.encoder.vel_estimate)
    pos.append(odrv0.axis1.encoder.pos_estimate)
 
    time.sleep(6)
 
    odrv0.axis1.controller.input_pos = 0
 
    velocity.append(odrv0.axis1.encoder.vel_estimate)
    pos.append(odrv0.axis1.encoder.pos_estimate)
 
    time.sleep(6)

but it doesn’t work, it only saves a single value each time the line is run. I tried to see how it is done in the utils.py at the start_liveplotter function, but I couldn’t comprehend the loop.

EDIT1: I just found these two topics, but they are somewhat old, is there any better practice?

Export data to CSV file

Hi, I can’t find any option to export or capture data like vel_estimate? any ideas how to do it ? Adrian.

Log timestamped data

Hi guys! Is it possible to log data (i.e. estimated velocity) with an accurate timestamp, so I know exactly at which point in time the data was recorded? And if so, what is the maximum frequency I can log data with? So far I have found this topic Export data to CSV file with which help I was able to write the data to a CSV file, but I did not find a way to get the timestamp of the data. My goal would be to log data with a fixed period of one microsecond or less. Thanks for your help

[solomondg]

Hi! You can write a loop like this:

import time
velocity = []
pos = []
t = []

t_sample = 0.01 # Sample at 100Hz
t_wait = 6 # Wait six seconds between setpoint changes

t_start = time.time() # Timestamp for when we started sampling
while True:
    odrv0.axis1.controller.input_pos = 10
    
    t_0 = time.time()
    
    while (time.time() - t_0 < t_wait):
        velocity.append(odrv0.axis1.encoder.vel_estimate)
        pos.append(odrv0.axis1.encoder.pos_estimate)
        t.append(time.time() - t_start)
        time.sleep(t_sample)
 
    odrv0.axis1.controller.input_pos = 0

    t_0 = time.time()
    
    while (time.time() - t_0 < t_wait):
        velocity.append(odrv0.axis1.encoder.vel_estimate)
        pos.append(odrv0.axis1.encoder.pos_estimate)
        t.append(time.time() - t_start)
        time.sleep(t_sample)

Your issue is that the sampling code only runs once before the time.sleep(6) blocks for the six seconds. Here, we move it into a polling loop, so that there isn’t any actual blocking calls.

That’ll additionally capture the timestamps of each datapoint.

Note two things:

1. The while True loop will never exit - you should have some exit condition, such as a keyboard command (ctrl-C for instance), or an amount of time elapsing
2. Since there’s no limit to the amount of data stored in pos and velocity, it’ll quickly grow in memory size and may cause memory errors
3. Since you’re continuously appending to a list, Python will have to re-allocate the memory for this list. If you can pre-allocate it before hand (i.e. make a list of known length full of zeroes), this’ll greatly speed up the code.

[mohdyaser]

Wow!! thanks a lot @solomondg I wasn’t expecting to get a full nice code!! Thank you again it works perfectly. I only added a break from the parent while to get data of a single cycle.