Need clear information about joint states

[ekorudiawan]

I use Darwin OP3 robot which uses XM430-W350 Dynamixel servo. I want to read current data in Ampere using ROBOTIS-Framework.
As I know, I can subscribe present joint states in topic

/robotis/present_joint_states

This topic publishes position, velocity, and effort. Is the effort contains value of joint torque? Because the servo motor doesn't have torque sensor, or it is just approximation?

In devices folder, there is XM430-W350.device that lists all information of the motor. I found a scaling factor

torque_to_current_value_ratio = 149.795386991

Where this value comes from?
How can I convert effort value in joint states to current (in Ampere)?

[onjinwook]

Hi, I'm Jin.

if you want to read the effort, you need to do something.

1. check this link -> OP3.robot
   : you may find and add "BULK READ ITEMS" -> prensent_current

After that you make use of "torque_to_current_value_ratio" in framework.
*ps : The" torque_to_current_value_ratio value" is an actual measurement and may not be accurate.

Regards,
Jin.

[ekorudiawan]

check this link -> OP3.robot
: you may find and add "BULK READ ITEMS" -> prensent_current

I am done with this, here an example reading

---
header: 
  seq: 880
  stamp: 
    secs: 25
    nsecs: 196000000
  frame_id: ''
name: - head_pan
- head_tilt
- l_ank_pitch
- l_ank_roll
- l_el
- l_hip_pitch
- l_hip_roll
- l_hip_yaw
- l_knee
- l_sho_pitch
- l_sho_roll
- r_ank_pitch
- r_ank_roll
- r_el
- r_hip_pitch
- r_hip_roll
- r_hip_yaw
- r_knee
- r_sho_pitch
- r_sho_roll
position: [8.420455125346393e-05, -0.0036501271747759745, -0.5074470885658897, -0.021233422276620395, -0.0005806854408909601, -0.9877315103525266, 0.02190427963862085, 0.006478857119786241, 1.2347772174846146, -0.2624526561998568, 1.3985380369619849, -0.505774797156783, 0.024472363280158227, 0.0006823349379478927, -0.9868821049928496, -0.021583271204584698, -0.007204199157222924, 1.2324199986224231, 0.2625453673115734, 1.3992656564533164]
velocity: [0.012730165317048889, 0.006602381264101093, 0.031290642219176754, -0.1671884450877372, 0.0012968547747411532, 0.012262565514619156, 0.03416588217015005, 0.0027814892795186043, 0.08419838527850326, 0.014732007522410162, -0.009373388961215968, 0.06461045145225715, 0.12410907878294766, 0.0014696812823669601, 0.028675257190019912, -0.009886637881570931, -0.00015261822575412913, 0.05049065780656754, -0.0047445688506732725, 0.010304926419331981]
effort: [-0.0003574316151899737, 0.01828303354011407, 0.27393444414847146, -0.23760928455174302, 0.005819751340796486, 0.265826754705234, 0.1535365403074973, 0.009660529969401921, -0.601800504936767, 0.007786059116590138, -0.022840644861021886, 0.19194477979218583, 0.07354303810855889, -0.00680874408325316, 0.22416885356322247, -0.16837590038334405, 0.026744237372655633, -0.4855997182856786, -0.008613453302279694, -0.02991980491987789]

My question, what is unit in effort value?
Is it an approximation of torque in Nm? Or it represents an "electric current" in Ampere?
If it is just approximation of torque value, how to scale this value to get actual "electric current"?

Actually, I want to read "electric current" data not "torque" data

[onjinwook]

Hi,
Nm is right.

There are two ways to obtain it.

1. <2.69 mA> per unit for XM430-w350 . =>

value = torque * torque_to_current_value_ratio
current = value * current_unit

2. There is a method to obtain from the process function of motion_module inside the framework.
   you can use "motion module" (it can be read with the "bulk read" in "robotis_controller").
   check it -> THORMANG example
   but be careful. THORMANG uses different DXL( -> 1mA per unit )

Regards,
Jin.

[ekorudiawan]

Are you sure about this?

1. <2.69 mA> per unit for XM430-w350 . => 2.69 * effort = current.

Check file XM430-W350.device

[type info]
torque_to_current_value_ratio   = 149.795386991

[control table]
   126 | present_current          | 2      | R      | RAM    | 0             | 1193        | N

Then check dynamixel.cpp

double Dynamixel::convertValue2Torque(int16_t value)
{
  return (double) value / torque_to_current_value_ratio_;
}

In function above you scale the "value" to "torque". "Value" in range 0-1193. So "torque" is equal to

torque = value / torque_to_current_value_ratio

From the datasheet, I got scaling unit for current = 2.69 mA

So, if I want to get "current" then I have to multiply "value" with "current_unit" right?
Then the final equation could be

value = torque * torque_to_current_value_ratio
current = value * current_unit

Is my equation above correct?

[onjinwook]

HI,

I'm sorry about it.
I got a misstake.

you right.

You find a way like below.

value = torque * torque_to_current_value_ratio
current = value * current_unit

That is great.

I'll change my comment as soon as possible.

Regards.
Jin