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set_desired_force()

Definition

set_desired_force(fd, dir, time, mod, vel_limit_dir, vel_limit)

Features

This function defines the target force, direction, transition time, and mode for force control based on the global coordinate.

Caution

  • In non-FTS models (A0509, A0912, A0509F, A0912F, E0509), force control is only possible in the translation direction, and the control error may be large.

  • If the command is used in a simulation environment without a robot, it may not operate normally.

  • Vibration may occur during force control when several of the following conditions overlap: a long TCP setting, a posture with the elbow (Joint 3) strongly folded, a light tool, and low compliance stiffness. See the Note below for countermeasures.

Parameters

Parameter Name

Data Type

Default Value

Description

fd

float[6]

[0, 0, 0, 0, 0, 0]

Three translational target forces [N]

Three rotational target moments [N·m]

dir

int[6]

[0, 0, 0, 0, 0, 0]

Force control in the corresponding direction if 1

Control compliance of corresponding direction if value is 0

At least one direction must be set to 1.

time

float

0

Transition time of target force to take effect [sec]

Range: 0 - 1.0

mod

int

DR_FC_MOD_ABS

DR_FC_MOD_ABS: Force control with absolute value

DR_FC_MOD_REL: force control with relative value to initial state (the instance when this function is called)

vel_limit_dir

int[6]

[0, 0, 0, 0, 0, 0]

Activate velocity limit in the corresponding direction if 1

Deactivate velocity limit in the corresponding direction if 0

vel_limit

float[6]

[0, 0, 0, 0, 0, 0]

Velocity limit in each direction: translation [mm/s], rotation [deg/s]

(If “vel_limit_dir” in that direction is 0, this value will be ignored)

Caution

In the Non-FTS A & E model, the data types of fd and dir parameters can be either float[6] or float[3]. (Rotational parameters are automatically set to the default values)

Return

Value

Description

0

Success

Negative value

Error

Exception

Exception

Description

DR_Error (DR_ERROR_TYPE)

Parameter data type error occurred

DR_Error (DR_ERROR_VALUE)

Parameter value is invalid

DR_Error (DR_ERROR_RUNTIME)

C extension module error occurred

DR_Error (DR_ERROR_STOP)

Program terminated forcefully

Note

  • This command takes effect only while task compliance control is active by task_compliance_ctrl(). If it is called without task compliance control active, the command is ignored, and if it is called while a task motion is in progress, it is rejected with a warning. Therefore, call task_compliance_ctrl() and then set_desired_force() while the robot is stopped.

  • The value of external force refers to the sensor measurement at terminating the force control (control mode transition to compliance control) by the command release_force().
    Therefore, the variation in external force can occur if the option mod=DR_FC_MOD_REL is applied.

  • Even if mod is set to DR_FC_MOD_REL, the relative reference applies only to the force control target; collision detection, tool weight estimation, and the external force value refer to the sensor measurement as is.

  • If vibration occurs during force control, set the force factor below 1.0 with set_force_factor() right after this command, together with a damping factor above 1.0 with set_damping_factor().

  • set_force_factor() can be set only in the force control state (after this command).

  • Both factors are reset to the default (1.0) when compliance control is started with task_compliance_ctrl(). See the Note in task_compliance_ctrl() for details.

Caution

  • No position control is performed on the axes for which force control is enabled by dir. Therefore, any component of the trajectory commanded by a motion command that overlaps with a force control axis is replaced by force control and is not executed; only the non-overlapping components are executed. This behavior is not limited to particular motion commands; it applies to all motion commands.

    The following are examples in which the partial execution of a trajectory is especially noticeable, making the resulting path differ from the intended one.

    • When Z translation is set to force control and a Z amplitude is specified for move_periodic(): the Z translation is not executed, and only the rz rotation is executed.

    • When Y translation is set to force control and move_spiral() is executed about the Z axis: the Y component of the XY spiral is lost, and the motion changes into a reciprocating motion along the X axis.

    If no component overlaps with a force control axis, the motion is executed normally. Using movel() or movec() in directions that do not overlap with the force control axes is a normal usage pattern (refer to Example #3).

  • To retain the accuracy in force control, it is recommended to start force control with setting mod=DR_FC_MOD_REL near the contact point, and to change the position/orientation only within a limited range during force control.

Example

Python
# Example # 1
# Executed in the global coordinate(tool coordinate)
# Zero force control in the z-axis direction of the tool, moment control in the z-axis direction of the tool, and compliance control in the other directions
# Force control with the relative value to the sensor measurement at starting the force control

set_ref_coord(DR_TOOL)
q0 = posj(0, 0, 90, 0, 90, 0)
movej(q0, vel=30, acc=30)
task_compliance_ctrl(stx=[500, 500, 500, 100, 100, 100])
fd = [0, 0, 0, 0, 0, 10]
fctrl_dir= [0, 0, 1, 0, 0, 1]
set_desired_force(fd, dir=fctrl_dir, mod=DR_FC_MOD_REL)   

# Example # 2
# Executed in the robot base coordinate(base coordinate)
# Force control in the z-axis direction, and compliance control in the other directions
# Activate velocity limit in the force control direction
set_ref_coord(DR_BASE)
q0 = posj(0, 0, 90, 0, 90, 0)
movej(q0, vel=30, acc=30)
task_compliance_ctrl()
fd = [0, 0, -30, 0, 0, 0]
fctrl_dir = [0, 0, 1, 0, 0, 0]
fctrl_vel_limit = [0, 0, 20, 0, 0, 0]
fctrl_vel_limit_dir = [0, 0, 1, 0, 0, 0]
set_desired_force(fd, dir=fctrl_dir, vel_limit=fctrl_vel_limit, \
                  vel_limit_dir=fctrl_vel_limit_dir) 

# Example #3
# 1. Move to initial posj: [J1, J2, J3, J4, J5, J6] = [0, 0, 90, 0, 90, 0]
# 2. Approach to the position to start force control: move -100mm along Base-z direction 
# 3. Start force control : apply -20N force along Base–z direction
# 4. Force & compliance control after detecting external force : while maintaining -20N force along Base-z direction (force control), move 200mm along Base-y direction. 
# 5. Retract 150mm in Base-z direction and move to initial posj

# 1. Move to initial posj
q0 = posj(0.0, 0.0, 90.0, 0.0, 90.0, 0.0)
set_velj(30.0)
set_accj(60.0)
movej(q0)

# 2. Approach to the position to start force control
set_velx(75.0)
set_accx(100.0)
delta_approach = [0.0, 0.0, -100.0, 0.0, 0.0, 0.0]
movel(delta_approach, mod=DR_MV_MOD_REL)

# 3. Start force control (apply -20N force along Base–z direction)
k_d = [3000.0, 3000.0, 3000.0, 200.0, 200.0, 200.0]
task_compliance_ctrl(k_d)
force_desired = 20.0
f_d = [0.0, 0.0, -force_desired, 0.0, 0.0, 0.0]
f_dir = [0, 0, 1, 0, 0, 0]
set_desired_force(f_d, f_dir)

# 4. Force & compliance control after detecting external force
force_check = 20.0
force_condition = check_force_condition(DR_AXIS_Z, max=force_check)
while (force_condition):
    force_condition = check_force_condition(DR_AXIS_Z, max=force_check)
    if force_condition == 0:
        break
delta_motion = [0.0, 200.0, 0.0, 0.0, 0.0, 0.0]
movel(delta_motion, mod=DR_MV_MOD_REL)

# 5. Retract 150mm in Base-z direction and move to initial posj
release_force()
wait(0.5)
delta_retract = [0.0, 0.0, 150.0, 0.0, 0.0, 0.0]
release_compliance_ctrl()
movel(delta_retract, mod=DR_MV_MOD_REL)
movej(q0)

Keyword

set / desired / force / set_desired / desired_force