Motor Control Modes

DaMiao motors support four different control modes, each optimized for different use cases.

Quick links:

MIT POS_VEL VEL FORCE_POS

Recommended learning path

To better understand the control modes in practice, use damiao gui and switch between modes while observing live feedback and charts.

Overview

Control modes determine how the motor interprets command messages. The control mode is set via register 10 (CTRL_MODE) and must match the command format being sent.

API method: DaMiaoMotor.ensure_control_mode()

Mode parameters marked as "Motor-specific" use the mapping limits for the selected motor type. See PMAX / VMAX / TMAX defaults. These registers are writable, but changing them is generally not recommended.

The ensure_control_mode() method automatically:

1. Reads the current mode from register 10

2. Writes the new mode if different

3. Verifies the write was successful

FOC Inner Current Control (not directly exposed)

At the motor communication level, users directly select and command these modes: MIT / POS_VEL / VEL / FORCE_POS. See Communication Protocol - Message Types for the CAN command frames.

Inside the motor firmware, all four modes ultimately drive the same inner FOC current loop:

• The selected communication-level mode computes a current target.
• That current target is sent to a fast inner FOC controller.

• The inner controller continuously compares target current vs measured current and corrects PWM voltage to reduce the error.

• \(i_{q,\mathrm{ref}}\): the target for the torque-producing current. Larger magnitude means stronger torque. Sign decides rotation direction.

• \(i_{d,\mathrm{ref}}\): the target for the flux-axis current. In these modes it is set to 0, meaning "do not add extra d-axis current."

Note

The motor tracks the calculated \(i_{q,\mathrm{ref}}\) to produce the desired torque, while keeping the d-axis current at \(i_{d,\mathrm{ref}} = 0\).

Mode	Easy interpretation of \(i_{q,\mathrm{ref}}\)	\(i_{d,\mathrm{ref}}\)
MIT	Calculated from position/velocity error + feedforward torque, then converted to current	0
POS_VEL	Position loop outputs a speed target, speed loop converts that speed error to current	0
VEL	Speed loop directly converts velocity error to current	0
FORCE_POS	Position loop and speed loop compute current, then it is clipped by current/force limit	0

MIT Mode (Impedance Control)

MIT mode (named after MIT's Cheetah robot) provides impedance control with position, velocity, stiffness, damping, and feedforward torque.

API method: DaMiaoMotor.send_cmd_mit()

Parameter	Range	Description
target_position	Motor-specific	Desired position (radians)
target_velocity	Motor-specific	Desired velocity (rad/s)
stiffness (kp)	0-500	Position gain (stiffness)
damping (kd)	0-5	Velocity gain (damping)
feedforward_torque	Motor-specific	Feedforward torque (Nm)

\[ T_{\text{ref}} = K_p \cdot (p_{\text{des}} - \theta_m) + K_d \cdot (v_{\text{des}} - \dot{\theta}_m) + \tau_{ff} \]

\[ i_{q,\text{ref}} = \frac{T_{\text{ref}}}{K_T}, \quad i_{d,\text{ref}} = 0 \]

Notation mapping: \(p_{\text{des}}\) = target_position, \(v_{\text{des}}\) = target_velocity, and \(\tau_{ff}\) = feedforward_torque.

where \(K_T\) comes from KT_Value (register 1).

POS_VEL Mode (Position + Velocity-Limit)

POS_VEL mode provides position-velocity control with trapezoidal motion profiles. The motor moves toward the target position, limiting velocity to the specified maximum, with automatic acceleration and deceleration.

API method: DaMiaoMotor.send_cmd_pos_vel()

Parameter	Range	Description
target_position	Motor-specific	Desired position (radians)
velocity_limit	Motor-specific	Maximum velocity during motion (rad/s)

\[ v_{\text{des}} = \text{clip}\!\left(K_{p,\text{apr}} (p_{\text{des}} - \theta_m) + K_{i,\text{apr}} \int (p_{\text{des}} - \theta_m) \, dt,\; -v_{\text{limit}},\; v_{\text{limit}}\right) \]

Notation mapping: \(p_{\text{des}}\) = target_position, \(v_{\text{limit}}\) = velocity_limit; \(v_{\text{des}}\) is an internal speed target generated by the position loop.

In the control-law diagram, velocity_limit directly sets the clip bounds \(\pm v_{\text{limit}}\).

\[ i_{q,\text{ref}} = K_{p,\text{asr}} (v_{\text{des}} - \dot{\theta}_m) + K_{i,\text{asr}} \int (v_{\text{des}} - \dot{\theta}_m) \, dt, \quad i_{d,\text{ref}} = 0 \]

where \(v_{\text{limit}}\) is the commanded velocity_limit, KP_APR (reg 27), KI_APR (reg 28) are position loop gains, and KP_ASR (reg 25), KI_ASR (reg 26) are speed loop gains.

VEL Mode (Velocity)

VEL mode provides pure velocity control. The motor maintains the commanded velocity. Positive values rotate in one direction, negative values in the opposite direction.

API method: DaMiaoMotor.send_cmd_vel()

Parameter	Range	Description
target_velocity	Motor-specific	Desired velocity (rad/s)

\[ i_{q,\text{ref}} = K_{p,\text{asr}} (v_{\text{des}} - \dot{\theta}_m) + K_{i,\text{asr}} \int (v_{\text{des}} - \dot{\theta}_m) \, dt, \quad i_{d,\text{ref}} = 0 \]

Notation mapping: \(v_{\text{des}}\) = target_velocity.

where KP_ASR (reg 25) and KI_ASR (reg 26) are speed loop gains.

FORCE_POS Mode (Force-Limited Position)

FORCE_POS mode (Force-Position Hybrid) provides position control with velocity and torque-limit-ratio constraints. The motor moves toward the target position while respecting these limits, providing safe position control with force limiting.

API method: DaMiaoMotor.send_cmd_force_pos()

Parameter	Range	Description
target_position	Motor-specific	Desired position (radians)
velocity_limit	0-100 rad/s	Maximum velocity during motion
torque_limit_ratio	0.0-1.0	Normalized torque-limit coefficient

\[ v_{\text{des}} = \text{clip}\!\left(K_{p,\text{apr}} (p_{\text{des}} - \theta_m) + K_{i,\text{apr}} \int (p_{\text{des}} - \theta_m) \, dt,\; -v_{\text{limit}},\; v_{\text{limit}}\right) \]

\[ i_{q,\text{ref}} = \text{clip}\!\left(K_{p,\text{asr}} (v_{\text{des}} - \dot{\theta}_m) + K_{i,\text{asr}} \int (v_{\text{des}} - \dot{\theta}_m) \, dt,\; -\tau_{\text{lim}},\; \tau_{\text{lim}}\right), \quad i_{d,\text{ref}} = 0 \]

Notation mapping: \(p_{\text{des}}\) = target_position, \(v_{\text{limit}}\) = velocity_limit; \(v_{\text{des}}\) is an internal speed target generated by the position loop.

In FORCE_POS mode:

\[ \tau_{\text{lim}} = \text{torque_limit_ratio} \cdot T_{\max} \]

Here \(T_{\max}\) is the max torque for the selected motor type (for example, for 4340, \(T_{\max}=28\) Nm, so torque_limit_ratio=0.5 gives \(\tau_{\text{lim}}=14\) Nm).

Also, \(v_{\text{limit}}\) is the commanded velocity_limit, KP_APR (reg 27), KI_APR (reg 28) are position loop gains, and KP_ASR (reg 25), KI_ASR (reg 26) are speed loop gains.