Torque controller: refactor calculations to be in accel space

[sshane]

Idea thanks to Harald. Moves everything to be in accel space until the very end. Instead of: #35780

This works because error is added onto feedforward, which is then converted at the right place on the non-linear curve for Bolt at the end.

Changes behavior slightly for Bolt, checking for linear cars...

opendbc: commaai/opendbc#2528

[haraschax]

This may limit max torque in ways we don't want

[sshane]

Isn't it the same effect? We were limiting from -1 to 1, which was after gravity_adjusted_lateral_accel was converted to torque. Now we limit to the torque that can achieve 3 m/s^2?

[sshane]

Assuming a perfectly linear torque car, I see that it could be a saturation problem if the car's lat accel factor is less than 3 m/s^2. Output could be 3 m/s^2 and torque_from_lateral_accel would convert it to 1.5x of the max torque which would be clipped to 1, and integral would windup outside the range. Is that what you mean?

[sshane]

We need an inverse of all torque_from_lateral_accel functions I think, but that's impossible for the neural function without binary search which is ugly. It's simple for linear

Maybe this is why the PID controller was in torque space

[sshane]

Actually we can revert #33970 to fix this.

[Copilot]

Pull Request Overview

This PR refactors the torque controller to perform all calculations in lateral acceleration space rather than converting to torque space early in the process. This approach allows for more accurate handling of non-linear torque responses, particularly for vehicles like the Bolt that have non-linear torque curves.

Key changes:

• Moves PID calculations to lateral acceleration space and converts to torque only at the final output step
• Simplifies error calculation by working directly with lateral acceleration values
• Updates saturation checking to use acceleration limits instead of torque limits

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
selfdrive/controls/lib/latcontrol_torque.py	Refactors torque controller calculations to work in acceleration space, simplifying PID error computation and deferring torque conversion
opendbc_repo	Updates subproject commit to support new ISO_LATERAL_ACCEL constant

Comments suppressed due to low confidence (2)

selfdrive/controls/lib/latcontrol_torque.py:30

• The variable name steer_max is misleading since it now represents a lateral acceleration limit rather than a steering/torque limit. Consider renaming to lateral_accel_max or accel_limit to reflect the actual units and purpose.

    self.steer_max = ISO_LATERAL_ACCEL

selfdrive/controls/lib/latcontrol_torque.py:69

• [nitpick] The variable name output_accel is more descriptive than the original output_torque, but consider using pid_output_accel or lateral_accel_output to be more explicit about what this represents in the control flow.

      output_accel = self.pid.update(pid_log.error,

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