Force sensitive grippers for teleoperation and imitation learning

SSG-48 Electric grippers
,
1

Bilateral teleoperation is a concept that Involves two-way communication where both the operator and the remote device send and receive feedback. The operator controls the device remotely and receives sensory feedback (such as force) from the device to adjust their actions.

The two grippers are virtually coupled to each other, allowing either to act as the master (the haptic interface) or the slave (the manipulator) in the system. We are not using a force sensor at the grippers but are relying on proprioception.

These kind of setup is perfect for:

  • Teleoperation
  • Imitation learning
  • Virtual Environments
  • VR

In example in this clip it is used on 2 SSG48 grippers capable of running that kind of algorithm.

2 ways we can run it:

  • Perform PD calculations on PC
  • Perform PD calculations on BLDC drivers.

From testing we found that doing calcs on PC works better in case of grippers.

We are using these formulas, we got them from amazing Ben Katz paper, creator of mini cheetah quad: Link to the paper

𝜏1,𝑖 = 𝐾𝑝(πœƒ2,𝑖 βˆ’ πœƒ1, 𝑖) + 𝐾𝑑( Λ™πœƒ2,𝑖 βˆ’ Λ™πœƒ1,𝑖) βˆ’ 𝐾( Λ™πœƒ1,𝑖)

𝜏2,𝑖 = 𝐾𝑝(πœƒ1,𝑖 βˆ’ πœƒ2, 𝑖) + 𝐾𝑑( Λ™πœƒ1,𝑖 βˆ’ Λ™πœƒ2,𝑖) βˆ’ 𝐾( Λ™πœƒ2,𝑖)

Note that the gains used are identical for both grippers, resulting in nearly identical motor torques for each. This means the force felt by the operator at the gripper is very close to the force on the slave side. Dynamically, the operator experiences the friction and inertia of both the master and slave sides, virtually coupled through the controller.

The achievable coupling stiffness was primarily determined by the communication rate between the two grippers. We achieved rates of 500 Hz to 1 kHz, which is sufficient.

Can I run this on my gripper?
You might be able to but you need to satisfy few requirements:

  • Low gear reduction (QDD drive) / high backdrivability
  • Good current sensing and FOC algorithm
  • Fast communication between devices
  • high bandwidth torque control
  • low reflected inertia
    Our ssg48 gripper is a rare case that can run this kind of algorithm because of its design.

You can see insides of the gripper here: Link
You can also see the complete assembly guide here: Link