# Contact Sensors

Genesis provides sensors for detecting and measuring contact forces. These are essential for manipulation tasks, grasping, and understanding physical interactions.

## ContactForceSensor

The `ContactForceSensor` measures the total contact force being applied to the associated rigid link in its local frame.

### Usage

```python
import genesis as gs

gs.init()
scene = gs.Scene()
robot = scene.add_entity(gs.morphs.URDF(file="gripper.urdf"))
finger = robot.get_link("finger_link")

# Add contact force sensor to gripper finger
contact_sensor = scene.add_sensor(
    gs.sensors.ContactForce(
        entity_idx=robot.idx,
        link_idx_local=finger.idx_local,
    )
)

scene.build()

# Simulation loop
for i in range(1000):
    scene.step()

    # Get contact force (plain tensor, not NamedTuple)
    force = contact_sensor.read()  # ([n_envs,] 3) force in Newtons
    print(f"Contact force: {force}")
```

### Configuration

```python
gs.sensors.ContactForce(
    entity_idx=robot.idx,           # Global entity index
    link_idx_local=finger.idx_local, # Local link index
    pos_offset=(0.0, 0.0, 0.0),    # Position offset from link frame
    euler_offset=(0.0, 0.0, 0.0),  # Rotation offset (degrees)
    min_force=0.0,                  # Min detectable absolute force per axis (below → 0)
    max_force=float("inf"),         # Max output absolute force per axis (above → clipped)
    noise=0.0,                      # White noise std
    bias=0.0,                       # Constant additive bias
    draw_debug=True,
)
```

### Output Format

`read()` returns a plain `torch.Tensor` (float32):

| Shape | Description |
|-------|-------------|
| `([n_envs,] 3)` | Total contact force in local link frame (Newtons) |

## ContactSensor

The `ContactSensor` detects whether the associated rigid link is in contact (boolean).

### Usage

```python
import genesis as gs

gs.init()
scene = gs.Scene()
robot = scene.add_entity(gs.morphs.URDF(file="robot.urdf"))

contact = scene.add_sensor(
    gs.sensors.Contact(
        entity_idx=robot.idx,
        link_idx_local=robot.get_link("base").idx_local,
    )
)

scene.build()
scene.step()
in_contact = contact.read()  # ([n_envs,] 1) boolean tensor
```

### Output Format

`read()` returns a plain `torch.Tensor` (bool):

| Shape | Description |
|-------|-------------|
| `([n_envs,] 1)` | True if link is in contact |

## API Reference

### ContactForceSensor

```{eval-rst}
.. autoclass:: genesis.engine.sensors.ContactForceSensor
   :members:
   :undoc-members:
   :show-inheritance:
```

### ContactSensor

```{eval-rst}
.. autoclass:: genesis.engine.sensors.ContactSensor
   :members:
   :undoc-members:
   :show-inheritance:
```

## See Also

- {doc}`index` - Sensor overview
- {doc}`/api_reference/entity/rigid_entity/index` - RigidEntity and links