(md_torque_sensor)=

# External Torque Sensor

By default, MD drivers estimate torque from motor phase current (see [Measurements](measurements)).
This works well for direct-drive and low-gear-ratio actuators, but on actuators with high gear ratios
or that are not easily back-drivable, the phase-current estimate becomes less accurate. For these
applications, MD supports connecting an **external in-line torque sensor**, which provides a direct
torque measurement independent of motor current and gearbox friction.

## Supported sensors

| Sensor | Interface | Notes |
| ------ | --------- | ----- |
| XJC X-ST319-75Nm-F | RS-485 | In-line torque sensor, polled internally at 500 Hz |

```{Note}
Custom sensor support.

If you would like to use a different torque sensor that is not currently supported, we are open to
introducing it to the MD Ecosystem. Please contact us at: `contact@mabrobotics.pl`.
```

## Connection

The external torque sensor is connected to the same RS-422 transceiver used by
[RLS RS-422 aux encoders](aux_encoders), via the **AUX2 (10-pin)** connector.

```{warning}
The torque sensor and an RS-422 aux encoder (RLS AksIM2 / Orbis) **share the same physical bus** and
cannot be used at the same time. Only one RS-422 device may be connected to AUX2 at once.
```

```{warning}
**RS-422 is an on-demand feature** of MD drivers, and by default it is not available on the board.
Contact MAB Robotics support at `support@mabrobotics.pl`, for more information.
```

```{note}
Due to the onboard transceiver, RS-485 is electrically emulated over the RS-422 lines by shorting
TX+ to RX+ and TX- to RX- on the cable side. Cables supplied by MAB Robotics, adjust for that. 
Refer to the sensor's wiring documentation when building a custom cable.
```

```{figure} ./images/torque_sensor_connector.jpg
:class: bg-primary mb-1
:align: center
:class: no-scaled-link
```

## Configuration

The sensor type is selected with the `torqueSensor` register:

| Value | Sensor |
| ----- | ------ |
| `0` | `NONE` (disabled, default) |
| `1` | `XJCSENSOR` |

```{important}
The sensor is only initialized at driver startup. After changing `torqueSensor`, save the
configuration (`runSaveCmd`) and reset the driver for the change to take effect.
```

If the sensor does not respond during initialization (e.g. not connected, or wired incorrectly), it
is simply marked inactive - no warning or error is raised, and `torqueSensorData` will read back `0`.
Verify wiring and the selected sensor type if the readout stays at zero.

## Zeroing

Before use, the sensor should be zeroed (tared) with no load applied, similarly to
[zeroing an encoder](candletool_commands). Running `runTorqueSensorZero` averages 100 samples from
the sensor and stores the result as an offset, which is subtracted from all subsequent readings. As
with other configuration values, save (`runSaveCmd`) to persist the offset across power cycles.

## Reading torque

Once configured and zeroed, the measured torque is available in the `torqueSensorData` register
(float32, Nm), updated at 500 Hz and smoothed with a low-pass filter.