ODrive/docs/endstops.md

# Endstops and Homing

By default, the ODrive assumes that your motor encoder's zero position is the same as your machine's zero position, but in real life this is rarely the case.  In these systems it is useful to allow your motor to move until a physical or electronic device orders the system to stop. That `endstop` can be used as a known reference point. Once the ODrive has hit that position it may then want to move to a final zero, or `home`, position.  The process of finding your machine's zero position is known as `homing`.

ODrive supports the use of its GPIO pins to connect to phyiscal limit switches or other sensors that can serve as endstops.  Before you can home your machine, you must be able to adequately control your motor in  `AXIS_STATE_CLOSED_LOOP_CONTROL`.

---

## Endstop Configuration
Each axis supports two endstops: `min_endstop` and `max_endstop`.  For each endstop, the following properties are accessible through `odrivetool`:

Name |  Type | Default
--- | -- | -- 
gpio_num | int | 0
offset | float | 0.0
debounce_ms | float | 50.0
enabled | boolean | false
is_active_high | boolean | false
pullup | boolean | true

### gpio_num
The GPIO pin number, according to the silkscreen labels on ODrive. Set with these commands:
```
<odrv>.<axis>.max_endstop.config.gpio_num = <1, 2, 3, 4, 5, 6, 7, 8>
<odrv>.<axis>.min_endstop.config.gpio_num = <1, 2, 3, 4, 5, 6, 7, 8>
```

### enabled
Enables/disables detection of the endstop.  If disabled, homing and e-stop cannot take place. Set with:
```
<odrv>.<axis>.max_endstop.config.enabled = <True, False>
<odrv>.<axis>.min_endstop.config.enabled = <True, False>
```

### offset
This is the position of the endstops on the relevant axis, in counts.  For example, if you want a position command of `0` to represent a position 100 counts away from the endstop, the offset would be `-100.0` (because the endstop is located at axis position `-100.0`).

```
<odrv>.<axis>.min_endstop.config.offset = <int>
```

This setting is only used for homing. Only the offset of the `min_endstop` is used.

### debounce_ms
The debouncing time for this endstop.  Most switches exhibit some sort of bounce, and this setting will help prevent the switch from triggering repeatedly. It works for both HIGH and LOW transitions, regardless of the setting of `is_active_high`. Debouncing is a good practice for digital inputs, read up on it [here](https://en.wikipedia.org/wiki/Switch). `debounce_ms` has units of miliseconds.

```
<odrv>.<axis>.max_endstop.config.debounce_ms = <Float>
<odrv>.<axis>.min_endstop.config.debounce_ms = <Float>
```

### is_active_high
This is how you configure the endstop to be either "NPN" or "PNP".  An "NPN" configuration would be `is_active_high = False` whereas a PNP configuration is `is_active_high = True`.  Refer to the following table for more information:

Typically configuration **1** or **3** is preferred when using mechanical switches as the most common failure mode leaves the switch open.

### pullup
Match the pullup value to the configuration.  If `true`, it enables the GPIO pullup resistor.  If `false`, it enables the GPIO pull*down* resistor.

![Endstop configuration](Endstop_configuration.png)  


### Example

If we want to configure a 3D printer-style (configuration 4) minimum endstop for homing on GPIO 5 and we want our motor to move away from the endstop about a quarter turn with a 8192 cpr encoder, we would set:

```
<odrv>.<axis>.min_endstop.config.gpio_num = 5
<odrv>.<axis>.min_endstop.config.is_active_high = False
<odrv>.<axis>.min_endstop.config.offset = -1.0*(8912/4)
<odrv>.<axis>.min_endstop.config.enabled = True
```

### Testing The Endstops
Once the endstops are configured you can test your endstops for correct functionality. Try activating your endstops and check the states of these variables through odrivetool:

```
<odrv>.<axis>.max_endstop.endstop_state
<odrv>.<axis>.min_endstop.endstop_state
```

A state of `True` means the switch is pressed.  A state of `False` means the switch is NOT pressed.  As simple as that. Give it a try. Click your switches, or put a magnet on your hall switch and see if the states change. 

After testing, don't forget to save and reboot:
```
<odrv>.save_configuration()
<odrv>.reboot()
```

---

## Homing Configuration
There is one additional configuration parameter specifically for the homing process:

Name |  Type | Default
--- | -- | -- 
homing_speed | float | 0.25f

`homing_speed` is the axis travel speed during homing, in [turns/second].  If you are using SPI based encoders and the axis is homing in the wrong direction, you can enter a negative value for the homing speed and a negative value for the minimum endstop offset. 


### Performing the Homing Sequence
Homing is possible once the ODrive has closed-loop control over the axis.  To trigger homing, we must enter `AXIS_STATE_HOMING`. This starts the homing sequence, which works as follows:

1. The axis switches to `INPUT_MODE_VEL_RAMP`
2. The axis ramps up to `homing_speed` in the direction of `min_endstop`
3. The axis presses the `min_endstop`
4. The axis switches to `INPUT_MODE_TRAP_TRAJ`
5. The axis moves to the home position in a controlled manner

It requires quite a few settings in addition to the endstop settings:

```
<odrv>.<axis>.controller.config.vel_ramp_rate
<odrv>.<axis>.trap_traj.config.vel_limit
<odrv>.<axis>.trap_traj.config.accel_limit
<odrv>.<axis>.trap_traj.config.decel_limit
```

We realize this is a little excessive and we will work towards minimizing the setup, but this works well for smooth and reliable behaviour for now.

### Homing at Startup
It is possible to configure the odrive to enter homing immediately after startup. To enable homing at startup, the following must be configured:

```
<odrv>.<axis>.config.startup_homing = True
```

## Additional endstop devices

In addition to phyiscal switches there are other options for wiring up your endstops - you will have to work out the details of connecting your device but here are some suggested approaches:

![endstop figure](endstop_figure.png)