load_cell: Load cell gram scale (#6729)

* Add gram scale features to load_cell
* Convert sensor counts to grams and make this available via unix socket and object status
* Basic GCodes for tearing and reading the load cell
* Guided Calibration
* Diagnostic gcode to check the health of the load cell
* Update load_cell Documentation
* Add API server load_cell/dump_force endpoint
* Update [load_cell] config with calibration fields
* Add G-Code commands for working with load cells
* Add status reference for load_cell objects

Signed-off-by: Gareth Farrington <gareth@waves.ky>

docs/Load_Cell.md

@@ -0,0 +1,122 @@
+# Load Cells
+
+This document describes Klipper's support for load cells. Basic load cell
+functionality can be used to read force data and to weigh things like filament.
+A calibrated force sensor is an important part of a load cell based probe.
+
+## Related Documentation
+
+* [load_cell Config Reference](Config_Reference.md#load_cell)
+* [load_cell G-Code Commands](G-Codes.md#load_cell)
+* [load_cell Status Reference](Status_Reference.md#load_cell)
+
+## Using `LOAD_CELL_DIAGNOSTIC`
+
+When you first connect a load cell its good practice to check for issues by
+running `LOAD_CELL_DIAGNOSTIC`. This tool collects 10 seconds of data from the
+load cell and resport statistics:
+
+```
+$ LOAD_CELL_DIAGNOSTIC
+// Collecting load cell data for 10 seconds...
+// Samples Collected: 3211
+// Measured samples per second: 332.0
+// Good samples: 3211, Saturated samples: 0, Unique values: 900
+// Sample range: [4.01% to 4.02%]
+// Sample range / sensor capacity: 0.00524%
+```
+
+Things you can check with this data:
+* The configured sample rate of the sensor should be close to the 'Measured
+samples per second' value. If it is not you may have a configuration or wiring
+issue.
+* 'Saturated samples' should be 0. If you have saturated samples it means the
+load sell is seeing more force than it can measure.
+* 'Unique values' should be a large percentage of the 'Samples
+Collected' value. If 'Unique values' is 1 it is very likely a wiring issue.
+* Tap or push on the sensor while `LOAD_CELL_DIAGNOSTIC` runs. If
+things are working correctly ths should increase the 'Sample range'.
+
+## Calibrating a Load Cell
+
+Load cells are calibrated using the `LOAD_CELL_CALIBRATE` command. This is an
+interactive calibration utility that walks you though a 3 step process:
+1. First use the `TARE` command to establish the zero force value. This is the
+`reference_tare_counts` config value.
+2. Next you apply a known load or force to the load cell and run the
+`CALIBRATE GRAMS=nnn` command. From this the `counts_per_gram` value is
+calculated. See [the next section](#applying-a-known-force-or-load) for some
+suggestions on how to do this.
+3. Finally, use the `ACCEPT` command to save the results.
+
+You can cancel the calibration process at any time with `ABORT`.
+
+### Applying a Known Force or Load
+
+The `CALIBRATE GRAMS=nnn` step can be accomplished in a number of ways. If your
+load cell is under a platform like a bed or filament holder it might be easiest
+to put a known mass on the platform. E.g. you could use a couple of 1KG filament
+spools.
+
+If your load cell is in the printer's toolhead a different approach is easier.
+Put a digital scale on the printers bed and gently lower the toolhead onto the
+scale (or raise the bed into the toolhead if your bed moves). You may be able to
+do this using the `FORCE_MOVE` command. But more likely you will have to
+manually moving the z axis with the motors off until the toolhead presses on the
+scale.
+
+A good calibration force would ideally be a large percentage of the load cell's
+rated capacity. E.g. if you have a 5Kg load cell you would ideally calibrate it
+with a 5kg mass. This might work well with under-bed sensors that have to
+support a lot of weight. For toolhead probes this may not be a load that your
+printer bed or toolhead can tolerate without damage. Do try to use at least 1Kg
+of force, most printers should tolerate this without issue.
+
+When calibrating make careful note of the values reported:
+```
+$ CALIBRATE GRAMS=555
+// Calibration value: -2.78% (-59803108), Counts/gram: 73039.78739,
+Total capacity: +/- 29.14Kg
+```
+The `Total capacity` should be close to the theoretical rating of the load cell
+based on the sensor's capacity. If it is much larger you could have used a
+higher gain setting in the sensor or a more sensitive load cell. This isn't as
+critical for 32bit and 24bit sensors but is much more critical for low bit width
+sensors.
+
+## Reading Force Data
+Force data can be read with a GCode command:
+
+```
+LOAD_CELL_READ
+// 10.6g (1.94%)
+```
+
+Data is also continuously read and can be consumed from the load_cell printer
+object in a macro:
+
+```
+{% set grams = printer.load_cell.force_g %}
+```
+
+This provides an average force over the last 1 second, similar to how
+temperature sensors work.
+
+## Taring a Load Cell
+Taring, sometimes called zeroing, sets the current weight reported by the
+load_cell to 0. This is useful for measuring relative to a known weight. e.g.
+when measuring a filament spool, using `LOAD_CELL_TARE` sets the weight to 0.
+Then as filament is printed the load_cell will report the weight of the
+filament used.
+
+```
+LOAD_CELL_TARE
+// Load cell tare value: 5.32% (445903)
+```
+
+The current tare value is reported in the printers status and can be read in
+a macro:
+
+```
+{% set tare_counts = printer.load_cell.tare_counts %}
+```