<p>Actually the sensor reads the bed correctly in terms of shape, not in actual reproducible distance.</p>
<p>Plotting the grids:</p>
<p><a href="https://i.sstatic.net/HoYurm.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/HoYurm.png" alt="enter image description here" /></a><a href="https://i.sstatic.net/o4H3Hm.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/o4H3Hm.png" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/TyZ2im.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/TyZ2im.png" alt="enter image description here" /></a><a href="https://i.sstatic.net/g2J4hm.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/g2J4hm.png" alt="enter image description here" /></a></p>
<p>Results in pretty much the same shapes, so, when correcting for the maximum displacement and plotting all results in a single graph gives:</p>
<p><a href="https://i.sstatic.net/3RTEe.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/3RTEe.png" alt="enter image description here" /></a></p>
<p>So the sensor does seem to produce reproducible bed geometry, but not the actual values.</p>
<p>Misinterpreting your initial question I assumed a default Anet sensor, this  official ROKO SN04-N sensor is reported to not be very accurate and has a maximum <a href="http://pliki.aksotronik.pl/sn04.pdf" rel="nofollow noreferrer">hysteresis</a> of 10 % error of the detecting distance, which is 10 % of 5 mm (so ± 0.5 mm). The overall maximum measurement is in grid 4 with a value of 0.949 mm and in grid 1 with a value of 0.124 mm;  0.949 mm - 0.124 mm = 0.825 mm is well within the ± 0.5 mm. This seems odd as the hysteresis should be in play on all measurements as the probe goes up and down. I've used this sensor myself, but replaced it for a more reliable sensor like the LJ18A3-8-Z/BX.</p>
<p>Now that it is clear that an accurate sensor is being used, there might be some play/backlash present in your printer. You should check the probe mount and the Z-axis.</p>

CC BY-SA 4.0
2020-11-16T14:35:25.277Z
Oh wow @0scar, how sweet of you to take this effort in the plotting! I agree with you that it seems to get the overal 'shape' of the bed. Note however that I am using the BLTouch, which seems pretty accurate seen from the `M48` results (I believe 0.0009 standard deviation).
I will see if I can do this test again, but then add a `G28` in between the results.. or maybe an `M500` command. I can imagine that the routine somehow accumulates all the measurements
zwep
CC BY-SA 4.0
2020-11-16T14:43:50.530Z
@zwep I read Anet A6 probe, now I understand that you mean printer Anet A6 and a BLTouch probe and BLTouch software! Sorry for the miscommunication, I'll alter the answer, but the main message will be the same, you need to check the bracket and the Z-axis for play or hysteresis/backlash.
0scar
CC BY-SA 4.0
2020-11-16T18:44:25.673Z
Hi, just did an additional test with `MULTIPLE_PROBING` set to 3 and this gave consistent results!
zwep
CC BY-SA 4.0
2020-11-16T12:27:36.407Z
2020-11-16T14:53:30.857Z
2020-11-16T14:53:30.857Z
0scar
<p>I want to check how stable the output of the bedleveling protocol <code>G29</code> was by running it multiple times. When running it four times, I got this as subsequent output</p>
<pre><code>Recv: Bilinear Leveling Grid:
Recv:       0      1      2
Recv:  0 -0.056 -0.475 -0.605
Recv:  1 +0.124 -0.349 -0.595
Recv:  2 -0.023 -0.344 -0.673
Recv:  

Recv:       0      1      2
Recv:  0 +0.218 -0.183 -0.325
Recv:  1 +0.399 -0.073 -0.328
Recv:  2 +0.251 -0.072 -0.399
Recv: 

Recv:       0      1      2
Recv:  0 +0.498 +0.090 -0.049
Recv:  1 +0.677 +0.211 -0.038
Recv:  2 +0.531 +0.217 -0.117

Recv: Bilinear Leveling Grid:
Recv:       0      1      2
Recv:  0 +0.772 +0.368 +0.222
Recv:  1 +0.949 +0.482 +0.231
Recv:  2 +0.806 +0.495 +0.154
</code></pre>
<p>Which are quite unstable, and strange results. It seems as if there is an increment of around +0.27/0.28 for each probe point in the next iteration. How can this happen? What are the right bed level settings right now?</p>
<ul>
<li>Printer: Anet A6</li>
<li>Probe: BLTouch</li>
<li>Software: Marlin 2.0</li>
<li>Bed level mode: Bilinear</li>
<li>Amount of probe points: 3x3</li>
<li>Probe surface: glass bed</li>
</ul>
<hr />
<p>For reference, here are the <a href="https://pastebin.com/tzwicptQ" rel="nofollow noreferrer">Configuration.h</a> and the <a href="https://pastebin.com/qpGVFD6j" rel="nofollow noreferrer">Configuration_adv.h</a>.</p>

<bed-leveling><bltouch><anet-a6>
Output of G29 bed leveling, Marlin 2.0
zwep
# Output of G29 bed leveling, Marlin 2.0

(Asked by: username_0 on 2020-11-13)

I want to check how stable the output of the bedleveling protocol `G29` was by running it multiple times. When running it four times, I got this as subsequent output

```
Recv: Bilinear Leveling Grid:
Recv:       0      1      2
Recv:  0 -0.056 -0.475 -0.605
Recv:  1 +0.124 -0.349 -0.595
Recv:  2 -0.023 -0.344 -0.673
Recv:  

Recv:       0      1      2
Recv:  0 +0.218 -0.183 -0.325
Recv:  1 +0.399 -0.073 -0.328
Recv:  2 +0.251 -0.072 -0.399
Recv: 

Recv:       0      1      2
Recv:  0 +0.498 +0.090 -0.049
Recv:  1 +0.677 +0.211 -0.038
Recv:  2 +0.531 +0.217 -0.117

Recv: Bilinear Leveling Grid:
Recv:       0      1      2
Recv:  0 +0.772 +0.368 +0.222
Recv:  1 +0.949 +0.482 +0.231
Recv:  2 +0.806 +0.495 +0.154

```

Which are quite unstable, and strange results. It seems as if there is an increment of around +0.27/0.28 for each probe point in the next iteration. How can this happen? What are the right bed level settings right now?

* Printer: Anet A6
* Probe: BLTouch
* Software: Marlin 2.0
* Bed level mode: Bilinear
* Amount of probe points: 3x3
* Probe surface: glass bed

---

For reference, here are the [Configuration.h](https://pastebin.com/tzwicptQ) and the [Configuration\_adv.h](https://pastebin.com/qpGVFD6j).

**Tags:** bed-leveling, bltouch, anet-a6


---

## 1 answers

### Answer by username_1 on 2020-11-16. Score: 1 | Accepted answer

Actually the sensor reads the bed correctly in terms of shape, not in actual reproducible distance.

Plotting the grids:

[![enter image description here](https://i.sstatic.net/HoYurm.png)](https://i.sstatic.net/HoYurm.png)[![enter image description here](https://i.sstatic.net/o4H3Hm.png)](https://i.sstatic.net/o4H3Hm.png)
[![enter image description here](https://i.sstatic.net/TyZ2im.png)](https://i.sstatic.net/TyZ2im.png)[![enter image description here](https://i.sstatic.net/g2J4hm.png)](https://i.sstatic.net/g2J4hm.png)

Results in pretty much the same shapes, so, when correcting for the maximum displacement and plotting all results in a single graph gives:

[![enter image description here](https://i.sstatic.net/3RTEe.png)](https://i.sstatic.net/3RTEe.png)

So the sensor does seem to produce reproducible bed geometry, but not the actual values.

Misinterpreting your initial question I assumed a default Anet sensor, this official ROKO SN04-N sensor is reported to not be very accurate and has a maximum [hysteresis](http://pliki.aksotronik.pl/sn04.pdf) of 10 % error of the detecting distance, which is 10 % of 5 mm (so ± 0.5 mm). The overall maximum measurement is in grid 4 with a value of 0.949 mm and in grid 1 with a value of 0.124 mm; 0.949 mm - 0.124 mm = 0.825 mm is well within the ± 0.5 mm. This seems odd as the hysteresis should be in play on all measurements as the probe goes up and down. I've used this sensor myself, but replaced it for a more reliable sensor like the LJ18A3-8-Z/BX.

Now that it is clear that an accurate sensor is being used, there might be some play/backlash present in your printer. You should check the probe mount and the Z-axis.

> **Comments:**
> **username_0**: Oh wow @username_1, how sweet of you to take this effort in the plotting! I agree with you that it seems to get the overal 'shape' of the bed. Note however that I am using the BLTouch, which seems pretty accurate seen from the `M48` results (I believe 0.0009 standard deviation). I will see if I can do this test again, but then add a `G28` in between the results.. or maybe an `M500` command. I can imagine that the routine somehow accumulates all the measurements  
> **username_1**: @username_0 I read Anet A6 probe, now I understand that you mean printer Anet A6 and a BLTouch probe and BLTouch software! Sorry for the miscommunication, I'll alter the answer, but the main message will be the same, you need to check the bracket and the Z-axis for play or hysteresis/backlash.  
> **username_0**: Hi, just did an additional test with `MULTIPLE_PROBING` set to 3 and this gave consistent results!  

---

