Sensors used in 3d printers

Automatic table calibration is a fairly common feature in desktop FDM 3D printers. However, most 3D printers with automatic table calibration use a non-contact inductive sensor, which is easy to install, operate and configure. These inductive sensors work well with metal 3D printer tables, but tables made from other common materials such as glass are virtually invisible to inductive sensors.

Perhaps it is because of this lack of inductive sensors that many people hear about BLTouch sensors: if you need to use a non-metallic 3D printer slot base but need automatic calibration, these sensors are a great option. Another plus of these sensors is its high accuracy. BLTouch raw readings have one of the lowest standard deviations of any sensor type for automating 3D printer bed calibration.

In this article we will look at the BLTouch sensor, its advantages and how to use it for a 3D printer (by the way, there are many clones of BLTouch sensors, but the original design belongs to Antclabs from South Korea).

What is a BLTouch sensor?

According to Antclabs, the original manufacturer of the sensor, "BLTouch is an automatic table calibration sensor for 3D printers that can accurately measure the angle of the table surface. " This sensor works on any type of surface, be it metal, glass, wood and others.

The 3D printer table tilt level sensor itself has a rather complex design. It consists of a microcontroller, a solenoid switch and a handpiece that is in direct contact with the table. The original BLTouch uses a Hall sensor for high accuracy, and this sensor, combined with the physical handpiece, allows it to be used with many types of tables.

In a sense, this sensor is equivalent to a microswitch mounted on a servo. When the tool head is lowered to "return" the nozzle in the Z-axis, the table pushes the tip slightly up, the Hall sensor is activated, and then the tool head rises.

BLTouch is one of the most accurate and reliable sensors available, so it's no wonder some manufacturers like MakerGear and CraftBot use this device on their high end 3D printers.

Now that we know exactly what we're dealing with, let's compare the BLTouch with the other two types of sensors.

Different types of sensors for automatic 3D printer table calibration

Inductive sensor

The key difference between inductive sensor and BLTouch is that the BLTouch sensor physically touches the table during the calibration process. The inductive sensor uses currents induced by magnetic fields to detect nearby metal objects, so there is no direct physical contact with the table.

But therein lies the problem: induction sensors only work with metal tables, as they only work when they come into contact with metal. So if you are using glass, then this type of sensor will not work for you.

Microswitch (mechanical limit switch)

Don't discount the old-fashioned physical microswitch either. Compared to the BLTouch, this sensor is less accurate and possibly less reliable in the long run because the measurement is dependent on physical parts that can wear out over time.

However, these mechanical switches outperform other options when it comes to cost and ease of setup. Since mechanical switches are very simple to implement, easy to set up and cheap, they have found quite a wide application.

Alternatives

Pinda Probe

Pina Probe is a sensor developed by Prusa Research which is an induction sensor with a thermistor to account for changes in bed temperature. The Pinda probe is specially designed for the Prusa line of 3D printers, which have special dot-mounted calibration markers on the table that help with alignment and skew correction.

This is said to be a very accurate sensor for 3D printers, and although there aren't many numbers on the internet, the reviews seem to be positive. So it is worth considering that this is a good option if your 3D printer has a metal table.

Ezabl Pro

Ezabl Pro is a capacitive sensor manufactured. It comes with a connection board that uses an optical isolator to prevent high voltage damage to the motherboard in case of connection errors.

In terms of accuracy, it can measure to one thousandth of a millimeter, which is what 3D printers need. Ezabl Pro also has useful features such as double shielding that prevents any interference from other signals.

The main disadvantage of Ezabl Pro is the cost, which is about 65 dollars. But this sensor can work with glass tables.

Piezo Sensors

Piezo Sensor uses the piezoelectric effect to detect changes in force, pressure or strain and convert those changes into an electrical impulse. Precision Piezo is one of the few companies in the UK that makes piezo sensors for 3D printers. They have an accuracy close to 7 microns, which is pretty accurate for a 3D printer.

The advantage of the piezoelectric sensor is that a nozzle can be used as a sensing element without the need for additional components. In addition, a piezoelectric sensor can be placed on the print head or under the pressure platform. As for the price, it is in the same range as BLTouch.

BLTouch Sensor Setup Basics

Before you start modifying your 3D printer for BLTouch, you need to work through the following points.

Mounting the sensor

The BLTouch sensor must be installed as close to the printhead as possible. You can find many designs available for download on the internet. If you would like to design the mounting structure yourself, be sure to read the BLTouch documentation to take into account the overall dimensions of the sensor.

Some users have experienced a problem where the nozzle hits the table on one side and prints perfectly on the other. The reason for this is that the BLTouch is mounted at an angle to the nozzle. Therefore, when designing and installing equipment, make sure that the BLTouch is mounted perfectly at right angles and aligned with the nozzle.

After installing the sensor, be sure to record the distance between the BLTouch sensor and the nozzle center in both the X and Y directions.

3D Printer Firmware Configuration

To run BLTouch, you will need to change the 3D printer firmware in several places.

We used Marlin firmware version 1. 1.9 which was updated in March 2020. Enter the marlin.ino file and go to the configuration.h tab.

The first step is to activate the BLTouch sensor by declaring it. Remove the two slashes to uncomment the define statement:

 #define BLTOUCH 

The next step is to set the BLTouch offset using the X and Y distances you should have measured when you installed the sensor. Z-shifting may seem like a more complicated procedure, as the BLTouch probe protrudes below the nozzle for direct measurements. You will need to find the distance between the sensor and the end of the nozzle when the sensor is fully extended.

Also, since the probe goes below the nozzle, the offset must be negative. We recommend setting it to around -2.5 for safe first runs and then adjusting. You need to change the values ​​of the following lines to match your numbers:

 #define X_PROBE_OFFSET_FROM_EXTRUDER 30 // X offset: -left + right [nozzles] #define Y_PROBE_OFFSET_FROM_EXTRUDER 20 // Y offset: -front + back #define Z_PROBE_OFFSET_FROM_EXTRUDER -2. 5 // Offset Z: -below + above 

After you're done with the offset, you'll need to choose a table alignment type. You can choose from five different compensation methods, below we will give an example using the bilinear method as it is easy to get started with.

To select the appropriate option, uncomment it by removing the slash. Please note that only one option can be left without comment. Your code should look something like this:

 //#define AUTO_BED_LEVELING_3POINT //#define AUTO_BED_LEVELING_LINEAR #define AUTO_BED_LEVELING_BILINEAR //#define AUTO_BED_LEVELING_UBL //#define MESH_BED_LEVELING 

You can also set the number of mesh points that you will base your calibration on. Default is 9points that form a 3-by-3 grid along the x and y axes. The number of points can be increased for more accurate results, but keep in mind that the probing time will also increase proportionally.

 #if ENABLED (AUTO_BED_LEVELING_LINEAR) || ENABLED (AUTO_BED_LEVELING_BILINEAR) 

 // Set the number of grid points per size. 

 #define GRID_MAX_POINTS_X 3 #define GRID_MAX_POINTS_Y GRID_MAX_POINTS_X 

Testing

After setting up the firmware, you will need to check if the BLTouch is working properly by doing a basic test. Once it's complete, you can start adjusting the Z offset. There's a great video from 3DMakerNoob that walks you through the process step by step.

Slicer setup

Be sure to add the G29 command to your Gcode right after the G28 command.

Troubleshooting

Troubleshooting

The creators of the original BlTouch at Antclabs mentioned that the original sensors come with a QR code built into the circuit board on the back. Other Chinese manufacturers, called TL Touch and 3D Touch, are also known to produce lower quality products, resulting in poorer quality. When buying online, be sure to purchase from one of the authorized resellers that are listed on the Antclabs website.

Known Issues with BLTouch Sensor

The latest version of BLTouch V3 had issues with Creality printers where it did not work properly. The Marlin firmware developers worked closely with Antclabs and determined that certain changes needed to be made to the firmware.

An excellent tutorial video on how to fix this problem was made by Teaching Tech. After making changes, many users finally solved the problem.

Disadvantages of BLTouch

Although the BLTouch is an extremely accurate level sensor, it also has several disadvantages.

When using the BLTouch, it is important that there is no debris on the surface of the bed. A mechanical sensor detects the surface of the table and any damage or debris on the surface will result in inaccurate readings.

The BLTouch is also known to suffer from interference from currents in the hot end heater wires. This interference causes incorrect BLTouch readings. The Marlin firmware has a special line of code that will help you turn off the heaters while the sensor is triggered. To activate it, find and uncomment the following:

 #define PROBING_HEATERS_OFF 

Passive Wi-Fi sensors printed on a 3D printer

A method has been developed to create passive sensors using 3D printing, they transmit data by reflecting the Wi-Fi signal, and do not require power supply themselves. In this way, you can create rotation sensors, buttons for controlling equipment and other devices, according to the work of American engineers presented at the SIGGRAPH Asia 2017 conference.

Almost all sensors use electricity to operate. Some of them are directly connected to the mains, while others work autonomously from the battery. But in both cases, such devices depend on electricity, so researchers are developing passive sensors. There are already widely used devices powered by radio waves emitted by a transmitter, such as RFID and NFC tags. But usually they require that the source of radio waves be within a few tens of centimeters, as well as the use of special equipment.

Engineers led by Shyamnath Gollakota of the University of Washington have developed 3D printed mechanical sensors that can transmit data using external radio sources and do not require power themselves. To do this, they used two types of filaments for the printer: regular plastic for most of the device, and plastic with the addition of copper particles to print antennas.

The sensor consists of a coil spring, a wheel that moves the end of the spring when rotated, and an antenna. The antenna and end of the spring are made of conductive plastic that reflects radio waves. When the end of the spring touches the antenna, its impedance changes, causing the reflected radio waves to change in amplitude. The engineers decided to use this property with the help of two Wi-Fi devices - a transmitter and a receiver. When the sensor is triggered (the wheel rotates), the receiver registers a change in the signal reflected by the sensor.

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