The present invention relates to the field of three dimensional printing used to print 3d model from a digital file, more specifically it relates to the leveling of the print bed used in fused filament fabrication based printers.
BACKGROUND OF THE INVENTION
Additive manufacturing or 3d printing is the technology to make 3d model by continuously adding one layer over the other until completion of part. 3d printing has seen a huge improvement over the last few years, it enables industries to test and validate their concepts as early as possible. It also enables to make the end use products. The most advantageous part of this technology is the on-demand customization in almost every field. Printers based on extrusion based technology like Fused Filament Fabrication (FFF) technique mainly comprises toolhead assembly, build platform and supporting frame. Build platform comprises print bed and a support frame wherein the print bed is the place where 3d model gets printed. It provides the complete flat surface to the bottom layer of 3d model which helps to stick the first layer of filament with the surface of the print bed. To get a perfect printed 3d model, there must be a proper and uniform distance between the nozzle and the print bed or lower surface. Uneven build surface will result in poor printing of the model which in turn results in not only the loss of material but also wastage of energy, manpower and as a result, the system becomes less productive as well as less efficient.

Patent No. US9233507B2 describes the embodiment which uses the photosensor and a software algorithm to level the build platform. However this technique is not very useful in surfaces beyond a certain size.
Despite using different mechanisms to level the print bed in the prior art, getting the same precision every time the print starts, is not an easy task.
To get the consistent precision, the present invention has employs a system to level the print bed before the start of any print, to increase the efficiency of the system and minimizing the losses.
SUMMARY OF INVENTION
The example embodiment herein provides a system to detect and correct the bed misalignment before printing. The present invention comprises of a print bed, support platform, toolhead assembly, sensor, supporting frame, ball screws, stepper motors and computing system.
In the example embodiment, print bed is placed on the support platform which can be leveled manually with the help of four levelling screws present beneath the print bed on the support platform.
Toolhead assembly comprising a sensor, the load cell in the preferred embodiment to sense the contact of nozzle with print bed. Nozzle touches the surface of the build platform at various predefined points and sensor sends the respective signals to the computing system.
Computing system analyses the data and gives instructions to the operator/personnel for the coarse leveling of print bed surface if the distance between nozzle and print bed surface is greater than a certain value. Otherwise the misalignment present in the print bed surface is automatically compensated by material extrusion process.
To perform the material extrusion process, computing system makes a map of contour of the surface of print bed and commands the toolhead to extrude filament material to correct the misalignment present between the nozzle and the print bed after defining the area on the print bed surface . The amount of material extrusion is calculated through the use of an algorithm.

DESCRIPTION OF DRAWINGS
Fig l One example embodiment showing the bed leveling mechanism
1 - Load cell: Transducer used to measure the force exerted by nozzle in upward direction (during contact with the build platform). It uses the strain gauge for measurement and gives output to the control system
2 - Nozzle
3 - Build platform: Platform where 3d parts are printed. In the present embodiment, glass as build platform is preferred

Fig 2 One example embodiment showing probe points on the print bed surface
(top view)
1 - Probe points
2 - Print bed
Fig 2 shows various probe points on the print bed surface to detect misalignment. Nozzle touches the print bed surface at these points to measure its distance from the surface.

Fig 3 One example representation showing the deposition of first layer of filament material
on print bed surface (top view)
1 - Print bed
2 - Deposition of first layer of filament material
Fig 3 represents the deposition of first year of filament material on bed surface. Deposition of filament material is in accordance with the contour mapped by the computing system. The computing system calculates the amount of filament material needed to deposit for the compensation of present misalignment

Fig 4 One example representation showing the deposition of second layer of filament
material on print bed surface (top view)
1 - Print bed
2 - Deposition of second layer of filament material
Fig 4 represents the deposition of second layer of filament material on print bed surface as instructed by the computing system to the toolhead.

Fig 5 One example representation showing the deposition of third layer on print bed surface
(top view)
1 - Unwanted space (No deposition of filament material)
2 - Deposition of third layer of filament material
Fig 5 represents the deposition of the third layer of filament material on print bed. It leaves some space (part l) over the bed surface since it is not required to print 3d model which helps to minimize the reduction of waste generation.

Fig 6 One example representation showing the print bed surface after complete alignment
of print bed surface
1 - First filament layer
2 - Second filament layer
3 - Third filament layer
4 - Print bed
Fig 6 represents the leveled bed after the deposition of all three layers of filament material. All the three layers are deposited with the same filament material which is not restricted to the main model material.

DETAILED DESCRIPTION
The terminology used herein is used for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well as the singular forms unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising”, when used in this specification, specify the presence of stated features, steps, operations, elements, and/or components, but do not include the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one having ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure and will not be in today in an idealized or overly formal sense unless expressly so defined herein.
The present document discloses a system to level the print bed before the start of any print on printing apparatus. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present system. It will be evident however, to one skilled in the art that the present system may be practised without the specific details. The present disclosure is to be considered as an example application of the invention and is not intended to limit the invention to the specific embodiments illustrated by the figures or description below.
The present system performs the following functions to ensure the perfect leveling of the print bed.
Before start of print, print bed moves toward the nozzle at constant velocity for software assisted coarse leveling. Coarse leveling in the preferred embodiment is a process of leveling the print bed in a manual way as instructed by software using the screws present beneath the print bed. Nozzle touches the print bed at predefined points to find out its distance from print bed. Contact of nozzle with print bed is sensed by sensor and sent it to the computing system which instructs the operator/personnel to level it manually. This process is performed only if the distance between nozzle and print bed is greater than certain value since it is not reasonable to compensate the misalignment through material extrusion.
Once the coarse leveling is done, nozzle (fig 2, part 3) touches the print bed surface (fig 2, part 1) at probe points (fig 2, part 2) which may not be the same as predefined points (used

for coarse leveling) before, to calculate its respective distance from print bed. Highest point of print bed surface i.e. closest point to the nozzle is assumed to be at zero point level and distance of all other points are to be calculated with respect to it. These distances are used to map contour of print bed surface using an algorithm. This contour is used to identify the misalignment present in print bed surface (less than certain value) and to define the area for the material extrusion that can be controlled as per the requirement of the 3d model. Selection of area is necessary, as there is no requirement to align the complete print bed surface unless 3d model occupies the whole print bed which ultimately helps in reducing the generation of waste.
The computing system i.e. brain of the printer analyses the contour and instructs toolhead to deposit the material filament for the compensation of misalignment. The toolhead, in turn deposits the filament material on the print bed layer by layer, wherever necessary.
Toolhead deposits three layers of plastic on the bed surface to compensate for the misalignment in the given example as represented in fig 3, 4, 5.
In another example embodiment, if during coarse bed leveling check, distance between the nozzle and print bed is less than certain value then the system directs the toolhead to perform the material extrusion leveling process without performing coarse bed leveling process. It involves the contact of nozzle at various probe points (fig 2, part 2), sensed by the sensor and the respective signals sent to the computing system.
The computing system analyses the data and maps the contour of print bed surface which enables it to identify the misalignment present in the print bed surface and selects the area of print bed for the material extrusion. Selection of area drives the process to its next step of instructing the toolhead to deposit the filament material on print bed surface.
In another example embodiment, if coarse bed leveling check failed, the system will not perform the further action and notifies the operator/personnel about the same. This avoids the wastage of material, energy, time and enables the operator/personnel to perform corrective action and restart the print.

References
1. Choi, Doo-won, 2014, Level aligning device of bed of 3d printer, KR101564554B1
2. Kim, Byung - Chang, Ryu, Myung - Hwan, Kim, Jae - Eun, Choi, Jin - Young, Ahn, Ki -Young, 2014, 3D printer with auto leveling system, KR101677452B1
3. Luo Xiaolong, Guo Jian, 2018, A kind of 3D printer automatic leveling platform, CN208682122U
4. Erik van der Zalm, Ultimaker B.V., 2018, Print bed leveling system and method for additive manufacturing, US20170057173
5. Chen Dingfang, Chen Dong, Tao Menglun, Shen Jupeng, Shi Dan, Chacheng, 2015, 3D printer work platform levelness monitoring system, CN205112416U
6. Wen Xin, 2017, Full auto leveling heat bed that 3D printer was used, CN207028178U

We claim
1. A method of leveling the print bed comprises a nozzle head assembly, sensor, stepper
motors, ball screw, print bed, support platform, computing system and the filament
material. The method comprising the steps of:
a. Moving the print bed and nozzle head assembly toward each other until nozzle
touches the print bed surface
b. Contact of nozzle with the print bed surface at various predefined points is sensed
by the sensor and the respective signals sent to the computing system
c. Computing system analyses the data and if the distance between the nozzle and
print bed surface is greater than a certain value then it instructs the
operator/personnel to level the bed manually or if the distance between the
nozzle and print bed surface is less than a certain value then the closest point to
the nozzle present on the build surface is assumed as zero level point and
distance of all other points are calculated with respect to it
d. Measurement of distance of different points with respect to zero level point
enables the computing system to identify the level of misalignment present in
print bed surface and defines the area on the print bed surface for material
deposition
e. Commands given to the toolhead to deposit the filament material on the selected
area of print bed surface for compensation of misalignment
2. Print bed according to claim 1, is the glass in the preferred embodiment, placed above the manually operated screws on the build platform of the printing apparatus.
3. Sensor according to claim 1, is the strain gauge based load cell in the preferred embodiment is mounted on the toolhead of the printing apparatus. It detects the contact of the nozzle with the surface of the print bed as shown in fig 1 and sends the respective signal to the computing system.
4. If the coarse bed leveling process fails, the computing system will not perform further process and notifies the operator/personnel about the same