Enhance students’ engineering ability based on virtual three coordinate design and measurement

: The three coordinate measuring machine (CMM) are increasingly widely used, but because of their high price, not easy to portable and other drawbacks, it is difficult to be widely used in university teaching procedures. Therefore, some disciplines such as measurement and control theories, measurement methods behind of the CMM are hard to well understood, which require students to have strong engineering ability. In order to solve the problem, the paper presents a virtual CMM way to improve the students engineering experience that improve the quality of course teaching as an assist tool. Firstly, use 3ds Max to model CMM, including probe, guide rail, frame, etc. Secondly,after the modeling development is completed, Hololens is used as a platform to develop scene interaction functions using Unity and c# scripts, that is, the connection between virtual scenes and students.Thirdly, in the virtual interface, students control the probe by the button to record and measure the coordinates of the object to be measured, and complete the measurement experiment. Students can master the measuring principle and measuring process of CMM faster through Hololens. So that students have a more intuitive understanding of CMM and are no longer limited by the traditional CMM teaching, it is of great significance to the cultivation of students' engineering ability.


INTRODUCTION
The development of the three coordinate measuring machine (CMM) has taken more than half a century.Its emergence is a historical necessity of industrialization.On the one hand, due to the development of high-efficiency processing such as automatic machine tools and CNC machine tools, and the processing of more and more complex shape parts, fast and reliable measuring equipment is required to match them; On the other hand, due to the development of electronic technology, computer technology, numerical control technology and precision processing technology, it provides a technical foundation [1].The advent of three coordinate measuring machines(CMM) made it possible to transition measuring instruments from manual to modern automatic measurement.However, the traditional CMM is expensive, due to financial problems can not be purchased in large quantities, and the equipment is inconvenient to carry, and the requirements for the training site environment are high, resulting in the lack of practical training operation in teaching, and cannot master the principle and use of the three coordinate measuring machine [2].
In order to improve this situation,we can use virtual reality technology to build a virtual CMM, and effectively combine it with the actual physical environment, to achieve the use of virtual CMM to complete the acquisition of the relevant size information of objects, and obtain the required parameter information through relevant data processing.
Virtual reality technology combines the virtual world with the real world, provides a richer presentation mode for education, and enhances the interaction and participation of learning [3,4].It makes knowledge and information more accessible and attractive to learners, and is becoming a new hotspot in the application of information technology in education.In addition to virtual reality technology, there are also technologies that combine the real world with virtual information, such as augmented reality technology, spatial mapping technology, and somatosensory interaction technology.
One of the characteristics of augmented reality technology is to blend virtual objects with the real world, and blur the boundary between the virtual world and the real world as much as possible, so as to improve the realism of virtual scenes [14].To achieve this goal, virtual objects need to be accurately displayed and positioned in the real world.This process is also known as the registration technique.During this period, by calculating the pose information of the camera or other types of sensors, the user's current position, line of sight and other information can be detected, so as to establish the relationship between the coordinate system of the virtual world and the coordinate system of the real world, and the virtual objects can be correctly displayed in the real world [15,18].The so-called tracking refers to the reconstruction of the spatial coordinate system according to the user's current position and attitude.The relationship between the two algorithms is that the tracking must depend on the registration technology, and the accuracy and performance of the two algorithms will eventually be reflected in the actual use effect [16,17].
This paper aims to introduce the development and application of a CMM simulation platform based on wearable devices and its application in teaching practice.The main function of the simulation platform is geometric measurement, which is used for practical teaching in universities.With VR integrated equipment as the carrier, students can enter the virtual CMM system anytime, anywhere.Students can operate the virtual CMM system through gestures and movements, measure parts or other objects, and learn about measurement.

THE USE OF VIRTUAL REALITY T ECHNOLOGY IN TEACHING
Nowadays, virtual reality technology has become a new educational means to promote the development of education.Traditional education is just blindly instilling knowledge in students, and now the use of virtual reality technology can help students create a vivid, realistic learning environment, so that students through real feelings to enhance memory, compared to passive indoctrination, the use of virtual reality technology for independent learning is easier for students to accept, this way is easier to stimulate students' interest in learning.In addition, universities have used virtual reality technology to establish virtual labs related to disciplines to help students learn better [20] .
There are many examples of combining virtual reality technology with teaching, and studies have found that virtual interaction can produce positive learning effects [5,19], especially when learning conceptual knowledge [3,6].Unity3D and VR technology can be combined, and the somatosensory technology can be applied to the teaching system to make the interaction of the teaching system more natural [7].The integration of virtual reality technology into the laboratory improves the efficiency of education and reduces the cost [8,9].In some equipment training can also be combined with virtual reality technology, and virtual training has considerable advantages in terms of conditions, costs and so on [10].In medicine, virtual reality technology has been used to a certain extent [11].HoloLens can also be used to assist staff in minimally invasive surgery training, which helps to improve efficiency [12].This technique also has great advantages in anatomical training [13].
In order to allow students to have a more efficient learning environment and create a foundation for research, this paper designs a virtual CMM based on HoloLens.The purpose of virtual CMM is to familiarize students with the working principle of CMM, and to show the composition and working process of CMM, improve students' interest in learning,and deepen their understanding of CMM professional knowledge.Next, the design and implementation of the virtual CMM will be explained.

3.DESIGN OF VIRTUAL COORDINATE MEASURING MACHINE
The idea of the whole project design is roughly divided into four parts: environment construction, modeling and development, interactive development, testing and release.In the environment construction stage, it is divided into the overall scheme design, software tool selection and development environment configuration, including the selection of a good development platform and development engine, and the collection of corresponding technical documents.The main development platform is Microsoft's HoloLens.The whole design flow chart is shown in Figure 1.When modeling the measuring machine, it can be divided into seven parts: probe, head shaft, guide rail, table, beam, pillar, carriage, and then drawn separately in the software, and finally assembled.Most of the parts of the measuring machine are cuboid structure, so most of the part models can be initially built to draw the cuboid, and then cut and supplement, stretch and other operations on the cuboid to obtain specific parts.Finally, the parts of each part are assembled to obtain a complete measuring machine model.
The color of each part of the preliminarily constructed model is different, and we modify it according to the actual material and color of each part of the measuring machine.For example, the table is usually made of granite, the probe and beam are made of ceramic, and the final 3D model can be obtained by rendering.
After the modeling development is complete, the interactive features can be developed.Using Unity combined with C# script, we can develop the function of scene interaction, which is the link between virtual scene and user, and coordinate the work and operation of the whole system.For a virtual CMM, each component should strive to be accurately restored.When the user makes certain instructions, the corresponding components of the virtual CMM move to simulate the measurement process, and the measurement results are displayed on the screen in real time to prompt the user for the next step.
When the X-axis of the virtual CMM moves, the sliding frame moves on the X-guide rail, and the probe and probe axis also move along the X-axis with the sliding frame, while the other parts of the CMM do not change.When the CMM moves on the Y axis, different from the X axis, the sliding frame, probe, probe axis and beam all move along the Y axis in addition to the column on both sides.When the Z-axis moves, only the probe and the probe axis need to be considered to move up and down the Z-axis.The scenario diagram is shown in Figure 2. The user interface design is mainly carried out by using Unity3D software.The 3D model of virtual CMM and Microsoft Mixed Reality Toolkit (MRTK) were imported into Unity3D software.Currently in version 2.6.1,MRTK is a set of components and features designed to accelerate the development of cross-platform mixed reality applications in Unity.It provides a cross-platform input system and modules for spatial interaction and UI, and provides functions such as hand tracking (such as solver), eye tracking (such as multi-scene manager, boundary system), UI controls (such as buttons, sliders, hand menu prompt bar, etc.), and spatial understanding.Use the button to control the recording and measurement of the probe coordinates, and every time the button is pressed, the probe position coordinates are imported into the system, and the data is saved.
In the whole design process of virtual CMM, it is necessary to have certain programming ability and be able to skillfully use modeling software.If you want to further improve the training of students' engineering ability, you can let students participate in the design of virtual coordinate measuring machine.Students independently learn programming development and modeling software, learn the types of CMM and their respective advantages and disadvantages by consulting materials, and choose the CMM they are interested in for modeling, and realize the final virtual CMM through programming.

4.VIRTUAL CMM MEASURING EXPERI MENTS
When students wear Hololens, the first thing they see is the virtual environment around them.In the environment, there is a coordinate measuring machine user interface, and there is a hand shape that plays the role of a mouse in the interface, which can be operated by dragging it with the hand.Place the hand on the coordinate measuring machine, press and hold the hand to move, the coordinate measuring machine will follow the direction of the hand, so that the coordinate measuring machine can be moved to the specified position for measurement.As shown in Figure 3.

Figure 3 measurement demonstration
The interface of CMM can choose to measure the length of parts or the radius of parts.This paper takes measuring the length of parts as an example to carry out experiments.Take the bench and metal parts as the measured objects, place the bench on a flat table or ground, drag the CMM to the top of the bench, adjust the position of the probe, touch the probe to an Angle of the chair, and then press the button, the system will record the position coordinates of the current probe.Then move the side head to the position of the other corner and press the button to record the current position of the probe.Press the button again to get the distance between the two coordinates, which is the length of the side of the chair.The steps and results for measuring the length of the side of the chair are shown in Figure 4, Figure 5and Figure 6.
Students should note that during the measurement process, the probe is gently touched by hand with the object to be measured.If the probe does not touch the object to be measured, the measured coordinates are invalid points.If the probe is too deep, the measured coordinates are inaccurate.Similarly, in real life, when the CMM is used for industrial measurement, the CMM needs to contact the measured object with the probe when measuring, and the information obtained without contact is invalid, but too deep contact will lead to surface damage of the probe and the measured object.Virtual CMM measurement is similar to industrial CMM measurement, and students will have a deeper understanding of the principle of CMM through virtual measurement experiments.After that, measure the side length of a metal workpiece.The measurement steps and results are shown in Figure 7, Figure 8, Figure 9 and Figure 10.
Relative error of metal workpiece measurement results is shown in Formula 2: We did a lot of follow-up measurements and found that the relative error was about 1.15%.The feasibility of this scheme is confirmed.

DISCUSSION AND CONCLUSION
At present, the simulation of measurement and teaching is mostly done by writing relevant software on traditional computers, thus the students' operating ability is not well cultivated, and is not intuitive in some specific aspects.In order to solve these problems, this paper proposes a virtual coordinate measuring machine method combined with HoloLens, and designs a human-computer interaction system using augmented reality or mixed reality devices by using augmented reality and spatial mapping technology.This paper combines virtual CMM with HoloLens, achieved a relatively free analog measurement, the user can need according to their own ideas and placed in a real scenario the virtual position of the three coordinate measuring machine, and then through the gestures and movements or eyes to manipulate the virtual three coordinate measuring machine, and according to the actual demand of the measured object.Measurement results are displayed in real time, and users can save and query these data.In the measurement of chair and iron parts, it can be seen that the accuracy of the virtual CMM still has room for improvement.In the subsequent design, it can be improved from the perspective of virtual image and physical object.

Figure1:
Figure1:Design process Model the measuring machine with 3ds Max.The views of 3ds Max are distributed as follows: top left view, top right front view, bottom left view left view, and bottom right perspective view.When you need to edit the model in different locations, you can work on the appropriate view.When modeling the measuring machine, it can be divided into seven parts: probe, head shaft, guide rail, table, beam, pillar, carriage, and then drawn separately in the software, and finally assembled.Most of the parts of the measuring machine are cuboid structure, so most of the part models can be initially built to draw the cuboid, and then cut and supplement, stretch and other operations on the cuboid to obtain specific parts.Finally, the parts of each part are assembled to obtain a complete measuring machine model.The color of each part of the preliminarily constructed model is different, and we modify it according to the actual material and color of each part of the measuring machine.For example, the table is usually made of granite, the probe and beam are made of ceramic, and the final 3D model can be obtained by rendering.After the modeling development is complete, the interactive features can be developed.Using Unity combined with C# script, we can develop the function of scene interaction, which is the link between virtual scene and user, and coordinate the work and operation of the whole system.For a virtual CMM, each component should strive to be accurately restored.When the user makes certain instructions, the corresponding components of the virtual CMM move to simulate the measurement process, and the measurement results are

Figure 2 .
Figure 2. Schematic diagram of the operation scenario