Üç serbestlik dereceli bir koordinat ölçüm cihanının tasarım ve imalatı
DesiGN and manufacture of a three dimensional coordinate measurement machine
Yazar: ÖMÜR BAÇ
Danışman: YRD. DOÇ. DR. ZEKİ YAĞIZ BAYRAKTAROĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Sistem Dinamiği ve Kontrol Bilim Dalı
Derece: Yüksek Lisans, 2013
Konu: Makine Mühendisliği = Mechanical Engineering ; Mekatronik Mühendisliği = Mechatronics Engineering

Özet:
Bu tezde üç boyutlu ölçüm yapabilen bir ölçüm cihazının tasarım ve üretimi anlatılacaktır. Tasarım aşaması kendi içinde alt gruplara ayrılmaktadır. Mekanik ve elektronik tasarım alt konularının tamamlanmasının ardından üretim süreci tamamlanacak ve ölçüm cihazının montajı yapılarak deneysel olarak elde edilen veriler incelecektir.
Üç boyutlu ölçüm, klasik ölçü aletleriyle yapılamayan ölçümlerin yapılabilmesini sağlamaktadır. Örneğin araba camı düşünüldüğünde, bu yapımın iki boyutlu ölçü aletleriyle kalite kontrolünün yapılmasının mümkün olmayacağı görülmektedir.
Üç boyutlu ölçüm cihazları sanayide ve üniversite laboratuvarlarında, genellikle kalite kontrol uygulamalarında kullanılmaktadır. Örneğin makine üretim endüstrisinde, üretimden çıkan parçaların boyutsal toleranslarının incelenmesi amacıyla üç boyutlu ölçüm cihazları kullanılabilir. Ayrıca tersine mühendislik uygulamalarında da bu tip cihazların kullanılması büyük kolaylık sağlamaktadır.
Ölçüm, bir operatör tarafından yapılacaktır. Bu da ölçüm kolunun eyleyici ihtiyacını ortadan kaldırmaktadır. Ölçülecek parçanın üzerinde bulunduğu yüzey, referans yüzey olarak kullanılır. Bunun için öncelikle bu yüzeye en az üç nokta tanımlanmalıdır. Daha sonra bu yüzey referans alınarak ölçümler gerçekleştirilebilir. 
Ölçüm cihazını temelde mekanik ve elektronik aksam olarak iki kısımda inceleyebiliriz. Mekanik aksamda; gövde konstrüksiyonu, rulmanlar, cıvatalar ve diğer mekanik bağlama elemanları, elektronik aksamda; enkoderler, USB-CAN dönüştürücü ve bilgisayar bulunmaktadır. Ayrıca yine CANbus haberleşmesi ve bilgisayar programı bu bölümde bulunmaktadır.
Mekanik tasarım, üç adet dönel mafsal kullanılarak oluşturulmuştur. Burada mafsallarda ölçüme etki edecek boşlukların oluşmaması için rulmanların ve millerin toleransları sıkı geçme olarak seçilmiştir. Ayrıca rulman seçiminde de yine boşluksuz tip rulmanlar kullanılmıştır.
Elektronik tasarım enkoder, USB-CAN dönüştürücü ve bilgisayar bileşenlerinin seçimi ve haberleşme programını içermektedir. Enkoderlerin dönel uzuvlarda mümkün olduğunca yüksek çözünürlükte açı bilgisi vermesi beklenmektedir. Ancak burada dikkat edilmesi gereken önemli nokta, bu üç açı bilgisinin aynı anda örneklenmesinin gerekliliğidir. Eğer açı örneklemeleri sırasında bir gecikme yaşanırsa, bu uç nokta hatası olarak karşımıza çıkacaktır. Uzuvlara 18 bitlik mutlak ve tek turlu enkoderler seçilmiştir. Senkron örnekleme için ise CANopen protokolü kullanılmıştır. USB-CAN dönüştürücü, CANbus? tan gelen sinyallerin USB girişli bilgisayarlara aktarılması için gereklidir. Bu cihazın seçiminde, üreticinin sağladığı API ve sürücülerin işletim sistemleri ile olan uyumluluğu dikkate alınmıştır. Enkoderler CANbus hattına bağlanarak USB-CAN dönüştürücü ile bilgisayara alınmış ve burada istenilen işlemler yapılmıştır.
Ortaya çıkması muhtemel problemlerin tasarım aşamasında giderilmesi için tasarım süreci uzun tutulmuş ve birçok problemin bu aşamada çözümü üretilmiştir. Üretim sürecinde karşılaşılan bazı problemlere ise bu süreçlerde müdahale edilebilmiştir.
Sistem tasarıma uygun olarak üretilmiş ve montajı yapılmıştır. Mekanik ve elektronik akşamlar tasarım aşamasındaki kriterlere uygun bulunmuş ve deneysel çalışmalar gerçekleştirilmiştir.

Abstract:
This thesis explains the design and manufacture of a three dimensional measurement arm. The design phase has two sub groups of mechanics and electronics. Manufacture is completed after the completion of the design, before which the components are assembled. After that, experimental works are done and obtained data are discussed. The recommendations to increase system performance are given in the last section.
Three dimensional measurements enable measurement that is not possible with classical measurement devices. For example, when we think of a car glass, it is not possible to assess a quality-control with classical two dimensional measurement devices. Coordinate measurement machines are known as immovable; however, portable CMMs enable this methodology to use in application which requires portability. For example, in casting applications, the products are not easy to relocate and there is a portable coordinate measurement necessity for this kind of applications.
The first coordinate measurement machine was made in Italy in 1960s. There was an accelerated production and traditional measurement machines could not respond with fast enough solutions for quality control applications. After this Italian company, another company named Ferranti in UK developed first cantilever coordinate measurement machine. The cantilever design is currently the most popular design. In fact, Ferranti was not a measurement company; it was a company in production machine industry. 
In these years, coordinate measurement technique became popular and other companies started developing industrial coordinate measurement machines.
Three dimensional measurement devices are used in industry and laboratories for quality control applications. For instance, in machine production industry, they are used to control tolerances of products after production. In addition, reverse engineering becomes easier when these devices are used. In reverse engineering, it is necessary to obtain 3 dimensional design parts from real objects. For example, if you want to design a crash helmet for a pilot, you need to obtain a 3D picture of the pilot?s head.
In portable CMMs, measurement is done by an operator, thus, this measurement arms do not need any actuator. The plane that the part is on, is used as the reference plane. Therefore, a minimum of three points must be determined on this plane. After that, measurement is achieved by using this reference plane.
Measurement arm can be separated into two fundamental areas: Mechanics and electronics. In the mechanics area, there are construction body, bearings, screws and other mechanical components. In the electronics, there are encoders, USB-CAN converter and a computer. In addition, CAN bus communication protocol, and computer program are the members of this group.
Mechanical design contains three rotary joints. To avoid clearance between bearings and shafts, tolerances are chosen as interference fit. In addition, bearings were chosen as clearance free to avoid clearance errors. Aluminum is used as the construction material, because it is light and rigid enough for its purpose. This construction must be rigid, because rigidity brings high construction frequency and measurement is not influenced from the construction displacements.
The mechanical construction is also designed to obtained flexible construction. There are several construction options in the design phase. For example, a design could be optimized for rigidity or stability. In this design, the first aim is to manufacture an arm which does not have dynamic error behavior. Hence, at the end of this work, it is known that the tip point error comes from constant parameter uncertainties.
Electronic design includes encoders, USB-CAN converter, computer selection and communication program. It is expected that encoders give high resolution angle output to decrease tip point error.  Also, it is very important to sample these three angles at the same time. If there is any delay between two samples, this error is shown as tip point error. After selection study, 18 bits absolute, single turn encoders are selected. For synchronous sampling, CANopen protocol is used. 
CANopen is an open communication protocol. It enables some services such as synchronous polling. It also has error detection and it has 1 Mbit/s baud rate. Communication protocol is important when selecting encoders and other components. From this point of view, CANopen is also a popular and widely used protocol.
USB-CAN converter is necessary for getting the data from CANbus via USB input. In the selection of the converter, CAN API, which the converter company supplies, and driver concurrencies are the important subjects. CAN API enables reading and writing CAN messages from the computer. There is a very important point in the driver selection. If this portable measurement arm is expected to be used in Linux based systems, CAN converter?s driver must have GPL License. The selection that was done, depends on this criteria.  Encoders are connected to the CANbus and by using USB-CAN converter, data is read from the computer. 
In the computer program, C++ is used as a programming language and Visual Studio is used as an IDE. Selection of correct programming language is very important in the beginning of the project. There are several language alternatives for this project, however, C++ is widely used and it is easier to get support from suppliers when C++ language is used. In this project, CANbus API is used to communicate between CAN network and computer. This API has several language alternatives, however, in C++ support is more efficient and faster. 
In this project, all programs are written in object oriented strategy. Object oriented programming brings several advantages in this kind of projects. In the programming section, a plane is obtained from the program in the sense of Least Square. After that, a plane to distance program was written and measurement results taken from this function. To obtain tip point coordinates from the encoders? angles, forward geometric model is required. This model is programmed in the programming section and used in Cartesian coordinate function.
To decrease probable problems, the design phase is kept longer and some problems are resolved in this phase. In the production period, some problems occurred, however, these problems could be solved in this period. For example, in the drilling application, two holes of the bearings are suggested to drill in one operation. For this purpose, snap ring was added to design and this increased the axial tolerances.
The system was produced and assembled in accordance with the desired design. Mechanical and electrical parts were approved and experimental works were accomplished. 
In the experimental works, there was a test setup which ensures known tolerances. For example, in the distance measurement test, a plate was used as reference plane that has a flatness under 1µm. After that a gauge block was used as a measured distance. It has 8mm distance with +0.04µm tolerances. Hence, this test setup is sufficient to test coordinate measurement machine. 
In the first test, a plane was obtained with measured data. After that, an algorithm was used to fit a plane to these points. There are several algorithms for this purpose. In this work, Least-Square algorithm is used as a fitting algorithm. In this experimental work, more samples enable more sufficient surface; because, Some errors might occur in the measurement time and more points make the calculated plane closer to real plane. 
To measure a distance to a plane, a point was measured on the gauge blog. Then distance to plane algorithm was used, and the minimum distance between measured point and plane was calculated. After measurement, the results are obtained as 7.9657, 8.0012.
These experimental results show that there is an error in the measurement. This error might depend on some geometric uncertainties and kinematic errors. These constant parameters are used in non-linear system model and their affects influence the system output non-linearly. Hence, the output error chance depends on the measurement motion. For example, if the points are measured close to each other, non-linearity minimally affects the system output. But if the points are measured far from each other, then rotary joints perform in a bigger degree and this affects the output error negatively. 
To decrease the tip point error, identification must be done. This identification will expose system parameters. There are several ways to identify system parameters: For example, a plane could be used as a constraint plane and sampled points may be used to obtain system parameters. In this process, it is known that all the sampled data are on the same plane and forward kinematic result must give an output that all the tip point coordinates must be on the same plane. 
In this thesis, it is seen that coordinate measurement machines are multi-disciplinary machines in the area of mechanical engineering, electronics engineering and computer engineering. There are several sub topics in this project and all must be done in a high level success to obtain sufficient measurement results.