71
72 Different experiment has been carried out to find the suitable isolation, that can be resistive to the clamping force, totally isolating, and have the right thickness to fit in the new location. Figure 43 shows the first isolation prototype that has been experimented. This prototype had a good resistivity and a perfect thickness but it was not adhesive enough to stick around the tool holder, the reason why a new isolation has been selected. It’s very important to note, that all the issues presented in this chapter are related directly to the isolation.
Figure 43. Isolation film fixed on the tool holder
73 5.4.2 Solution for the noise
To solve the issue of the noise, I had concluded that theoretically reducing the constant resistance of the measurement circuit R2 can imperatively improve the output signal of the measured voltage 𝑉𝑚𝑒𝑠 , and also may reduce the noise. The proposed solution can be proved using the Eq (4) and (5)
𝑉𝑚𝑒𝑠 = (𝑅 1 + 𝑅2) · 𝐼 − E1 (4)
𝑅2 = 𝑟1 + 𝑟2 + 𝑟3 ··· +𝑟𝑛 (5)
r1 represent resistance of the rotary contact mercury. r2 is the resistance of the supplement wires connectors that connect the mercury contact with a vice of the CNC machine, r3 can be represented as the resistance of the bearing and finally rn which can be represented as the constant resistance of the rest parts of milling machines body.
𝑅 =
𝜌
𝐿/𝐴 (6)σ = 1/
𝜌
(7)
Reducing the resistance R2 will reduce the voltage and the resistivity in the circuit, this mean that final results which is the contact resistance measurement range will also be reduced. The result can be demonstrated using Eq (6) where
𝜌
is theresistivity, L the length of the wire, and A is the wires cross sectional area. By reducing the resistivity of the circuit, the conductivity (σ) represented in Eq (7) will automatically increase.74 5.4.3 Replacement of contact mercury
As the location of the isolation has been changed, the mercury contact has to be removed and replaced by another contact component that consider the location of the new isolation and in same time satisfied the condition of the transmission, which is low resistivity.
Tentative 1: Based on the mentioned condition, different experiment has been conducted in order to find out the best contact component. The first idea was the carbon graphite brush and copper ring. This transmission system is commonly used in DC motor to transmit the current from rotor to stator, which match exactly with a function of the previous contact mercury. To apply this system to the milling machine, mechanical support prototype has been designed by 3D printer to fit the size and the function of the selected carbon brush as shown in Figure 44 and Figure 45.
75 Figure 44. Designed carbon brush support
Figure 45. Carbon brush support realized with 3D printer.
76 Figure 46 illustrates the carbon brush and copper ring in action. After experiment.
It can be observed that contact area of the carbon brush with ring is very small due to the hardness of the carbon. Figure 47shows the condition of the carbon brush after experimentation, and Figure 48 illustrates the results of the experiment. Even after a long period of use, the results were the same. From same Figure 48, it can be observed that output signal is very noisy and not effective.
Figure 46. Carbon brush in action
77 Figure 48. Output signal using single carbon brush
Figure 47. Carbon brush after experiment.
78 After investigation, we concluded that reason of the failures was determined as follow:
1. The shape of the carbon brush was rectangular and flat, which does not match with a curved shape of the copper ring as shown in Figure 47. This reduced the contact area between the spindle and the carbon brush.
2. The pressure of the lateral force was carry out by the integrated spring delivered with a carbon brush. But it was not sufficient to ensure a good transmission of the current between the tool and the measurement unit.
3. The contact resistance of the carbon brush was very high, even superior to the measured resistance R1,
Tentative 2: In the second experiment, the carbon brush N1 has been replaced by a double silver graphite brushes shown in Figure 49, and the copper ring by a silver coated ring as shown in Figure 50.
Figure 50. Silver coated ring.
Figure 49. Silver graphite brush.
79 Same procedure, a new support also has been designed to fit the second carbon brush and also the new ring as shown in Figure 51. This time, the carbon brush was low in resistance comparing to the previous one N1.
Figure 51. Double graphite brushes and Coated silver ring in action.
80 The second tentative delivered better performance than first experiment than carbon brush. The noise was reduced and the voltage drop more stable than the previous tests with carbon brush and copper ring. Figure 52 shows the results of the output signal.
However, the results were not exactly sufficient, for the following reason:
1. The lateral pressure delivered by spring force was also adapted for the brushes but not enough to obtain the suitable signal. The mentioned performance has been obtained because addition lateral force has been Figure 52. Double silver graphite brushes and coated silver ring output.
81 added manually same as previous experiment, as it can be seen, that this issue also affects the improvement of the noise, which was still persisted.
2. The voltage drop was more stable than results with carbon brush, but not stable enough to deliver a clear output signal required for the signal processing and allow a suitable calculation of the contact resistance. in an acceptable level. Which made the signal still not stable. The variation was very important and sensitive to the lateral pressure force.
Tentative 3: Fiber brush
From the previous results, we concluded that successful contact has to be lower in contact resistance comparing to the carbon brush and silver graphite brushes. And must have an excellent contact-pressure ratio. Based on this conclusion, we decided to use a fiber brush Ag-Cu instead of graphite brush as shown in Figure 53, the fiber is for the flexibility and good contact-pressure ratio. And the coating is for the low resistance and a good conductivity. As the slip ring is an important part of this systems combination, it has been also replaced by Au coated ring to increase the performance of the system.
Figure 54 illustrates the new coated slip ring.
Figure 54. Au Coated slip ring.
Figure 53. Ag Cu Coated fiber brush.
82 After multiple experiment, a very good results have been obtained. with a new combination system (Ag Cu fiber brush and Au slip ring). Figure 55 illustrates a partial zone of the experimental setup used in this experiment.
Figure 55. Installation of Ag-Cu Fiber brush and Au slip ring
83 Figure 56 shows the output signal with a new transmission system. It can be observed that noise was almost totally illiminated. In addition, the level of the voltage drop was stabilized.
As the result of the experiment was very successful, the new transmission system has been approved and adopted for the accomplishment of the new developed tool wear detection system.
Figure 56. Output signal with a coated Ag Cu fiber brush and Au slip ring.
84 5.4.4 Solution for the implementation system
It has been mentioned that, the old system was very complex to implement. It’s required a heavy aluminum support, access to the shaft, supplement wires connectors and so on. After recognizing all the mentioned issues, and also considering all the new proposed solutions, a new support has been designed. It’s flexible and adjustable arm, with a powerful magnetic base. As shown in Figure 55 This system has been approved in the previous experiment with a Ag Cu fiber brush and Au slip ring. Actually, this system is commonly used for a precision measurement. It’s very stable and able to be fixed easily on any metallic surface, which is the case in our experiment Figure 57 illustrates the prototype idea of how the new magnetic support should be. Able to be readapted to fit our new developed system with a suitable CNC machine.
Figure 57. Fiber brush support unit
85