2.875 - Fall 2001
Mechanical Assembly and Its Role in Product Development
Term Project: Report #2
Analyzing the Assembly Design of a Computer Mouse Using Datum Flow Chains
October 31, 2001
Annabel Flores
James Katzen
Photos taken from:
Analyzing the Assembly Design of a Computer Mouse[1]
Using Datum Flow Chains
The Microsoft Mouse Version 2.0 is an ergonomic, dual-button mouse. The simple, ten-part design provides an opportunity to analyze the product’s assembly characteristics. In addition, we can apply assembly analysis tools such as a liaison diagram and datum flow chains to evaluate the current design and propose product redesigns. Below is a figure of the computer mouse.
Figure 1: Computer Mouse Semi-Exploded View
The Liaison Diagram for the mouse is shown below:
Figure 2: Liaison Diagram for Microsoft® Mouse v2.2A
Note that this diagram shows the numerous key characteristics that must be delivered in this device. A designer must pay attention to numerous aspects of the assembly in order to properly deliver the important functional and customer requirements. This diagram appears fairly straightforward, and shows that there is no coupling of key characteristics. Thus, we assume that each of these key characteristics can be met under nominal conditions.
In observing the liaison diagram, as well as various different parts of a mouse, we found a number of key characteristics that we could have focused on for this report. For example, the alignment between the mouse base and cover is a key characteristic particularly of importance to the end user who would like a smooth transition between the parts. This key characteristic is therefore an appearance characteristic. Another key characteristic is the positioning of the mouse cover with respect to the circuit board, so that the buttons properly actuate the electronic switches. This key characteristic is therefore a functional characteristic.
In choosing a key characteristic for the purpose of this analysis, we were compelled to choose one that was crucial in the functional performance of the mouse. We determined that other than the monitoring of switch inputs, the key driver of mouse functionality is its ability to convert mechanical motion into optical information that could be transferred onto a computer screen.
A user will move the mouse on a surface that, in turn, will move the ball within. Refer to the figure below to follow our nomenclature. The ball transfers up/down and left/right motion to two independent gear axles that are in constant contact with the ball. The gear head rotates according to the ball’s speed and direction. The teeth on the gear head pass through optical sensors that record the speed and direction of the ball and hence the user’s movement.
Figure 3: Gear/Sensor with Co-ordinate System
The position of the gear’s teeth with respect to the sensor is critical to provide the sensitivity necessary to capture the user’s movement. If the gear is grossly out of position in or about any direction, the sensors will not be able to process the data accurately. However, for the purpose of this analysis, we are focusing on the key characteristic that is the relative position of the gear and sensor in the direction parallel to the gear’s rotational axis. This key characteristic, and its associated coordinate system are denoted in the figure above. Note that we have chosen to analyze only one gear/sensor pair instead of both as the same analysis can be applied to both.
The parts that deliver the key characteristic are the mouse base (part #8), circuit board (part #11), optical sensors (subassembly on part #11) and gear (part #7). The most likely root for this datum flow chain is the mouse base as this part locates all others. The datum flow chain for the gear/sensor subassembly is as follows:
Figure 4: Datum Flow Chain Delivering Key Characteristic
As the assembly root, the mouse base is the starting point to locate all other parts. The table below lists the important features on the base that locate the circuit board and the gear.
FeatureNumber / Feature / Part
1 / Support / Base
2 / Pin 1
3 / Pin 2
4 / Tab 1
5 / Tab 2
6 / Bottom Surface / Circuit Board
7 / Hole 1
8 / Hole 2
9 / Hole Set 1
10 / Hole Set 2
11 / Pegs / Photo Eye Sensor
12 / Pegs / LED Sensor
13 / Sensor Spacing / Sensors
14 / Teeth / Gear
15 / Peg End 1
16 / Peg End 2
Table 1: Features Delivering the KC
The Support, Pin 1 & Pin 2 define the location of the circuit board within the mouse base. Tabs 1 & 2 locate the position of the gears with respect to the base. The injection-molded features of the base are shown in the figure below. The part’s coordinate frame locates the coordinate frame of the features.
Figure 5: Mouse Base Features Delivering the KC
The circuit board features that connect to those on the mouse base are the bottom surface and Holes 1 & 2. The circuit board’s coordinate frame is located in the assembly through these features. The part coordinate frame defines the frame of the Hole Sets 1 & 2 that define the position of the optical sensors.
Figure 6 locates the above features on the circuit board. The circuit board’s coordinate frame is located through these features.
Figure 6: Circuit Board Features Delivering the KC
The two sensors that translate the mechanical motion of the gears are the photo eye sensor and the LED Sensor. The position of these sensors, and the spacing between them, is defined through the position of the circuit board’s hole sets.
Figure 7: Sensor Features Delivering the KC
As shown in Figure 8, the gear’s coordinate frame is located through its pegs and the respective tabs on the mouse base.
Figure 8: Gear Features Delivering the KC
The datum flow chain developed in Figure 4 is a direct representation of the assembly scheme. The circuit board and gear are located through features on the mouse base. The circuit board subassembly locates the sensors. The position of the gear with respect to the sensor spacing is the key characteristic under analysis.
In studying the assembly scheme of the mouse, it became apparent that parts in the assembly were over-constrained. Adding the degrees of freedom to the datum flow diagram developed in Figure 4 results in the following figure.
Figure 9 Datum Flow Chain with Assembly Degrees of Freedom
There are a number of potential redesigns of the computer mouse that can eliminate the over-constrained conditions. In addition, a product redesign can improve the deliver of the key characteristic. We have developed a number of design proposals that improve the product assembly with a more robust design and are described in detail below.
Design Proposal 1: Revise Circuit Board – Mouse Base Locating Method
Currently, the circuit board (Part #11) is oriented with respect to the mouse base (Part #8) through the use of two peg-hole mates. These are shown in the picture below:
Figure 10: Assembly Features linking the mouse base and the circuit board
As we have seen in this class, this is an inherently over-constrained design. The over-constraint in the x and y directions will cause numerous problems, such as assembly difficulty, and the locking-in of internal stresses.
This over-constraint can be overcome by introducing a peg-slot mate at one of the features. This design is shown below:
Figure 11: Proposed Assembly Features linking the mouse base and the circuit board
Note that since the two pegs do not share a common x or y location, the slot must be placed on an angle. The long axis of the slot must be parallel to the line that connects the two center points of the pegs. This will result in a completely constrained assembly, with no over-constraint. The circuit board will be completely positioned and oriented properly with respect to the mouse base. This will likely result in reduced assembly efforts, and reduced “locked-in” stresses.
Design Proposal 2: Revise Optical Encoder Sensors’ LED Unit – Circuit Board Locating Method
Currently, the optical encoder sensors’ LED units are oriented with respect to the circuit board (Part #11) through the use of two peg-hole mates. These are shown in the picture below:
Figure 12: Assembly Features linking the optical encoder sensors’ LED units and the circuit board
This is recognized as an inherently over-constrained design. The over-constraint in the x and y directions will cause numerous problems, such as assembly difficulty, and the locking-in of internal stresses. It is assumed that these problems occur very regularly, since both the LED units are visibly misoriented about their x-axes on the unit we examined. We assume that this misorientation does not affect the amount of light produced by the LED units in the direction of the photo eye (due to a wide field of illumination). However, this over-constraint likely causes additional effort than should be needed in assembling these relatively simplistic parts.
This over-constraint can be overcome by introducing a peg-slot mate at one of the features. This design is shown below:
Figure 13: Proposed Assembly Features linking the optical encoder sensors’ LED units and the circuit board
Although it has not been confirmed, it is assumed that the through holes in the circuit board are large enough to have clearance between the sensor element leads and the hole edges. If this is the case, the item is not over-constrained. However, if these items are soldered one lead at a time rather than both at once, the item will be over-constrained, since an adjustable contact feature was fixed before the mates were fixed. But since it is known that this part is made with a wave-soldering process, we can assume that the contacts are all affixed at the same time.
This new design will result in a completely constrained assembly, with no over-constraint. The LED unit will be completely positioned and oriented properly with respect to the circuit board. This will likely result in reduced assembly efforts, and reduced “locked-in” stresses.
Design Proposal 3: Revised Optical Encoder Sensors’ Photo Eye Unit – Circuit Board Locating Method
Currently, the optical encoder sensors’ photo eye units are oriented with respect to the circuit board (Part #11) through the use of three collinear peg-hole mates. These are shown in the picture below:
Figure 14: Assembly Features linking the optical encoder sensors’ photo eye units and the circuit board
As with the circuit board – mouse base and the LED units – circuit board mates, this is recognized as an inherently over-constrained design. The over-constraint in the x and y directions will cause numerous problems, such as assembly difficulty, and the locking-in of internal stresses. It is assumed that these problems occur also very regularly, since both the photo eye units are visibly misoriented about their x-axes on the unit we examined. We assume that this misorientation does not affect the amount of light sensed by the photo eye units (due to a wide sensing field). However, this over-constraint likely causes additional effort than should be needed in assembling these relatively simplistic parts.
Eliminating one of the three pegs and introducing a peg-slot mate at one of the features can overcome this over-constraint. This design is shown below:
Figure 15: Proposed Assembly Features linking the optical encoder sensors’ photo eye units and the circuit board
This will result in a completely constrained assembly, with no over-constraint. The photo eye unit will be completely positioned and oriented properly with respect to the circuit board. This will likely result in reduced assembly efforts, and reduced “locked-in” stresses.
It is not known whether the new two-lead photo eye units would be compatible with the existing circuitry and sensing algorithms. Therefore, this change to correct an over-constrained condition may require additional analysis and changes.
The same concern regarding the soldering process exists for this item, as it did for the LED units.
Suggested Redesigns That Improve KC Delivery
The Key Characteristic that has been identified is the alignment of the gear, the optical encoder sensor LED unit and the optical encoder sensor photo eye unit. This alignment must be tightly controlled, since the mechanical motion of the mouse is directly transferred to electrical signals via the use of the optical encoder. Figure 16 shows this nominal case.
Figure 16: Nominal case where the light transmitted by the optical encoder sensor LED unit is sensed by the optical encoder sensor photo eye unit
If this alignment were in error, the amount of light passing through the gear (Part #7) would be affected. If large misalignments occur, no light could pass through, and even though the mouse is in motion, the unit would not sense the movement. The condition of lateral error is shown below:
Figure 17: Effect on the amount of light sensed by the optical encoder sensor photo eye unit as the result of lateral misalignment of the optical encoder sensor LED unit and the optical encoder sensor photo eye unit
The condition of angular error is shown below:
Figure 18: Effect on the amount of light sensed by the optical encoder sensor photo eye unit as the result of angular misalignment of the optical encoder sensor LED unit and the optical encoder sensor photo eye unit
Multiple design proposals are being offered to make the sensing system more robust to variation in the alignment of the optical encoder sensor LED unit and the optical encoder sensor photo eye unit.
Design Proposal 4: Increase Diameter of Toothed Portion of Gear
Since the passages that allow the passage of light are slots rather than point holes, the current design of the gear (part #7) is tolerant of some lateral error in the x-direction. A non-robust design for the gear, with point holes is shown below:
Figure 19: Non-Robust design for optical encoder gear
However, due to geometric concerns, the current gear cannot be made more tolerant of lateral error in the x-direction, since expanding the slots would soon weaken the gear tremendously. But, if the overall diameter of the gear is increased, the slots could then be lengthened, making the gear much more tolerant of lateral error. This design modification is shown below:
Figure 20: Proposed design improvement for optical encoder gear
Note that the accuracy of the device would be negatively affected as the lateral error in the x-direction increases. Knowing the elapsed time over which the light passes through a gap in the gear teeth, a rotational velocity can be calculated, as long as the distance of the gap is known. However, if this gap increases, the relation between the time and the corresponding rotational velocity is affected, and false readings can be created. This is precisely what would happen if the gear placement were in error. Because the width of the slot increases with increasing radius, this increased gap would fool the electronics into thinking that the mouse was moving slower than it was. However, existing computer software utilities exist that allow the computer user to customize the behavior of the mouse. Therefore, the customer could correct the slight calculation error in mouse position, and this integration of this design proposal is therefore feasible.
Design Proposal 5: Decrease Distance Between Optical Encoder Sensor LED and Photo Eye Unit
Changing the distance between the optical encoder sensor LED and sensing field of optical encoder sensor photo eye units will affect the robustness of the light sensing performance. Shortening this distance will result in an increased ability of the photo eye to detect the light illumination, even in the presence of LED / Sensor misalignment. The same angular misalignment that was shown in Figure 18 is repeated in Figure 21. However, in Figure 21, the distance between the LED and the photo eye unit is decreased.