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Example Problems
AutoCAD Modeler:
The remote positioner, an introduction to open loop specifications.
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CREATING THE ASSEMBLY DRAWING

The first few steps of 1) defining parameters, 2) defining parts, 3) defining joints, and 4) defining closed loops have been completed for you so that we may continue with the new modeling elements--open loop design specifications. Begin by opening the drawing remotel.dwg in AutoCAD.

Figure 6-1. Remote positioner with closed loops defined.

If you would prefer to complete these previously done steps on your own, begin with the drawing remote.dwg. Your final figure should look like figure 6-1. (This figure also shows the point at which the example problem will start.)

DEFINING A PARALLELISM SPECIFICATION

Design specifications allow tolerances to be placed on assembly dimensions to ensure the assembly functions properly. The position of the right tip of PART_5 with respect to the Ground as well as the parallelism of PART_5 with respect to PART_1 is critical to the remote positioner's function. The parallelism specification constrains (with a tolerance) two parts to be parallel. PART_1 of the remote positioner must remain parallel to PART_5 to within 0.100 inches to assure accuracy of the parallelogram linkage. The .1in. tolerance refers to the width of a tolerance zone parallel to a reference line on PART_1 and in which a specified line of interest on PART_5 must be contained. A tolerance "bandwidth" such as this is also used for the relative orientation specification.

Creating a parallelism specification.

Enter CATS to invoke the CATS menu.
Select SPECS from the main CATS menu.
Select NEW_S to begin creating a design specification.
Select PARallelism from either the icon or pop-up menu.
Select the CEN of Joint 16 to specify the location of a datum on PART_5's line of interest.
Select Rectangular from the FeatDtm menu.
Select PART_5's DRF.
Enter Return for a direct path back to the DRF.
Enter Return to indicate that the path is correct.
Select CEN of the right tip of PART_5 to be the other endpoint on PART_5's line of interest.
Select PART_1's DRF to be one endpoint on PART_1's reference line.

For this and in other selections, it may be necessary to ZOOM IN to distinguish one DATUM or JOINT from another on top of it. The MV Label option in DISPLAY can also be used to separate "cluttered" labels.

Select Joint 8 as the location of a 2nd point on the reference line of PART_1
Enter 22.0 as the characteristic length (the distance from Datum 17 to Datum 18).
Enter 0.1 for tolerance bandwidth on the parallelism specification.
Enter 0,-35 for a symbol location.

MANUALLY DEFINING AN OPEN LOOP

Before creating additional specifications, an open loop will be created manually for the parallelism specification just completed. Results derived from open loop analysis are dependent on the direction of the loop; therefore, the direction of open loops is critical. The loop must run from a reference node to the point of interest. Be sure to select the point of interest as the first specification endpoint. To manually define an open loop path, the joints connecting the parts between the last specification endpoint and the first must be sequentially selected. It may be helpful to 'turnoff' the Loops using the DISPLAY option.

Open loop for Parallelism Spec

Select LOOPS from the CATS menu.
Select NEW_L from the Loops menu.
Enter Open for the loop type.
Select Parallelism Specification symbol to begin the open loop.
Select Joint 8 as the first joint.
Select Joint 16 as a continuation joint.
Select Datum 18 as the last node in the open loop.
Enter Yes or Return to verify that the loop is correct.
Enter 0.005 for the tolerance on the nominal length of 22.000.
Enter 0.005 for the default tolerance on the nominal length of 22.000.
Enter 0.005 for the tolerance on the nominal length of 10.400.
Enter 0.02 for the tolerance on the nominal angle of 30.00deg..
Enter 0.01 for the tolerance on the nominal length of 49.300.
Enter 0.005 for the tolerance on the nominal length of 12.900.
Enter Return for the default tolerance on the nominal length of 12.900.
Enter 0.02 for the tolerance on the nominal angle of 42.60deg..
Enter 0.005 for the tolerance on the nominal length of 22.000.
Enter -10,-20, a location for the loop name.
Enter No to discontinue creating loops.

The parallelism spec open loop should appear as in Figure 6-2. Refer also to Figure 2-13.


DEFINING A POSITION SPECIFICATION

The position of the tip of PART_5 with respect to the Ground is also critical to the remote positioner's function. The tip of PART_5 must be at its nominal position within a circular tolerance zone with a diameter of 0.2 inches. Notice the point of interest must be selected first.

Creating a position specification.

Select SPECS from the main CATS menu.
Select NEW_S to begin creating a design specification.
Select POSition from either the icon or pop-up menu.
Select Datum 18 to be the specification's point of interest.
Select PART_5's DRF.
Select Datum 17 for the next point along the path back to the DRF.
Enter Yes or Return to indicate the path is correct.
Select GROUND's DRF to be the specification's reference location.
Select GROUND's DRF.
Enter Return for a direct path back to the DRF.
Enter Yes or Return to verify the path is correct. (There is no visible path here)
Enter 0.2 for the diametral tolerance zone on the position specification.

This will require the tip of PART_5 to be within a circle of diameter .2 in. centered about its nominal position

Enter 0,-40, a location for the specification symbol.

Figure 6-2. Remote positioner with parallelism specification and its associated open loop.

We will now use the autoloop generator to create the remaining loop.

DEFINING OPEN LOOPS WITH THE AUTOLOOP GENERATOR

An open loop must be created for each open-loop specification. We will use the autoloop generator to automatically create the open loop which needs to be created for the position specification. The autoloop generator automatically starts at the last specified endpoint (the reference) and finds a vector path to the first (the point of interest). It also could have been used to generate the open loop for the parallelism specification. After creating this open loop, use the DISPLAY option to redisplay all of the loops. The model should look like Figure 6-3.Open loop for Position

Select LOOPS from the CATS menu.
Select AUTOLOOP from the Loops menu.

Wait for the Autoloop Generator to generate the open loop.

Enter Yes or Return to verify that the loop is correct.
Enter 0.02 for a tolerance on the angle of 95.10deg.
Enter 0.005 for the tolerance on the nominal length of 22.0000.
Enter 0.005 for the tolerance on the nominal length of 10.4000.
Enter 0.02 for a tolerance on the angle of 30.00deg..
Enter 0.01 for a tolerance on 49.3000.
Enter 0.005 for a tolerance on 12.900.
Enter Return for a tolerance on 12.900.
Enter 0.02 for a tolerance on the angle of 42.60deg..
Enter 0.005 for a tolerance on 22.000.
Enter -10,-25, a location for the loop name.

Neutral File

Save the neutral file with the name REMOTE.NF and proceed to the analysis.

Figure 6-3. Remote positioner with all loops and both parallelism and position design specifications.


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Verification: Clutch | Stack Blocks | Remote Positioner | Bike Crank | Ratchet | Parallel Blocks | NFOV

CATIA

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