Figure 1: Visual comparison . Steppers (Left) are dense and simple. Servos (Right) are longer and include a visible feedback encoder housing on the rear. The Million Dollar Question: "Which Motor Do I Need?" If you are designing a CNC machine, a packaging robot, or a conveyor system, you face the same dilemma every time: Stepper or Servo? Make the wrong choice, and you face two disasters: The Stepper Trap: Your machine "loses steps" (positional error) without knowing it, scrapping parts. The Servo Trap: You spend $5,000 on a system that could have been done for $500, blowing your budget. This guide bridges the gap between mechanical requirements and electrical reality. 1. The Stepper Motor: The "Digital Ratchet" Think of a Stepper Motor like a very strong, magnetic ratchet. It divides a full rotation into equal steps (typically 200 steps per revolution, or 1.8°). Pros: Incredible Holding Torque: Ste...
In real-world engineering, a mechanism often needs to guide a part through more than just a start and end point. It usually requires passing through 3 specified positions to clear obstacles or perform complex tasks.
This technique is known as 3-Position Motion Generation. We can extend the logic from our previous post [Four-bar linkage Synthesis using Unigraphics NX4 Sketch] to solve this problem geometrically within the CAD environment.
The Design Challenge
Assume we must design a mechanism to move Link AB through three specific positions (A1B1, A2B2, A3B3) while avoiding an obstacle (represented by the rectangle below).
Step-by-Step Synthesis
1. Define the Positions:
Draw Link AB in its three design positions: A1B1, A2B2, and A3B3.
2. Geometric Synthesis for Pivot O2:
To find the fixed pivot that allows point A to move through all three locations, we must find the center of the circle that passes through A1, A2, and A3.
- Draw construction lines connecting A1 to A2 and A2 to A3.
- Create Perpendicular Bisectors for both lines.
- The intersection of these bisectors is the fixed pivot O2.
- Draw line O2A1. This is your Input Link (Link 2).
3. Geometric Synthesis for Pivot O4:
Repeat the exact same logic for point B to find the Follower Link pivot.
- Draw construction lines connecting B1 to B2 and B2 to B3.
- Create Perpendicular Bisectors for both lines.
- The intersection is the fixed pivot O4.
- Draw line O4B1. This is your Follower Link (Link 4).
Building and Validating the Kinematic Chain
Now we construct the linkage geometry to verify the motion path.
- Draw line O2M equal in length to O2A1.
- Draw line O4N equal in length to O4B1.
- Draw coupler line MN equal in length to A1B1.
- Set an angular driving dimension (e.g., 15 degrees) between O4B1 and O4N.
Simulation via "Animate Dimension":
In the NX Sketcher, select the angular dimension and set the limits.
Lower Limit: 0 | Upper Limit: 56.355 | Steps: 150
Video Result: 3-Position Synthesis
Watch how the synthesized mechanism perfectly guides the link through all three positions while avoiding the obstacle.
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