Motion Profile Selection in Precision Motion Control

[Erkan Teskancan]

## Motion Profile Selection in Precision Motion Control

Motion profile design determines how precisely and efficiently motor-driven axes will move in automation machines. This directly impacts vibration, positioning accuracy, and cycle times in industries such as robotics, semiconductor processing, and laboratory automation.

### Motion Profile Mathematics and Industrial Automation

A motion profile defines how position, velocity, and acceleration change over time along a motor axis. In industrial automation, this is typically used in closed-loop systems that maintain positional accuracy with feedback from encoders or sensors.

Modern motion controllers and servo drives can automatically generate trajectories, considering limits such as travel distance, maximum velocity, and acceleration. However, relying solely on these automatic profiles can create undesirable effects such as overshoot, vibration, position deviation, or prolonged settling times.

Engineers' understanding of the mathematical structure of motion profiles allows for precise adjustment of performance parameters during commissioning and optimization phases. This shortens commissioning time and ensures long-term stability in production environments.

### Trade-offs Between Trapezoidal and S-Curve Trajectories

One of the most common motion patterns in industrial machines is point-to-point motion. In this method, the axis accelerates from a standstill, moves at a constant velocity, and decelerates at the target position.

• Trapezoidal motion profile: Consists of three phases: linear acceleration, constant velocity, and linear deceleration. Acceleration changes instantaneously, and motion time is minimized.
• Triangular motion profile: Further shortens motion time by eliminating the constant velocity phase.

However, in these profiles, instantaneous changes in acceleration (jerk) can lead to vibration and oscillation, increasing the risk of position errors and overshoot. Additional settling times are required to mitigate these issues. Furthermore, high jerk can cause mechanical stress and increase maintenance needs.

S-curve motion profiles offer seven-stage transitions to smooth acceleration, reducing the jerk effect and increasing stability. Compared to trapezoidal profiles, they do not cause longer delays and provide efficiency in overall cycle time.

### Profile Adjustment Based on Load and Process Characteristics

Motion profile selection depends on the application's characteristics:

• High-speed pick-and-place systems: Prefer a short acceleration smoothing phase (typically 5-15%), prioritizing speed with acceptable vibration levels.
• Systems working with sensitive materials: Further smoothing of acceleration reduces vibration and damage, especially in sensitive processes like medical fluid systems.

Sinusoidal motion profiles, on the other hand, further reduce the jerk rate by replacing the linear acceleration ramps in standard S-curves with continuous curves derived from sine functions.

### Special Trajectory Development and Feedforward Control in CNC Systems

In some applications, especially in CNC machines, profiles can be pre-calculated based on load and motor characteristics and stored as motion vector tables. These tables can also include feedforward parameters that send corrective commands before a position error occurs. This method increases response time and accuracy in applications requiring speed and precision.

### Balance in Motion Profile Optimization

In motion profile optimization, jerk, motion time, and mechanical factors (inertia, friction, structural compliance) must be balanced. These parameters typically become clear during the commissioning phase, and adjustments are made iteratively.

### Control Platforms Supporting Multiple Trajectory Models

Manufacturers like PMD (Performance Motion Devices) offer controllers, drives, and positioning devices that support trapezoidal, S-curve, and sinusoidal trajectories. These systems, with the C-Motion® control software library, support motion development in medical, laboratory automation, semiconductor equipment, and robotic applications.

INMOCO provides mechanical engineering and hardware support services to balance positional accuracy and machine efficiency in complex motion applications.

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