What questions should we answer before writing a precision motion specification?
One thing I’ve learned from working with precision motion and machine automation is that it’s easy to start specifying hardware before we fully understand what the machine needs to do.
We start discussing accuracy, repeatability, resolution, velocity, acceleration, and controller performance. These are all important considerations, but are they the right specifications for the application?
Start with the process.
Is the machine positioning a sample for imaging? Does it follow a continuous path during laser processing? Does it make hundreds of short moves per minute? Or does it position a component and hold it stable while taking a measurement?
Each application can have very different motion requirements, even when they all demand high precision.
Sometimes, the most impressive specifications on a positioning stage aren’t the ones that matter most to the process.
And I can point to a project where not starting with the application led us in the wrong direction.
A Lesson From a Medical Tube Cutting Project
A couple of years ago, I worked on a precision laser cutting system for medical tubing. The project involved upgrading an existing machine whose motion controller had become obsolete.
Our initial approach seemed reasonable. We reviewed the existing motion system’s specifications and selected a replacement controller that seemed to meet them.
But we made a mistake. We started with the hardware specifications rather than fully understanding the application.
The original controller supported cubic spline motion profiles in its G-code programming. That capability was important to the existing cutting application, but it wasn’t included in the standard functionality of the controller we selected. Supporting it would require an additional controller upgrade.
We had evaluated the new controller against the original system’s hardware specifications without identifying an important requirement embedded in the existing motion program.
The lesson was straightforward: replacing a motion controller isn’t simply a matter of matching axis counts, encoder interfaces, positioning accuracy, and motion performance. You also need to understand how the application uses the controller.
What motion profiles does the process require? How are complex trajectories generated? Which controller functions does the application depend on?
Had we examined the application and its G-code requirements first, we would have identified the spline capability before selecting the replacement platform.
It was a good reminder of why the process needs to come before the motion specification.
1. What Does the Process Actually Need to Do?
Before selecting motors, stages, encoders, or controllers, I want to understand how the machine operates.
Is it a point-to-point positioning application where the machine moves to a location, settles, and performs an operation?
Is it a continuous-motion application where the machine must follow a trajectory while coordinating with a laser, camera, or measurement device?
Or does it require multiple axes working together, possibly with complex motion profiles and real-time synchronization?
These differences can strongly influence the architecture.
For example, in laser processing, maintaining a consistent relationship between motion and laser energy delivery may be more important than achieving the smallest possible static positioning error.
In microscopy, the priorities may be very different. Rapid positioning, short settling times, and exceptional stability during imaging can be critical to throughput and image quality.
Understanding the operation is the first step toward understanding the motion requirements.
2. How Much Accuracy and Repeatability Do We Really Need?
Precision motion specifications can be confusing because several terms are often discussed together.
- Resolution: The smallest position increment the system can command or detect.
- Accuracy: How closely the actual position matches the intended position.
- Repeatability: How consistently the system returns to a position.
- Stability: How well the system maintains its position over time under operating conditions.
These are different performance characteristics.
A positioning system with extremely fine encoder resolution does not necessarily provide comparable positioning accuracy.
And that raises another important question:
Where are we measuring the performance?
At the encoder? At the motion stage? Or at the actual point where the manufacturing or measurement process takes place?
Mechanical alignment, angular errors, Abbe offsets, structural compliance, and thermal effects can introduce errors between the measurement point and the workpiece.
The motion specification should ultimately relate to the process, not simply the capabilities of an individual component.
3. What Really Determines Throughput?
It’s tempting to specify higher velocity and acceleration to improve machine performance.
But maximum speed isn’t always the limiting factor.
Consider an imaging system that repeatedly moves between locations, waits for the stage to settle, and captures an image.
The overall cycle time depends on more than travel speed.
It includes acceleration, deceleration, settling time, and the time required to complete the imaging operation.
A system that moves extremely fast but takes too long to settle may actually have lower throughput than a system with more moderate motion characteristics.
An interesting example appears in the Machine Design article, “Actuators and Drives: Foundations of High-Performance Motion Systems” [1].
The article discusses an Aerotech application involving next-generation DNA sequencing, where acceleration, settling time, and position stability are important factors in overall machine performance.
This illustrates why understanding the entire process cycle is essential.
Sometimes reducing settling time is more valuable than increasing maximum velocity.
4. How Important Is Motion Stability?
Different processes place different demands on the motion system.
A laser cutting application may require accurate path following at a specified velocity.
A high-resolution imaging application may require the stage to remain exceptionally stable after reaching its target.
Other applications may require both.
This brings mechanical design and control-system performance into the same discussion.
We need to consider:
- Structural stiffness and mechanical resonance
- Servo bandwidth and control-loop tuning
- Encoder and feedback performance
- Vibration from the machine and surrounding equipment
- Thermal expansion and temperature stability
- Payload, mounting, and center of gravity
These factors are closely related.
For example, increasing servo gain may improve dynamic response, but the mechanical structure and its resonances can limit how far you can go.
Likewise, a stage capable of exceptional positioning performance in a controlled laboratory environment may behave differently after being integrated into a larger machine.
The completed machine, not just the stage, must meet the process requirements.
5. How Does Motion Interact With the Rest of the Machine?
This is an area where I’ve spent much of my career.
In laser manufacturing, the motion system rarely operates independently.
It may need to synchronize with laser pulses, modulate laser power based on velocity, respond to sensors, or coordinate several axes while maintaining a prescribed trajectory.
Similar considerations apply to imaging and inspection systems.
A camera may need to acquire an image at an exact position. A measurement system may require a stable trigger relationship to motion. Multiple axes may need coordinated movement rather than independent point-to-point commands.
These requirements influence controller architecture, interfaces, feedback, and software design.
They also raise an important question:
Which functions belong in the motion controller, and which belong elsewhere in the machine control system?
No single answer fits every application.
But making these architectural decisions early can simplify integration, improve performance, and reduce software complexity.
My medical tube cutting experience is one example of why this matters. The original application’s reliance on cubic spline G-code was more than a programming detail. It was a functional requirement that influenced controller selection.
6. Should We Use a Standard Motion Platform or Develop Something Custom?
I believe this question deserves attention early in a project.
Modern precision motion manufacturers offer increasingly capable integrated solutions, including positioning stages, drives, feedback systems, controllers, and software tools.
Often, much of the required functionality already exists.
The engineering challenge is determining whether a standard platform satisfies the application or whether customization is actually justified.
That decision involves more than the initial hardware cost.
We should consider integration effort, development time, technical risk, maintainability, and long-term support.
An overly customized solution may meet the immediate requirements but create unnecessary difficulties throughout the machine’s life.
On the other hand, forcing an application into an unsuitable standard architecture can be equally expensive.
The goal should be to customize what makes the machine unique while using proven technology wherever practical.
7. How Are We Going to Prove the System Meets the Requirements?
This may be one of the most important questions, and it’s worth asking before finalizing the design.
How will we measure accuracy and repeatability?
What are the acceptable settling times?
How much following error can the process tolerate?
What environmental conditions must be considered?
And how will we verify that all of these requirements are satisfied when the system is fully integrated?
The answers may influence the mechanical design, feedback selection, controller architecture, and even the equipment required for testing.
A requirement that cannot be measured or verified is difficult to manage.
Early testing, simulation, and prototyping can also help identify technical risks before you commit substantial resources.
In my experience, resolving a major architectural issue during concept development is far less expensive than discovering it during commissioning.
Precision Motion Is a System-Level Engineering Problem
An interesting article in Machine Design, “Actuators and Drives: Foundations of High-Performance Motion Systems,” examines several of these engineering challenges through examples involving Dover Motion, Aerotech, and PI (Physik Instrumente) [1].
The article illustrates how motion performance depends on the interaction of mechanical components, actuators, feedback, drives, and control technology.
It also highlights how different applications demand different engineering solutions.
That’s an important point.
A microscopy system, semiconductor manufacturing tool, laser processing machine, and automated inspection system may all require precision motion, but the best architecture for one may not fit another.
I’ve spent much of my career working on the controls and integration side of machine development, and one lesson continues to stand out:
The best motion system isn’t necessarily the one with the most impressive individual specifications. It’s the one that reliably meets the process requirements without unnecessary complexity or cost.
And the best way to get there is to understand the process before writing the specification.
My medical tube-cutting project reminded me that even when we’re replacing an existing system, we shouldn’t assume its hardware specifications tell the whole story. Sometimes the most important requirements hide in the application software, motion programs, or how the machine actually operates.
I’d be interested in hearing from other machine builders, motion-control engineers, and system integrators.
What’s the first question you ask when defining a precision motion system?
References and Further Reading
[1] Machine Design (October 20, 2025). “Actuators and Drives: Foundations of High-Performance Motion Systems.” Discusses high-performance motion architectures and examples involving Dover Motion, Aerotech, and PI.
[2] Dover Motion. “Move and Settle Time.” Discussion of motion profiles, positioning performance, and settling behavior in high-throughput applications.
https://dovermotion.com/resources/motion-control-handbook/move-and-settle-time
[3] PI (Physik Instrumente). “The Difference Between Position Accuracy and Repeatability, and Methods to Reduce Position Errors.” Discussion of positioning specifications and error sources.
[4] Aerotech. “Understanding Precision Linear Stages for Optimal Performance.” Discussion of stage selection, performance characteristics, and mechanical considerations.
https://go.aerotech.com/in-motion/understanding-precision-linear-stages-for-optimal-performance
[5] ACS Motion Control. “Motion Profile Generation.” Discussion of trajectory generation, dynamic performance, following error, and settling behavior.