Where to Start with Motion Systems Design?

Motion systems encompass a variety of systems from basic XY gantries to complex multi degree of freedom robotic arms. IDC’s team has wide experience of designing robotic systems and understands the issues which commonly present themselves, as well as the best design solutions.

IDC Design Engineer, Nick Brown, discusses the five essential points to think about when planning mechanical design for motion systems.

1. Motor Selection

The two most important factors when selecting a motor are torque and speed, these are often simple to calculate based on the forces required to move the motion system at the desired accelerations and velocities. Keep in mind, an often-overlooked factor is the inertia ratio between the motors rotor and the load being moved. If the inertia of the load is far greater than that of the motor, it may be near impossible to develop a stable control system with fast settling times.

2. Open Loop vs Closed Loop Control

Basic motion systems often use stepper motors as their primary drive, for example 3D printers. Stepper motors are generally used in open loop control configurations, where the controller sends a command for the motor to move to a position at a certain speed and there is no feedback that the motor has done what was asked. This is acceptable as long as performance targets are still being met, and it isn’t absolutely critical that the system moves exactly as commanded.

If performance requirements exceed the capabilities of an open loop stepper motor system, closed loop PID control with servomotors may be required. In a closed loop system, the controller commands the motor to move to a position at a set velocity with a set acceleration, an encoder on the motor shaft then feeds back to the controller reporting its actual velocity and position. The controller decides if the motor needs more or less power based off the error between the commanded position and velocity versus the actual position and velocity.

A well-tuned PID control loop will result in faster, more accurate positioning of the motion system whilst ensuring the commanded movements have been completed, albeit at the cost of greater controller complexity compared to an open loop system.

3. Mechanical Design to Avoid Backlash and Free Play

There is little point in developing an advanced control system and specifying motors with top of the range encoders if the mechanical system has significant backlash and free play resulting in inaccuracies, poor repeatability and a sloppy look or feel.

The two primary sources of free play in a motion system are bearing arrangements and gear backlash (among other system specific factors). Gear backlash can be overcome during component selection, a strain wave gear will have zero backlash, precision planetary sets will have minimal backlash. Otherwise direct drive systems avoid the problem altogether, though this isn’t always feasible.

Free play from bearing joints can often be eliminated through correct specification of components and application of preload. Jointed arms can have all free play eliminated by using preloaded angular contact ball bearings, and linear guides and rails can be made to feel solid through using spring loaded polymer bearings.

4. Mechanical strength and alignment

The more precise or high performance a motion system is, the more emphasis needs to be placed on mechanical strength of components and geometrical tolerances of assemblies to guarantee performance. Taking the example of a simple XY motion system using close fitting polymer bearings on a pair of linear rails; the slightest misalignment of those rails will result in the system jamming, misalignment can arise during manufacture, assembly or as a result of deformation due to normal operating loads.

5. Friction, wear and life

Wear of parts in relative motion will cause a change in performance over time, or at worst a catastrophic failure. Designing with emphasis on bearing life for both metallic roller bearings and polymer bearings allows service intervals to be determined and life estimations to be made. When designing systems for long life with infrequent servicing, polymer bearings can offer a large advantage over their steel counterparts. Steel bearings rely on lubrication to limit or eliminate wear of metallic surfaces, as the lubricant becomes contaminated, friction increases and performance is reduced. Dry running polymer bearings are able to maintain the same friction characteristics over the course of their useable life.