Back to basics: The motion-system design process

In this back-to-basics primer, we outline the essential elements of an automated motion system … whether it’s a piece of robotics or machine design dedicated to a specific routine.

All motion systems are electric-motor-driven designs built around two kinds of components:

• Mechanical power-transmission components — including gears and gearsets, kinematic linkages, belt and pulley sets, chain drives, linear screws, rotary bearings, slides, guides, and more.
• Electrical and electronic motion-control components — including electric motors, motor drives, the electronics known as motion controllers, sensors, pneumatics (which after all rely on electrically compressed air for operation), and connectivity components such as cables, network switches, power supplies, and more.

The motion industry often uses the term power-transmission designs to indicate machine builds for which there’s emphasis on raw torque (or force) and speed — and domination of the build by the mechanical components.

The motion industry often uses the term motion systems to indicate machine builds for which there’s emphasis on positioning and controlled motion profiles defining the speeds, accelerations, jerk, speed-torque profiles, and homing routines of the machine’s various axes.

Both design types are at the heart of myriad consumer products, office automation equipment, medical devices, laboratory automation equipment, and (in the industrial space) discrete factory automation. In fact, this is yet another distinction to be made. Discrete factory automation differs from the other type of factory automation — that associated with process or analog automation.

Discrete automation (not to be confused with discrete motion control) involves the working, machining, assembling, testing, capping, wrapping, packaging, labeling, counting, scanning, and transporting of separate (discrete) workpieces or other individual products. Each workpiece might be subject to several different operations before completion. Automotive manufacture, electronics production, automated warehousing operations, and packaging are all discrete automation industries.

In contrast, process automation involves vats of liquids, uninterrupted systems of fluids, and other batches subject to continuous chemical reactions, temperature controls, metering, and flowing operations that are generally difficult to stop and start. Process automation industries — in which these types of operations dominate — include mining, energy (via both coal and nuclear), oil and gas, food processing, pharmaceutical and chemical manufacture, glass and ceramic production, plastics, textiles, and wood and paper.

Motion system designs are associated with discrete automation — to deliver on the precise placement and transport of discrete workpieces or other individual products.

Parameters pertinent to the motion system design process

Automated motion designs begin with documentation of the loads to be moved and inertias involved, along with the friction the motorized axis will need to overcome without introducing undue sluggishness.

Recall that inertia is an object’s resistance to speed changes — accelerations or decelerations. Total inertia includes that of the workpiece load as well as the axes’ moving parts. This inertia can be quite low in the case of a torque motor (direct-drive motor) or linear motor having no intermediate mechanical components.

It’s higher anywhere an axis’ electric motor is fitted with a gearset for speed reduction … and (in some instances) servotuning through inertia matching. Inertial values vary for axes involving belt-and-pulley, rack-and-pinion, or chain-and-sprocket sets — which are the so-called tangential forms of power transmission. Inertial values also vary for linear actuators incorporating one of the three main linear-screw types used in motion — leadscrews, ballscrews, and roller screws. Various calculations essentially quantify how the attached inertia affects the system at the very critical location of the axis’ motor-output shaft.

In short, engineers tasked with designing motion systems should at this stage sum all the axis components’ inertias and what’s reflected back to the axis’ motor shaft. One motion design that requires additional analysis is that involving a changing inertia — such as robotic axes executing dispensing or machining tasks. Here, the motion controls may need to include additional servotuning to compensate for the changing dynamics.

The next step in motion system design is (after the required mechanisms and their friction and inertia are quantified) to define require motor output force (in the case of a linear motor) or (more commonly) torque. That allows proper sizing of the electric motor on each axis of the design. Besides the defined acceleration torque, a servo axis’ motor must deliver sufficient torque over the axis’ whole move — including any constant torques during run phases and deceleration torques during stopping.

Motor and motion controls in motion designs

Various motor actions are commanded by system motor controls or motion controls — two technologies that have seen convergence over the last two decades. Both types of electronics accept low-power signal that inform their prompting of actuation-level electrical output to drive the involved motors. In fact, if we use a zoological analogy to describe automated motion systems, motion controllers serve as the system’s brains to monitor, command, and integrate:

• Appendages and axis muscles — moved by electric motors and drives, actuators, and attached mechanical components.
• Senses — in the form of industrial sensors and other encoder and even machine-vision feedback.
• Informational input from the external environment to inform adjustments — from machine operators as well as IIoT networks communicating enterprise-level order and throughput requirements.

Just like the nervous systems of animals, motion controllers can work to command systems arranged for:

• Centralized control (like the human body) with most all components networking back to the main controller.
• Distributed control systems using (like the diffuse neurological structures in an octopus) engineered cabling and electronics integrated into axis components to execute logic at the periphery.

In fact, choosing between centralized controls (with controls, motors, drives, and complementary electronics housed together in a cabinet) or distributed drive-based controls is a choice that’s often made during reiterative design processes. Machines with many axes over a large area may benefit most from decentralized motion control — as that can trim wiring issues and costs plus make for a smaller control cabinet. In contrast, compact machine installations may benefit from the economies associated with traditional centralized control.

No matter the control architecture, each motor’s drive (motor control or amplifier) must have the ability to provide acceleration and continuous current sufficient to satisfy the motion design’s duty-cycle requirements. This includes power to both move the load and dissipate heat to slow the load … ultimately dependent on rms current or acceleration current while accounting for any increase in the motor’s winding resistance due to heat.

The special case of servosystems

Motion system designs that take the form of closed-loop systems (servosystems) tend to be dynamic machines involving quick accelerations and decelerations and operation in all four design quadrants — an industry concept indicating a design controls an axis over all positive and negative torques and speeds. Sizing such servosystems necessitates a systems approach that concurrently accounts for mechanical, electrical, and software parameters. The drives supplying power to axes’ servomotors must be capable of delivering all necessary continuous (rms) and acceleration (peak) currents.

Motion system design environmental considerations

Where a motion design will operate in hot, dusty, washdown, vacuum, explosive, or other challenging industrial environments, the power-transmission, electromechanical, and electronic components should be chosen for their ruggedness. Ratings specifically list whether a given component can withstand the anticipated conditions …

Component features for washdown environments
IP ratings versus NEMA ratings
Motion-component features for cleanroom environments

Feedback and connectivity for motion designs

Many motion system designs rely on sensors and encoders to track position, velocity, and other axis values during machine operation. Here, Hall-effect sensors, resolvers, incremental and absolute encoders, and various forms of mechanical, optical, and magnetic feedback devices track axis behavior. Then these devices convert their collected signals into (in many cases) digital signals for the motion controls to employ in their trajectory (and correction) calculations. The accepted communication protocols of the controls partially dictate which feedback devices are allowable in the system …

Connectivity and IoT in motion and general automation

Though beyond our scope here, it’s worth noting that the cables and connectors transmitting power and data between components must also satisfy specific electrical and physical requirements. Cable runs should be kept as short as possible to avoid issues associated with voltage drop, impedance, and (especially if pulse-width modulated or PWM drives are nearby) electromagnetic interference or EMI. That’s especially true if feedback data rides over high-speed protocols vulnerable to impedance and noise.

Quick note on motion profiles

Supreme in the world of motion systems are motion profiles — the mathematically defined trajectories that motion controllers command axis drive-and-motor systems to follow. Many motion profiles define sequences of position commands over time to tell the axis how and where to position attached load … as well as the necessary accelerations, move times, and dwell times. Here, motion-controller electronics also convert created trajectories to generate torque commands to be used by the drive powering the motor …

What is a motion profile?
What’s the difference between accuracy and repeatability?

Of course, a design’s required accuracy, resolution, and repeatability are just as important as its motion profile. These measures of precision dictate the level of mechanical backlash and compliance are acceptable and what feedback components may be necessary.