Motion Control

What is Motion Control?

Motion Control is the branch of industrial automation that precisely controls the position, speed, acceleration and sometimes the force of moving machine parts using motors, drives and feedback. A motion controller calculates where each axis must be at every moment, a servo amplifier drives the motor, and an encoder reports back where the axis actually is. This allows machines to achieve repeatability down to micrometres with cycle times of a fraction of a millisecond.

Motion Control is essential in applications such as robots, Pick-and-place machines, CNC machines, packaging lines and sortation systems.

🎯 What does Motion Control do?

• Positioning - moving an axis from point A to point B and holding it there, for example a rotary table or linear slide
• Controlling speed and force - maintaining a constant web speed or a limited torque, as in winding or pressing
• Following motion profiles - accelerating and decelerating along a calculated profile so the mechanics are not stressed unnecessarily
• Correcting through feedback - a feedback loop with encoders continuously eliminates deviations between commanded and actual position
• Synchronising - aligning multiple axes or machines exactly, through electronic gearing, electronic cams or path interpolation

🔧 Which components make up a motion system?

The system typically consists of a Controller, sensors, actuators, drives and software that coordinate movements precisely.

A stepper motor often runs open loop: the controller counts steps without any feedback. That is inexpensive, but under overload the motor can lose steps without anyone noticing. A servo system always runs closed loop and detects such a deviation immediately as a following error.

There are two main types of feedback device: the encoder (optical or magnetic, incremental or absolute single-turn or multi-turn) and the resolver, a rugged rotary transformer with no electronics inside the motor. Modern servo motors use digital encoder protocols such as EnDat, BiSS or HIPERFACE DSL, often over a single cable together with the motor power.

🔄 How does cascade control work in a servo axis?

A servo axis is controlled by a cascade of three nested control loops. Each inner loop has a bandwidth 5 to 10 times higher than the loop around it, so the outer loop can treat the inner one as nearly ideal.

In practice, then, this is not a single classic PID controller but a combination of P and PI controllers supplemented by feedforward of velocity and acceleration. Feedforward outputs the expected torque in advance, so the controllers only have to correct the remainder and the following error stays small. The current and velocity loops almost always run inside the drive itself; the position loop sits in either the drive or the motion controller, depending on the architecture.

📈 Which motion profiles and forms of synchronisation exist?

The motion controller converts a desired move into a series of setpoints per cycle, known as the motion profile:

• Trapezoidal profile - constant acceleration, constant velocity, constant deceleration. Simple and fast, but the step change in acceleration (infinite jerk) excites vibration.
• S-curve - also limits jerk, so transitions are gradual. Slightly slower, but with less vibration, less wear and shorter settling time.
• Interpolation - several axes jointly follow a path: linear, circular or spline-based. This is the basis of CNC machining and robot paths.
• Electronic gearing - a slave axis follows a master axis at a fixed velocity ratio, without a mechanical gearbox.
• Electronic camming - the slave follows the master via a cam table; a classic for flying saws, rotary knives and packaging machines.

For multi-axis coordination, as in robotics, a kinematic model is added that converts joint angles into a tool position in space.

🧱 How do you program motion according to PLCopen?

PLCopen Motion Control standardises function blocks on top of IEC 61131-3, making motion programs less dependent on a single vendor. Part 1 (version 2.0 from 2011, into which Part 2 was merged) defines blocks such as MC_Power, MC_Home, MC_MoveAbsolute, MC_MoveVelocity, MC_Stop, MC_GearIn and MC_CamIn, plus a fixed state machine per axis. There are also Part 3 (user guidelines), Part 4 (coordinated motion and robot kinematics; version 2.0 was released in 2026), Part 5 (homing) and Part 6 (fluid power axes).

At drive level, IEC 61800-7 (2007) standardises four drive profiles: CiA 402, CIP Motion, PROFIdrive and SERCOS. This lets a controller command drives from different manufacturers in a comparable way.

🌐 Which real-time networks are used for motion?

Motion requires deterministic behaviour: in every cycle all axes must receive their setpoint at exactly the same moment. Low jitter matters even more than low latency. Ordinary fieldbuses and standard Ethernet are therefore not sufficient; motion uses real-time industrial Ethernet.

In addition, TSN is gaining ground: it enables deterministic traffic on standard Ethernet and can combine motion and IT traffic on a single network.

🔐 Which safety functions do servo drives offer?

Moving axes are a major source of hazards. IEC 61800-5-2 (edition 2, 2016) describes the functional safety functions that a drive can perform itself, up to and including SIL 3:

• STO (Safe Torque Off) - safely removes torque; corresponds to stop category 0 in IEC 60204-1
• SS1 (Safe Stop 1) - controlled deceleration followed by STO (stop category 1)
• SS2 / SOS - deceleration to a monitored standstill with torque maintained (stop category 2)
• SLS (Safely Limited Speed) - monitors a safe maximum speed, for example during setup
• SDI, SLP and SBC - safe direction, safely limited position and safe brake control

These safety functions are commanded by a safety PLC or safety relay, often through a safety protocol over the same network (such as FSoE, PROFIsafe or CIP Safety). Validation follows ISO 13849 or IEC 62061. From 20 January 2027 the Machinery Regulation (EU) 2023/1230 applies, which also sets requirements for protecting control systems against corruption, including cybersecurity.

🏭 How do you approach a motion application in practice?

1. Define the motion profile - stroke, cycle time, mass and required accuracy per axis.
2. Size motor and drive - calculate peak and RMS torque and an inertia ratio between load and motor that typically stays below about 10:1.
3. Choose network and cycle time - often 1 ms for packaging and handling, 250 µs or less for CNC and highly dynamic applications.
4. Carry out the risk assessment - determine which safety functions (STO, SS1, SLS) are needed and which PL or SIL is required.
5. Tune the control loops - from the inside out: current first, then velocity, then position, then feedforward.
6. Secure the network - separate the motion network from the office network according to IEC 62443 and restrict who may change drive parameters.

Typical applications:

• Robots and cobots, typically with six coordinated axes
• Packaging machines and palletisers
• CNC milling, turning and cutting machines
• 3D printers and laser cutters
• Vision-guided pick & place
• High-speed labelling and dosing systems

✅ What are the benefits of Motion Control?

• Highly accurate and reproducible motion, down to micrometres
• Higher speed and efficiency in production thanks to shorter cycle times
• Less mechanical wear through S-curves and by replacing mechanical cams and gears
• Fast changeovers - a new format is a different cam table rather than a mechanical rebuild
• Improved product quality and safety through safety functions integrated in the drive

❓ Frequently asked questions

What is the difference between a servo motor and a stepper motor?

A servo motor always operates with feedback from an encoder or resolver and continuously corrects deviations, which allows high speeds and dynamics. A stepper motor turns in fixed steps and usually runs without feedback. Stepper motors are cheaper, but under overload they can lose steps unnoticed.

What is following error in Motion Control?

Following error is the difference between the position commanded by the motion controller and the position measured by the encoder. A small following error indicates well-tuned control with feedforward. If the following error exceeds a configured limit, the drive stops the axis with a fault.

What is Safe Torque Off (STO)?

Safe Torque Off is a safety function from IEC 61800-5-2 that safely interrupts the energy supply to the motor so that it can no longer produce torque. STO corresponds to stop category 0 and often replaces a contactor between drive and motor. An axis loaded by gravity also needs a safe brake.

Why does Motion Control use EtherCAT or PROFINET IRT instead of ordinary Ethernet?

Motion Control requires all axes to receive their setpoints at exactly the same moment in every cycle, with jitter that must stay below one microsecond in highly dynamic applications. Standard Ethernet does not guarantee a fixed delivery time and can delay packets. EtherCAT, PROFINET IRT and SERCOS III deliver deterministic cycles with minimum cycle times of 12.5 µs (EtherCAT) and 31.25 µs (PROFINET IRT and SERCOS III).

What is PLCopen Motion Control?

PLCopen Motion Control is a specification of standardised function blocks, such as MC_MoveAbsolute and MC_GearIn, for programming motion in IEC 61131-3. As a result, motion programs on different PLC platforms look very similar. Many vendors, including environments based on CODESYS, support these blocks.

What is electronic camming?

Electronic camming is a form of Motion Control in which a slave axis follows the position of a master axis via a software cam table. It replaces mechanical cam discs and enables format changes simply by loading a different table. Typical applications are flying saws, rotary knives and packaging machines.

📌 In summary

Motion Control automates and optimises the movement of machines and components, with high precision and reliability - through cascade control in the servo drive, standardised function blocks, real-time networks with microsecond synchronisation and integrated safety functions such as STO.

Motion Control is therefore the fastest layer of machine control and indispensable for robots, CNC and packaging lines. As it becomes linked to Industry 4.0 applications such as condition monitoring and digital twins, securing drives and motion networks is becoming increasingly important.