Servo and Motion Control Basics for PLC Engineers

Servo versus VFD, in one paragraph

A VFD spins a motor at a commanded speed and does not generally know or care where the shaft is. A servo system knows exactly where the shaft is at all times, because it has a feedback device, and it closes a loop to hold or move to a commanded position.

Speed control: VFD. Position control: servo. That distinction is the whole of it, and it decides which job an advert is describing.

The parts of a servo system

• Servo motor. Usually a permanent magnet synchronous motor, built for rapid acceleration and low inertia.
• Feedback device. An encoder or resolver on the motor shaft, reporting position continuously.
• Servo drive (amplifier). Closes the control loops and delivers current to the motor.
• Controller. Often the PLC itself on modern platforms, issuing motion commands over a network.

The loops are nested: a position loop outside a velocity loop outside a current loop. Each runs faster than the one outside it. Tuning is usually done from the inside out.

Encoders: incremental versus absolute

Incremental encoders output pulses as the shaft turns. Count the pulses and you know how far it has moved, but not where it started. Power off, and the position is lost.

Absolute encoders report an actual position value. Multi-turn absolute encoders remember position through a power cycle, usually with a battery or a mechanical gear count.

This is why homing exists.

Homing, and why it comes first

With an incremental encoder, the machine has no idea where the axis is at power-up. Before any motion, it must find a known reference: drive slowly towards a home sensor or a hard limit, find the switch, often then find the encoder's marker pulse for precision, and set that position as the datum.

Everything afterwards is relative to that datum. Which means:

• A machine that has not homed must not run a positioning move. Interlock it.
• A bad home sensor produces an axis that is confidently wrong, which is more dangerous than an axis that refuses to move.
• Absolute encoders remove the need to home, which is a large part of why they cost more.

Homing routines are a common first task given to a junior engineer, and getting one wrong is a common first mistake.

Motion vocabulary that appears in adverts

• Point to point. Move to a position and stop.
• Interpolation. Two or more axes moving in coordination so the tool follows a path. Linear and circular interpolation.
• Electronic gearing. One axis follows another at a ratio, replacing a mechanical gearbox.
• Camming. One axis follows another according to a profile, replacing a mechanical cam. Very common in packaging.
• Jerk limiting / S-curve. Smoothing acceleration changes to reduce mechanical shock.
• Registration. Adjusting a move based on a sensor seeing a printed mark. Standard in film and label handling.

Camming and registration in particular are worth naming on a CV if you have touched them. They are exactly what packaging machine builders search for.

Safe torque off

Servo drives normally provide STO, a hardwired safety input that removes the drive's ability to produce torque without cutting the main power. It lets a machine stop safely and restart quickly without a full power cycle.

STO is a safety function and is wired accordingly, not commanded over the network. See emergency stop and safety circuits.

Tuning, briefly

Tuning means adjusting gains so the axis follows its command accurately without oscillating. Modern drives auto-tune reasonably well as a starting point, and the residual work is usually:

• Too soft: the axis lags its command, following error grows, positioning is slow.
• Too stiff: the axis oscillates, buzzes audibly at standstill, or overshoots.
• Following error is the measured difference between commanded and actual position. Most drives will fault if it exceeds a limit, which is a useful early warning of a mechanical problem: a stiffening bearing shows up as growing following error before it shows up as a breakdown.

Where this sits in a career

Motion control is a specialisation within automation and it pays for itself. Machine builders, packaging, printing, converting, textiles and electronics assembly all need it, and far fewer engineers are comfortable with it than with basic PLC work.

If you are choosing what to go deep on after the fundamentals, motion is one of the better bets, alongside robotics and process control.

Learn it on real axes

Motion is unusually hard to learn from a simulator, because half of it is mechanical behaviour. On the Automation Engineer Program you work with real servo and drive systems as part of the hardware phase.

Related: VFD parameter setup, Fanuc robot programming for beginners.