Servo Drives and Motion Control: What to Learn, From Tuning to Electronic Cams

Motion control training teaches you to control the position, speed or torque of a machine axis precisely, usually with a servo motor, a servo drive and a PLC or motion controller. The core skills are understanding the drive's control loops, commissioning and tuning an axis, and programming moves, gearing and electronic cams.

Servo, stepper or VFD?

Not every moving axis needs a servo. The right choice depends on whether position matters and what happens if the load fights back.

A VFD varies the speed of an induction motor and suits pumps, fans and conveyors where speed matters more than position; our VFD start-up walkthrough shows a first commissioning. A stepper moves a fixed angle for each pulse and is cheap and simple for light indexing, but if it is overloaded it can miss steps without the controller knowing. A servo measures its actual position with an encoder or resolver and corrects continuously, so it can hold position against a load, follow complex profiles and report when it cannot keep up. Packaging machines, pick-and-place, printing, cutting and CNC axes use servos for that reason. Our PACK EXPO 2026 preview shows where packaging motion is heading, including linear motor transport.

In this short lesson by AutomationDirect.com, from our free servo course, different servo system options for motion control are compared. Use it alongside the table above when deciding what an axis needs.

How a servo loop works

A servo drive runs three control loops, one inside another:

• Current (torque) loop, the innermost and fastest. It controls the motor current, and so the torque.
• Velocity loop. It compares the actual speed with the demanded speed and asks the current loop for torque.
• Position loop, the outermost. It compares the actual position with the commanded position and asks the velocity loop for speed.

The difference between commanded and actual position is the following error. It is the single most useful number in motion work: if it grows, the axis is undersized, mistuned or mechanically binding. Feedforward terms let the drive act on the commanded motion before an error builds up, which is how a well-tuned axis follows a profile closely.

Commissioning an axis

Sizing comes first. The motor must deliver the peak and continuous torque the motion profile needs, and the ratio between load inertia and motor inertia affects how well the axis can be tuned. Vendors publish sizing tools that do this from the mechanics and the profile.

Wiring covers motor power, the feedback cable, the holding brake if fitted, and the safety input for safe torque off. Keep feedback and power cables separated and use the shielding the manufacturer specifies; many servo faults are noise faults.

Configuration sets the units the program works in (millimetres, degrees), the scaling from motor revolutions through the gearbox and pulley to those units, and the software travel limits.

The hook-up test runs the motor a short distance to check that positive command gives positive movement and that feedback counts in the same direction. Only then is it safe to move under position control.

Homing establishes where zero is, using a home switch, a hard stop or an encoder marker. An absolute encoder can keep its position through power loss.

Tuning usually starts with the drive's autotune and is refined by moving the axis and watching position, velocity and following error on a trend.

Programming motion

Most controllers now use the same ideas, often following the PLCopen motion control function blocks: enable the axis (MC_Power), home it (MC_Home), move it (MC_MoveAbsolute, MC_MoveRelative, MC_MoveVelocity) and stop it. Siemens technology objects in TIA Portal use MC blocks of this kind. Rockwell's Logix controllers use their own instructions, such as MSO to enable, MAH to home, MAM to move and MAG to gear.

Three techniques go beyond single moves:

• Electronic gearing locks a follower axis to a master at a fixed ratio, replacing a mechanical gearbox or line shaft.
• Electronic camming makes the follower's position a function of the master's position through a table. Changing products becomes a software change instead of a new mechanical cam.
• Registration captures the exact axis position when a sensor sees a print mark or product edge, so the next move can correct for it. It relies on fast, hardware-latched inputs rather than the PLC scan.

Fillers, labellers and case packers in food and beverage automation rely on all three.

Coordinated multi-axis motion, where axes move together along a line or arc, underlies gantries, robots and CNC machines. CNC programming in G-code is a specialism of its own, but the motion concepts carry over.

Networks and safety

Servo drives are usually connected over a deterministic motion network: EtherCAT, PROFINET IRT, EtherNet/IP with CIP Motion, or SERCOS III. These deliver setpoints to every axis on a fixed, synchronised cycle.

Drive-based safety functions are defined in IEC 61800-5-2. The one you will meet on every machine is safe torque off (STO), which prevents the drive from producing torque. Others include safe stop 1 (SS1) and safely limited speed (SLS). STO is not electrical isolation: the drive can still be live, and maintenance work needs proper isolation. For how drive safety functions fit with safety PLCs and SIL ratings, see safety PLCs, SIL and SIS explained.

Common commissioning problems

Most first-day motion faults have ordinary causes:

• The axis runs away or faults immediately. Motor direction and feedback direction disagree; the hook-up test exists to catch this.
• Following error faults at speed. The acceleration is higher than the motor can deliver, the tuning is too soft, or the mechanics bind.
• The axis hums or whines at standstill. Gains are too high, or the load is coupled through a flexible belt or long shaft.
• Position drifts after a power cycle. The axis has an incremental encoder and was not re-homed.
• Random feedback faults. Cable shielding, earthing or routing next to motor power cables.

A learning order

1. A VFD first, if you are new to drives: power wiring, parameters, control sources and fieldbus control.
2. Servo fundamentals: loops, wiring, homing, tuning and the standard move blocks.
3. One platform in depth, the one your target employer uses: Rockwell Kinetix with Studio 5000, Siemens SINAMICS with TIA Portal, or Beckhoff with TwinCAT and EtherCAT.

Free motion control training courses

Servo Drives and Motion Control covers what a servo system is and how to wire, mode, home, tune and network one, with motion programming in Studio 5000 and TIA Portal technology objects. Then go deeper with Allen-Bradley Kinetix Servo Motion for gearing, registration and cams, Siemens SINAMICS Drives for G120 and S120 commissioning and drive safety, or Beckhoff TwinCAT 3 for PC-based control. See every drive course on the drives and motion page; if you need a VFD start, the VFD course is there too.

Every course is free in full, with credited video lessons, notes, practice tasks and one final assessment. The optional EDWartens Certificate of Completion for these intermediate courses is a small one-off fee, and checkout shows your price. Anyone can check it at edwartens.com/verification. It is not a Rockwell, Siemens or Beckhoff certification and is not accredited. Create a free account to start.

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