Motion Control Basics

The parts of a motion system

Every motion axis, from a 3D-printer carriage to a packaging line, is the same chain of parts. Knowing what each does makes a fault or a sizing problem easier to place. This guide follows the chain from the load back to the controller: Mechanical Transmission: Screws, Belts and Gearboxes and Choosing a Motor Technology first, then drives and Encoders, Resolvers and Feedback, and finally Sizing a Motion Axis and Braking, Safe Stopping and Troubleshooting.

Part	Job	Typical devices
Controller	Generates the motion command: positions, speeds, sequences, synchronization	PLC with motion instructions, motion controller, CNC, microcontroller, PC
Drive (amplifier)	Turns the low-power command into the current or voltage the motor needs; closes the fast loops	Servo drive, stepper driver, variable-frequency drive, H-bridge, brush-motor controller
Motor	Converts electrical power to torque and speed	Servo, stepper, induction, brushed DC, linear motor, voice coil
Transmission	Matches motor speed and torque to the load and changes rotary motion to linear	Gearbox, ball screw, lead screw, belt, rack and pinion, coupling
Load and guides	The thing being moved, and the bearings that constrain it	Slide, table, conveyor, arm, linear guide rails
Feedback	Reports position and speed so the system can correct itself	Encoder, resolver, Hall sensors, tachometer, linear scale

Open loop and closed loop

	Open loop	Closed loop
How it works	The controller sends commands and assumes the motor follows	A feedback device reports the real position or speed and the drive corrects the error
Typical motor	Stepper, or an induction motor on a basic drive	Servo, closed-loop stepper, vector-controlled induction motor
Cost and wiring	Lower: no feedback device	Higher: encoder, more wiring, a more capable drive
If it is overloaded	Loses position silently (a stepper skips steps)	Reports a following-error fault or pushes harder, up to its limit
Best when	Loads are known and well within the motor’s capability	Loads vary, speed must be held, or position must be guaranteed

The choice is not only about accuracy. A closed loop also gives you information: a drive that sees the load pushing back can raise an alarm before a part is scrapped, which an open-loop system cannot. See Stepper Motors and Drivers for the limits of open loop and Servo Systems and Tuning for how the loops work.

Speed, torque and power

ω = 2π × n ÷ 60    P = T × ω    T = 9.549 × P ÷ n

ω is angular speed in radians per second, n is speed in revolutions per minute, T is torque in newton meters and P is power in watts. The constant 9.549 is 60 ÷ 2π, so a motor producing 1 kW at 1 500 rpm delivers 9.549 × 1 000 ÷ 1 500 = 6.37 N·m.

Quantity	SI unit	Convert
Torque	N·m	1 N·m = 8.851 lbf·in = 0.7376 lbf·ft = 141.6 oz·in
Speed	rad/s	1 rpm = 0.10472 rad/s; 1 rev/s = 60 rpm
Power	W	1 hp = 745.7 W; 1 W = 1 N·m/s
Inertia	kg·m²	1 kg·m² = 10 197 kg·cm² = 5 467 oz·in²
Linear speed from rotation	m/s	v = ω × r for a wheel; v = n × lead ÷ 60 for a screw

Motion profiles

A motion profile says how speed changes during a move. The profile you choose sets the peak speed, the peak acceleration and therefore the torque the motor must deliver. The most common is the trapezoid: accelerate, cruise, decelerate. For a move of distance d in total time T with ramps of time t_a each:

v = d ÷ (T − t_a)    a = v ÷ t_a

From the load to the motor: torque and RMS

The motor must supply torque to accelerate the total inertia, overcome friction and hold any weight: T = J × α + T_friction + T_gravity, with J the total inertia seen by the motor (kg·m²) and α the angular acceleration (rad/s²). Two ratings matter: the peak torque (available for short bursts, limited by the drive current and magnet saturation) and the continuous (RMS) torque (limited by heating). The RMS torque over a cycle is the square root of the time-weighted mean of the squared torques: T_rms = √(Σ T_i² t_i ÷ T_cycle).

The four quadrants

A drive can run a motor with torque in the same direction as speed (motoring) or opposed to it (braking). Plotting speed against torque gives four quadrants. Most simple drives only work in one or two. Applications with vertical loads, fast decelerations or overhauling loads need the braking quadrants, where energy flows back from the load to the drive, and the drive must have somewhere to put it: a braking resistor, a regenerative front end or a shared DC bus (see Braking, Safe Stopping and Troubleshooting).

A design checklist

• Define the move: distance, time, cycle rate, payload and its orientation (horizontal, vertical, rotary).
• Pick a profile that meets the time with the lowest peak acceleration, and compute v and a.
• Reflect the load through the transmission: inertia, friction and gravity at the motor shaft.
• Compute the peak and RMS torque and the speed they occur at; add a margin (typically 20 to 30 percent).
• Check the inertia ratio of load to motor (see Servo Systems and Tuning and Mechanical Transmission: Screws, Belts and Gearboxes).
• Choose the motor technology (Choosing a Motor Technology) and a drive with the right current, voltage and feedback.
• Plan the stopping: brakes, safe torque off and the energy of a stop (Braking, Safe Stopping and Troubleshooting).

Common mistake	Result	Fix
Sizing for cruise torque only	The motor stalls or faults during acceleration	Include J × α and use the peak torque at speed
Ignoring the speed at which torque is needed	Rated torque exists only below base speed	Check the torque-speed curve, not just the catalog torque
Using peak torque as continuous	The motor overheats	Compare the RMS torque with the continuous rating
Forgetting reflected inertia of the load	Overshoot, oscillation, poor tuning	Reflect through the transmission and check the ratio
No margin	Works on the bench, fails at temperature or with wear	Add 20 to 30 percent, then test hot

Related

• Motion Control and Actuators: the overview of this guide.
• Choosing a Motor Technology and Sizing a Motion Axis: choosing and sizing the motor.
• Motor Torque and Inertia Matching Calculator and the Gear Ratio Calculator.
• Motors, Drives and PID Control in the PLC guide: wiring a drive to a PLC.
• Gears and Belts and Pulleys.

FAQ

What is the difference between peak and continuous torque?

Peak torque is the most the motor can produce for a short time (seconds or less), limited by the drive current and the magnetic circuit. Continuous torque is the most it can deliver indefinitely without overheating. Size for the peak at the right speed and for the RMS against the continuous rating.

Why use an S-curve?

A trapezoid has sudden changes of acceleration, which excite resonances in the frame, the coupling and the load. An S-curve rounds them, giving smoother motion and less settling time at the cost of a slightly higher peak acceleration.

Do I always need feedback?

No. A stepper or an induction motor running a known load is often fine open loop. Add feedback when the load varies, when a lost position is costly, or when you need speed regulation or fault information.

How much margin should I add?

Commonly 20 to 30 percent on torque and speed, more when the load is uncertain, the duty is severe or the temperature is high. Test with the real mechanism before relying on the numbers.

References

• Sciavicco, L. and Siciliano, B., Modeling and Control of Robot Manipulators, Springer: motion profiles and dynamics.
• Lyshevski, S. E., Electromechanical Systems, Electric Machines, and Applied Mechatronics, CRC Press.
• Motor manufacturers’ sizing guides (Parker, Kollmorgen, Siemens, Yaskawa) for the load-torque method and margins.

Last reviewed: October 2026. Values are worked examples; use the motor and drive datasheets for the real ratings.