What is motion control?

What is position control?

Moving a load to a defined, repeatable position

Position control moves an axis or mechanism to a specified linear position or angular position. Examples include moving a cutting stop to 250 mm, indexing a rotary table by 90 degrees, or feeding a drilling axis to a defined depth. In servo systems, the objective extends beyond turning the servo motor: the load must move within the required position tolerance. Position control is one part of motion control, which also covers velocity, acceleration and coordinated axis movement.

How does a closed position loop work?

The controller compares the instantaneous position setpoint generated during the move with the measured position. Their difference is the following error; it is not simply the remaining distance to the final target. In a common cascaded servo architecture, the outer position loop generates a velocity demand, the velocity loop generates a torque demand, and the inner current loop regulates motor current. These functions may be distributed between the drive and motion controller. Position controllers do not all use the same PID structure.

Control method and actuator technology are different

In open-loop positioning, a command is issued without correcting the actual position through feedback. A stepper motor is a common example: steps lost under excessive load can produce a position error. However, closed-loop stepper systems also exist. Electric, hydraulic and pneumatic describe the actuation technology. With suitable measurement and valve control, hydraulic or servo-pneumatic positioning can also be implemented. These terms therefore do not belong in a single list of mutually exclusive control methods.

Where is position measured?

A motor-mounted encoder measures the motor shaft. Gearbox backlash, coupling compliance or screw errors can leave the load in the wrong position even when the motor position is correct. A linear encoder can measure linear position at the load, but the control architecture for this additional feedback needs appropriate design. An absolute encoder does not automatically establish the machine zero. The home reference relates encoder position to the machine coordinate system; retaining that reference depends on the capabilities of the complete system.

Absolute targets, relative moves and command interfaces

From an initial position of 100 mm, an absolute target of 140 mm and a relative move of 40 mm can reach the same endpoint. Incorrect starting coordinates or reference information change the result. With pulse/direction commands, the pulse count is converted into a position demand according to the configured electronic gearing. Sending pulses does not prove that the load reached its target. Networks such as EtherCAT can exchange position, velocity and status data cyclically; industrial communication timing matters when coordinating multiple axes.

Scaling pulses, revolutions and millimetres

Consider a motor directly coupled to a screw without a gearbox. The screw lead is 5 mm per revolution, and the drive's effective command scaling is 10,000 pulses per revolution. The theoretical command increment is 0.0005 mm per pulse, so a 20 mm move requires 40,000 command pulses. This command pulse count is distinct from the encoder feedback count. Gear ratios, pulse evaluation and drive scaling must all be accounted for.

Motion profiles and gain adjustment

Velocity, acceleration, deceleration and jerk limits define how the load moves; jerk is the rate of change of acceleration. Abrupt profile changes can excite vibration in compliant mechanisms. Increasing servo loop gain does not always improve performance: excessive gain can cause oscillation, while insufficient gain can allow large following errors. Check mechanical connections and scaling first, then evaluate performance under load using the manufacturer's recommended tuning procedure.

How is arrival at the target confirmed?

The next operation should not depend solely on a move command having been issued. Evaluate move-complete status, the in-position signal, position tolerance, dwell time and fault status according to the drive or controller definitions. An inductive sensor used for homing does not by itself provide continuous position measurement. In a drilling machine, the positioning axis controls feed depth while spindle speed is a separate variable. Pressing and clamping may require force or torque control in addition to positioning.

Practical checks when position is incorrect

Start with reference validity, movement direction, mechanical transmission and engineering-unit scaling. Then compare commanded and actual position, following error, motor current and changes in load. Software travel limits and an ordinary in-position indication do not, by themselves, constitute safety functions. Emergency stopping and protective functions require separate consideration within the machine's required safety architecture.