Motor control loop

All movement and extrusion in 3D printer are done by electric motors.

These motors need to deliver a precisely known displacement or rotation.

There is different possibilities to obtain such precision.

An ensemble of a physical device, its control and power driver and eventual sensor system is called a loop.

It is a generic denomination which apply to any system, being it a unit generating a movement, a heater system, light generator or anything producing a physical effect.

An open loop is a system where an order to a device is sent without controlling the effective execution of this order.

If a feedback sensor is installed, the control system can adjust its command to have the device effectively comply with the order. This is called a 'closed loop system'.

While it does exist linear motors, all 3D printers are using rotating motors.

Sending orders to the motors

In the 3D printer world, orders are send in the form of requirement movements steps, which are calculated by a movement unit. It does exists others protocols where more simple orders of position or movement with acceleration and speeds are sent to the drivers, which are then doing detailed calculation to directly command the motor.

We will only expose systems where orders are sent with two signals called 'step' and 'direction'. A change in step signal give the order to move of a step (or fraction of step) , while the direction signal decide of the movement direction.

Simple stepper control

The most frequently used system is an open loop unit driving a stepper motor. Bipolar steppers with two coils are used in 3D printers. Change in current direction in the coils of a stepper makes the motor moving one step. controlling the relative current one one coil and the other allow a finer control and this is managed by dividing main steps in what is called 'microsteps'. It can give a precise positioning system without having any measurement of the effective position. The wiring is simple as only one cable of 4 wires is needed to command the motor. The torque of a stepper depends from its speed, but in open loop system, the current is generally set constant, with only sometimes reduction when idling.

'Hybrid' stepper control

This is operating with steppers like in simple stepper control, but the stepper is equipped with a sensor giving information about the effective movement. The sensor also divide the movement in steps, but at a higher resolution than the stepper resolution. Having this feedback allow constant current adjustment and sending power only when it is really required. Most of the time, the required torque is much lower than the nominal torque so the stepper motor is less noisy and heat less. This is commonly used in CNC world but in 3D printer world, this kind of setup is extremely rare.