Motor Types#
CNC machines use stepper motors for precise movement of each axis. The stepper motors are able to turn a fraction of a degree at the instruction of the CPU.
Here you can see side by side an open-loop motor and a closed-loop motor.
Stepper motors contain 2 electromagnetic coils. The coils are energized in sequence, which in turn rotates the shaft one step at a time.
The coils require high current and high voltage (compared to logic-level signals that run at 5 volts). The actual voltage depends on the motor size, with smaller motors needing 12 volts of electricity and larger motors using 24, 48 volts, or even higher.
The coils are connected to a device called “stepper driver”. It receives signals from the CPU about how many steps to rotate and in which direction. Based on the step signals, it modulates the voltage it gets from the power supply and sends it to the motor coils.
There are two main types of stepper motors used in CNC machines - open-loop and closed-loop.
Open-loop motors#
When open-loop motors are used, the stepper drivers are located at the base of the machine, with cables running through the drag chains to reach each motor.
There is one cable for each motor, with each cable containing 4 wires - 2 for each coil.
Because the wires carry relatively high voltage, they are more resistant to electrical interference.
Closed-loop motors#
With closed-loop motors, the driver is part of the motor. The motor has an encoder that measures the shaft orientation and feeds it back into the driver. This way the driver is notified when the motor has lost a step, and can try to recover. This feedback loop is why these motors are called “closed loop”.
Closed-loop motors require 2 cables each - one for power and one for control signals. The power cable uses 2 thick wires directly connected to the power supply.
The signal cable has 4 thinner wires at the minimum, 2 for the step counter and 2 for the direction.
There could be 2 additional wires to send a signal back to the control box when there is an alarm condition (if the motor fails to recover from a lost step).
There could be 2 additional wires for disabling the motor completely when the machine is idle. They are often omitted because closed-loop motors are efficient at holding their idle position.
Due to their relatively low cost and because of their higher efficiency, the closed-loop motors have become standard in most hobby-level machines.
Which motor type is better?#
Each motor type has advantages and disadvantages.
Closed-loop motors are more modern. They can run faster and quieter, especially during jogging because if they happen to overshoot their target the driver can compensate. They can recover from a lost step during a difficult cut and get back on track.
They also use less power to hold the idle position because the driver can detect when it is pushed away and can temporarily increase the power to counter the external force when necessary.
Since the open-loop motor cables carry higher voltage and higher current than the closed-loop signal cables, they are more resistant to electromagnetic interference (EMI). That’s why grounding and shielding are that much more critical for machines that use closed-loop motors.
In short, open-loop motors are more resistant to EMI, but can lose steps.
Closed-loop motors are faster, quieter, and better at recovering from lost steps. But they are more susceptible to EMI.
Read more about EMI here: Electromagnetic Interference
About alarms#
There is one quirk of the closed-loop motors that could be a pro or a con, depending on how they are used. If the motor encounters an obstacle that it can’t overcome (like if you try to push the cut too hard, or if the axis hits the end of the travel limits), the motor will lock up and send an alarm signal to the control board.
Many commonly used control boards simply ignore the alarms. They may not even have the necessary wiring to receive it. If the motor for one axis locks up, while the rest continue to chug along, it could quickly make a bad situation worse.
The correct behavior is to abort the job after one of the motors locks up. Only some more advanced control boards handle this correctly.