Which type of motors are best for a plotter format laser cutting machine

This question often arises during the later stages of evaluation just prior to purchasing a laser cutting machine because some manufacturers make a big issue of the types of motor that their machines incorporate.

Generally there are two types of motor used in lower cost, low power, small format laser cutting machines:

1. Servomotor

2. Stepper motor

The differences between the two can be vast or minor depending on the performance criteria of the motor itself.

Older motor technology incorporates brushes that physically contact to transfer the electrical current. This type of motor can still be found on very low cost laser cutting machines but should be avoided because the brushes will wear-out. More modern technology uses a non-contact electromagnetic design to pick-up the electrical current. These are called brushless motors and they will often give many years of service without maintenance. In almost all cases the current used to power motors in a laser cutter will be DC

A typical servomotor in a laser cutter will be quite small and low in performance compared to, for example, the same make of servomotor used in a larger or heavier machine such as a CNC router or milling machine.

Signals to a servomotor can be analogue or digital. As the signal is sent to the servomotor it will rotate either clockwise or anticlockwise. Coupled with the servomotor will be some form of encoder. This can be linear (external to the motor) or rotary (usually positioned below the main shaft). The encoder can be read magnetically or optically with the magnetic reader being better as it is not susceptible to error through contamination whereas an optical sensor is.

The function of the encoder it to provide feedback to determine if the motor has moved as commanded. This is particularly useful if the motor is used at high speeds or under heavier loads because if any positioning is lost the feedback from the encoder can be used to correct. This makes servomotors potentially more accurate and more reliable especially when used at higher velocities.

Servomotors operate well with a variable velocity, so a servomotor is a better solution if the system is to be used at high velocities

A modern stepper motor simply receives a signal to move a certain number of steps in a particular direction: clockwise or counter clockwise. It will normally work at a set velocity. Stepper motors can lose accuracy, especially if they are overloaded with a motion system that is too heavy or if they are driven too fast

Therefore, you would think that the servomotor is the only option to choose, however, a high quality stepper motor can often provide for a smoother cut as, unlike the servomotor alternative, it is not in a constant state of agitation. Stepper motors are also often of a significantly lower cost so that the equipment is lower cost to purchase and replace  

Furthermore, on lower cost systems it is often possible to drive the axis directly off of the main shaft of a stepper motor whereas most servomotors are always connected to the axis via some kind of intermediary gearing mechanism. When such mechanism wears accuracy is lost and if the servomotor is using a rotary encoder this means that the electronics thing all is Ok but in fact the machine is working with an error.   

In summary, it is our view that there is no advantage to using a servomotor unless the system is to be driven very fast and/or if the motor is moving a very heavy motion system. For these reasons, aside from our largest format machine most of the plotter lasers that we manufacture incorporate stepper motors.