Stepper or Servo Motors?
Should you buy a CNC plasma cutter with stepper motors or servomotors? Depending on what company you are considering buying your system from and what motors they offer you will hear two completely different stories.
Here is some information you can use to help you make your decision as to which system you might want. If you need to keep the cost down in order to build a low cost CNC Plasma Cutter, most manufacturers will employ stepper systems. Stepper systems are inexpensive and provide a degree of accuracy when used in well-engineered systems.
A Stepper motor translate a series of received individual electrical pulses into degrees (steps) of motor shaft rotation. Steppers, depending on the quality of the motor and electronics combination can provide either full, half or microstep motion control. Full steps typically divide one shaft rotation into 200 increments. Half steps into 400 increments and microsteps into 800-256000 increments. By dividing the motor shaft rotation into the smallest increments possible could allow more resolution for the control. Without a feedback device, there is NO guarantee that the microstep was or is ever performed. Also, stepper motors lose torque in microstep operation and operate much closer to rated power in full step operation. Full and half step motion control is very coarse compared to microsteps.
The stepper methodology most commonly uses Open Loop technology. With Open Loop technology the computer sends step and direction information to an external amplifier (driver) then onto the individual motors. The major problems with this technology are there is only one-way computer-motor communication and stepper motors have severe limitations on the torque they can produce. Stepper motors miss performing steps even in the most well engineered CNC systems. As an example, a stepper motor losing steps in a ramping operation can stall completely. A bit of dirt, slight drive train resonance, excessive momentary torque demands or any of a dozen reasons can cause a stepper motor to miss steps. If the computer knew of these missed steps it could correct for them. With Open Loop’s inherent one-way communication the computer remains unaware of the errors. The computer thinks the torch is in one location while in reality it is somewhere else. The errors generated in cutting one part are passed on when cutting the next part while adding more errors. Pretty soon the accumulation of errors will cause a failure of the cutting operation.
A stepper motor is rated in "holding torque". This is the torque that can be supplied to the shaft when the motor is stationary. In other words holding torque is the amount of force the motor can generate to resist turning its shaft. This can also be known as the point of guaranteed failure, where if one additional in-oz of torque is required, the motor will fail.
However. once the shaft starts to turn, the actual amount of torque drops off sharply. It is usually only about 75% of the holding torque, and continues to drop as the rpm increases. Depending on the driver circuitry, some go down to 20% at 1000 rpm. When you consider the friction and other torque variations, the instantaneous torque load could be 50% higher than the average load. Stepper driven gantry systems must have their rapid motion speeds restricted because the torque needed for quick direction changes is not available. What this means to you is, you must waste a great deal of time waiting for the gantry to relocate to the next cut location.
Plasma cutting requires employing the correct torch motion speed for the material being cut. Stepper motors however lose torque when asked to speed up. You can imagine the problems if your car engine lost torque when you stepped on the gas. The loss of torque production is a big reason stepper systems miss steps. The torque needed for rapid, momentary changes of direction (to overcome inertia) may or may not be available while operating at high speed. Some stepper systems use step down drive trains which require the stepper motor to turn faster to deliver the called for gantry travel speed exacerbating an already bad situation. A typical stepper motor torque curve from a leading stepper motor manufacturer is shown below.
TORQUE-RPM GRAPH FOR A TYPICAL STEPPER MOTOR
The above graph reports a 64% loss of available torque at 2000 RPM with a stepper motor. Any momentary demands for torque beyond that shown in the graph will result in position errors being generated.
Our decision to use Intelligent Servomotors was based on solving the biggest problems inherent in the other logical choice, stepper motors.
Our Brushless Servomotor drive is unique even to other Servomotor offerings. We use intelligent motors. With intelligent motors the host computer communicates directly with the Intelligent Servomotors on the gantry. No problem prone intermediary electronics are needed. The motor itself interprets the computer instructions and translates them directly to motor rotation.
Our Intelligent servomotor system uses a Closed Loop methodology. Built into the Intelligent Servomotors are position encoders. These position encoders report back to the on board servo control computer on the actions of the Intelligent Servomotors. If for any reason the gantry has not moved to the required location the onboard computer is aware of it and corrects the motor, hence, no accumulated errors. The Closed Loop system assures accuracy with the first part cut and any others in the series. There is no motor system that is error proof, however, Intelligent Servomotors are as close to error proof as technology allows. Intelligent Servomotors are less error prone than steppers and can correct errors should they occur automatically.
Intelligent Servomotors do not loose torque as rapidly as stepper motors. See below for a typical torque/RPM graph for Intelligent Servomotors.
CONTINUOUS TORQUE-RPM GRAPH
FOR A TYPICAL INTELLIGENT SERVOMOTORContinuous service rating. Extra torque up to 570 in-oz@ 0 RPM and 490 in-oz@ 2000 RPM available for momentary bursts of power if needed.
The above CONTINUOUS SERVICE graph reports a 9% loss of available torque at 2000 RPM with an Intelligent Servomotor. However, large reserves of auxiliary power are available for momentary needs such as rapid changes of gantry direction..
