Why CarveWright Uses Servo Motors Instead of Stepper Motors
CarveWright’s CNC systems from the early models to the new RAVEN are the only CNC machines in size capacity and price range using servo motors. You usually only find servos on large, expensive industrial machines that require accuracy and high torque at high speeds. To understand why we use this technology in our machines rather than stepper motors like our competitors, it’s necessary to understand the differences between the two motor types.
Stepper Motors
Stepper motors are a basic motor design that uses electromagnets to divide a full rotation into a number of equal steps. Basically, they are commanded by a controller to move a certain number of steps at a specified speed. There isn’t much more sophistication to the motors than that.
Their advantage lies in their simplicity. However, this simplicity has a few serious drawbacks. Most importantly, stepper motors do not utilize position feedback and cannot react or adjust to changes in load. The motor torque in a stepper motor also decreases as its speed increases, requiring very slow speeds and shallow cuts. Moving too quickly or with too deep a cut will result in missing steps, and without any feedback, the controller has no way of compensating for it.
For these reasons, stepper motors are not well-suited to the sophisticated project and material types of high-end CNC operations demanded by our community of users.
| Stepper Motor CNC | |
|---|---|
| PROS | CONS |
| • Generally lower in cost • Good low speed torque and accuracy • Low duty cycle | • No feedback to report positioning • Cannot sense or react to changes in load • Prone to losing position and crashing if not run at slower speeds • Requires shallow cuts in multiple passes • Torque decreases as speed increases |
Servo Motors
The servo motors found in CarveWright CNC’s are high-torque DC motors with an encoder mounted on the back shaft to detect the position and speed of the rotor. The feedback from the encoder enables high-resolution, high-response positioning operations.
This sophisticated motor design offers some real advantages over stepper motors. Unlike steppers, servos can apply peak torque on demand at any speed. Our ServoSmart Controller monitors and regulates the motor’s current for more precise torque control, allowing the machine to adapt to changes in load in real time without stopping. Furthermore, the machine’s position is constantly being reported by our high-resolution encoder (1296 PPR), allowing the machine to detect and auto-correct any errors. The result is that CarveWright CNCs offer faster, more precise carves than competing CNC machines.
Servo motors allow our machine to carve a lot faster because excess load or backlash situations are automatically compensated for. Our machines produce different commands based on feedback from the three-axis servo motors. This makes servo motors a superior choice for variable-load systems like a CNC machine, which is why all professional-grade CNC machines use servo motors just like CarveWright does.
| Servo Motors | |
|---|---|
| PROS | CONS |
| • Feedback loop for real-time sensing • Senses slips and auto-corrects • Reports issues instead of crashing • High torque at high speeds • High resolution encoder ensures accuracy | • Generally higher in costs • Proprietary to specific controller |
Feeling a little lost? Understandable! Here’s what you really need to know: Thanks to our ServoSmart controller and servo motors, CarveWright offers the fastest and most precise consumer CNC routes in the world. That’s why the performance of CarveWright CNC machines is unmatched.
Have questions about how our machines work? We’ve got answers! Contact us today for more detailed information on how CarveWright machines outperform the competition.
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One Comment
As a Retired Navy Gun Fire Control Tech that started working with Servos in 1976 and purchased my first CarveWright in 2007, this is an excellent comparison of Steppers to Servos. The feedback via the encoder is the magic that make this so accurate. When the Y and Z do it’s homing the axis bump into the left and upper mechanical stop telling the controller. This is home and 0000 on the encoder. As the encoder moves it counts in pulses due to the lines in the encoder changing with the position of the axis in the computer from 0000 to 0001 to 0002 to 0003…. And so on in + or minus directions. If you look at the thin lines on the clear encoder disk you can see the steps or fine resolution of the encoder. I purchased this new CW to add to my 12 I currently own but only use one for now. I have big plans for the future. The CW was instrumental in my Fire Department Name Tag business using the CW to cut two sided plastic tags for printing along with the wood handles. The CW also excels in repeatability. I used a Jig Sled to cut side slots in a Accountability Board after it was printed. Using place on end as the most accurate way for me to position a sled for repeatability. Place on end also saves time as “place on center” of board measures the board length twice… Once to find the length of the board and a second time to find center. That is almost double the servo time and more wear on the drive. At least that is the way the basic designer works that I use. So “Place on End” or “Place on Corner” is my setup of choice. Lastly I use Centerline Text often. When I first started with my CW in 2007, I was tasked to reproduce the block text wood signs that were made in the past by using plastic letter guides and a hand held router for name signs in a 4000 home development. I did close to 50 back ordered double sided name signs using a Single Stroke Font and told the machine to use outline the text mode. The bit followed around the outside of single stroke font making a 3 inch high text letter with a 3/8 end cutter bit. The bigger the text the bigger the bit and the smaller the text the smaller the bit. I did a shallow flat bottom letter that painted well. Excited at getting my new CW.