Blog › Open-loop vs closed-loop stepper motors
1 November 2026 · 2 min read
Open-loop vs closed-loop stepper motors
Most hobby CNC machines assume their motors did what they were told. Some check.
Nearly every desktop CNC router uses stepper motors. A stepper turns in fixed steps, so the controller can move an axis a known distance by sending a known number of pulses. The question spec sheets answer with "open loop" or "closed loop" is whether anything checks that the steps actually happened.
Open loop
Wikipedia's article on stepper motors puts it simply: they "can be set to any given step position without needing a position sensor for feedback", and such systems "track position simply by counting the number of steps that each motor has been instructed to take." The weakness is in the same article: under some conditions "steps may be missed", and the motor can "end up in the wrong position". The controller has no way of knowing.
Closed loop
A closed-loop stepper adds a sensor that watches the motor. Sienci Labs' page for its closed-loop kit explains it in plain terms: the driver sends steps "while simultaneously monitoring the position of the motor using a sensor", and if the motor falls behind it "will receive more power to ensure that the motor gets to the specified position without any skipping". An open-loop motor in the same situation, the page says, "will 'skip steps' and will essentially lose track of where it thinks the motor is".
In the record
| Machine | Motors | As published |
|---|---|---|
| SainSmart 3018-PROVer V2 | Not published | 32-bit board, GRBL (version not published). Power supply 24V/4A. |
| SainSmart 3020-PRO MAX V2 | Not published | Grbl V1.1 with 32-bit TB67S109 drivers; upgraded offline controller |
| SainSmart 4040-PRO | Not published | Offline controller CTL4040; GRBL on a 32-bit MCU, TB67S109 drivers. NEMA 17 steppers. |
| FoxAlien Masuter Pro | Not published | Control box with e-stop and home buttons; firmware not published. 12V/5A supply. Steppers 1.3 A, 0.25 Nm. |
| FoxAlien Masuter Pro Max 2x2 | Not published | Control box included; firmware not published. NEMA 17 steppers. |
| SainSmart 4040-PRO MAX | Not published | GRBL on a 32-bit MCU, TB67S109 drivers. Offline controller CTL4040 sold separately. |
| Onefinity Apprentice | Open loop | Redline CNC controller. Open-loop 1.2 Nm NEMA 23 motors. Computer required. |
| FoxAlien Masuter 3S | Closed loop | Controller box with shortcut buttons; supports an offline controller. Four NEMA 23 closed-loop steppers, 2.2 Nm. |
| Sienci Labs LongMill MK3 30x30 | Closed loop | SLB-LITE controller, grblHAL. Closed-loop NEMA 23, 1.2 Nm. |
| Onefinity Gen 2 Elite Woodworker | Closed loop | Redline CNC controller. Closed-loop 2 Nm X/Y, 1.2 Nm Z. Computer optional (15" screen option); WiFi, Ethernet. |
| Carbide 3D Nomad 3 | Not published | Not published. Carbide Motion control software. |
| Sienci Labs AltMill MK2 4x4 | Closed loop | SLB-EXT controller with e-stop. Closed-loop steppers; "over 600IPM". |
| Carbide 3D Shapeoko 5.1 Pro 4x2 | Not published | Not named. Optically isolated electronics, internal supply, 110/220V. |
| Sienci Labs AltMill MK2 2x4 | Closed loop | SLB-EXT controller with e-stop. Closed-loop steppers on all three axes; about 600 in/min. |
Sienci Labs fits closed-loop NEMA 23 motors on the LongMill MK3, and sells the closed-loop kit separately for $560.00. Onefinity's compare page lists open-loop motors on the Apprentice and closed-loop on the Gen 2 Elite.
What NEMA 17 and NEMA 23 mean
These are frame sizes, not power ratings. Wikipedia gives the rule: NEMA 17 means "a 1.7 by 1.7 inches (43 mm × 43 mm) faceplate". By the same naming, NEMA 23 is a 2.3 inch faceplate. A bigger frame usually allows a bigger motor, but the torque figure (in Nm) is the one that describes strength, and only some makers give it: Sienci Labs and Onefinity give 1.2 Nm and 2 Nm figures; FoxAlien gives 0.25 Nm for the Masuter Pro and 2.2 Nm for the closed-loop motors on the Masuter 3S.
Sources
Read on 1 November 2026. Quotes are copied from these pages.