Q10Industrial Electronics

(a) Full Stepping vs Half Stepping

Full stepping refers to a stepper motor excitation sequence in which each successive input pulse advances the rotor by exactly one full, nominal step angle (as determined by the motor's basic mechanical construction, such as a 1.8° step angle for a common 200-step-per-revolution hybrid stepper motor). Full stepping can itself be implemented in either of two excitation patterns: single-phase-on (wave) excitation, in which only one stator phase winding is energized at any given time, stepping sequentially from one phase to the next; or two-phase-on excitation, in which two adjacent stator phases are energized simultaneously at all times, stepping the pair of energized phases sequentially — two-phase-on excitation produces a higher holding torque (typically about 1.4 times that of single-phase-on excitation, since two phases jointly contribute to the total torque) and is therefore the more commonly-used full-step excitation pattern in practical industrial applications, at the cost of correspondingly higher winding power dissipation (since two windings are continuously energized rather than one).

Half stepping is achieved by alternating between the single-phase-on and two-phase-on excitation patterns in sequence, effectively inserting an intermediate rotor equilibrium position between each pair of successive full-step positions — since this intermediate position lies exactly halfway (angularly) between the two adjacent full-step positions, half stepping causes the rotor to advance by only half of the nominal full-step angle for each input pulse, doubling the number of discrete steps (and hence the angular resolution) achievable per revolution compared to full stepping, using the identical physical motor hardware. Half stepping also generally provides smoother, quieter operation and reduces the risk of a mechanical resonance phenomenon (mid-band resonance) that can occur with full-step excitation at certain stepping rates, since the reduced step-to-step angular and torque discontinuity inherent in half stepping produces less abrupt rotor motion at each step transition — the corresponding practical trade-off is a somewhat more complex drive/sequencing circuit required to generate the alternating single/double-phase excitation pattern, and a torque output that varies cyclically between the full-step single-phase and two-phase torque levels as successive half-steps are taken, rather than the more uniform torque profile of a pure full-step (particularly two-phase-on) excitation scheme.

Microstepping as an Extension Beyond Half-Stepping

Microstepping extends the same underlying principle further, by proportioning the current in the two energized phases according to a sine/cosine (or other smoothly-graduated) profile rather than merely switching each phase fully on or off — by driving the two phase currents through a sequence of intermediate current ratios (rather than the simple 0%, 100% combinations used in full and half stepping), the rotor's equilibrium (torque-balance) position can be placed at any of a large number of intermediate angular positions between adjacent full-step positions, commonly subdividing each full step into 8, 16, 32, 64, or more microsteps. Microstepping drives yield extremely smooth, low-vibration rotor motion (effectively eliminating the discrete step-to-step jerk and associated resonance issues of full/half stepping), and a finer nominal positioning resolution — although the actual achievable positioning accuracy at very fine microstep subdivisions is ultimately limited by the motor's manufacturing tolerances, detent-torque nonlinearity, and load-torque-dependent position error rather than by the microstep count alone, so microstepping is used primarily for motion smoothness and audible-noise reduction rather than to claim a correspondingly large increase in true positioning accuracy. The drive electronics required for microstepping are correspondingly more sophisticated than simple full/half-step drives, requiring a current-regulated (chopper-based) driver capable of accurately sourcing multiple intermediate current levels into each phase winding, typically under microcontroller or dedicated microstepping-driver-IC control.

Servo Motor vs Stepper Motor for Closed-Loop Applications

When a closed-loop position control scheme is required (as discussed below), industrial designers frequently face a choice between a closed-loop stepper motor system and an alternative AC or DC servo motor system. A stepper-based closed-loop system retains the stepper motor's simple, discrete-step-based operating principle while adding feedback purely to detect and correct step-loss events, and is generally more economical and simpler to control at low-to-moderate speed and torque requirements. A true servo motor system (typically a brushless PM AC servo motor or a PM DC servo motor, driven by a dedicated servo amplifier operating continuous current/torque control based on continuous position and velocity feedback) is designed from the outset for closed-loop operation, and generally provides substantially higher achievable speed, higher torque-to-inertia ratio (faster acceleration/deceleration), and better dynamic response and disturbance rejection under varying load conditions than an equivalently-sized stepper motor, at correspondingly higher system cost and control complexity — servo systems are therefore generally preferred for high-speed, high-dynamic-performance, or high-precision industrial applications (such as high-throughput CNC machining centers and robotic manipulators), while closed-loop stepper systems remain an attractive, lower-cost alternative for moderate-speed, moderate-precision positioning tasks where the stepper's inherently simpler open-loop-like operation and lower cost outweigh the servo system's superior dynamic performance.

(b) Open Loop and Closed Loop Control Schemes for Stepper Motor

Open loop control: in an open-loop stepper motor control scheme, a pulse generator (or a microcontroller/indexer) simply outputs a stream of step pulses at the desired rate directly to the motor's driver circuit, which in turn energizes the appropriate motor windings in the correct excitation sequence, without any feedback signal confirming that the rotor has actually reached (or is correctly tracking) the commanded position. This approach relies entirely on the assumption that the stepper motor faithfully executes every commanded step without ever 'missing' a step (a step-loss event, in which the rotor fails to keep up with the commanded pulse rate, most commonly caused by excessive load torque, excessive acceleration/deceleration rate, or operation at a pulse rate coinciding with a mechanical resonance frequency of the motor-load system) — provided the drive system is conservatively designed and operated well within its torque and speed capability, open-loop control offers a simple, low-cost, and inherently stable control scheme (with no feedback loop stability concerns), widely used in moderate-precision industrial positioning applications such as printers, X-Y plotting tables, and simple indexing mechanisms.

Closed loop control: in a closed-loop stepper motor control scheme, a position (and often also velocity) feedback sensor — typically an incremental or absolute shaft encoder mechanically coupled to the motor shaft — continuously reports the rotor's actual instantaneous position back to the controller, which compares this measured position against the commanded (desired) position and adjusts the pulse timing/rate (or, in more sophisticated closed-loop stepper drive systems, directly commutates the motor windings based on actual measured rotor position, similar in principle to a brushless DC or synchronous motor drive) to correct for any detected discrepancy, actively preventing or immediately correcting step-loss events rather than simply hoping they do not occur. Closed-loop control substantially improves the reliability and achievable dynamic performance (higher acceleration, higher speed, ability to handle variable or uncertain load torque) of a stepper motor system compared to open-loop operation, at the cost of the additional expense and complexity of the feedback sensor and the more sophisticated control electronics required to process the feedback signal and adjust drive commands accordingly — closed-loop stepper control is therefore generally reserved for higher-performance or higher-reliability industrial applications (such as CNC machine tool axes or robotic positioning systems) where the risk and consequence of an undetected step-loss error would be unacceptable, whereas simpler open-loop control remains adequate and more economical for the large majority of moderate-precision industrial stepper motor applications.

Practical Encoder Types Used in Closed-Loop Systems

The position/velocity feedback sensor most commonly used in closed-loop stepper (and servo) drive systems is a shaft encoder, available in two principal types. An incremental encoder generates a train of pulses (typically two channels in quadrature, labeled A and B, plus often a single once-per-revolution index/marker pulse Z) as the shaft rotates, with the pulse count giving relative position change and the quadrature phase relationship between the A and B channels indicating direction of rotation — incremental encoders are comparatively simple and inexpensive, but only provide relative position information, requiring a homing/reference procedure at power-up (typically using the index pulse or a separate limit/home switch) to establish an absolute position reference. An absolute encoder, by contrast, outputs a unique digital code (via multiple parallel tracks, in older Gray-code-disk designs, or via a serial digital protocol in modern optical/magnetic absolute encoders) directly corresponding to the shaft's actual absolute angular position, retaining valid position information even after a power interruption, without requiring any homing procedure — at correspondingly higher cost and interface complexity than an incremental encoder. The choice between incremental and absolute encoder types in a given closed-loop stepper or servo application depends on whether the system can tolerate a homing sequence after every power-up (favoring the simpler, cheaper incremental encoder) or requires immediate, power-loss-tolerant absolute position awareness (favoring the absolute encoder), a common consideration in industrial CNC and robotic positioning system design.