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How Servo Motors Hold Position

2023-12-27 10:06:13

Closed-Loop Position Control

At the heart of a servo motor's ability to hold position precisely is closed-loop position control. This control method uses real-time feedback to maintain the motor's shaft angle.

A key component that enables positional feedback is the encoder. An encoder is usually fitted directly on the motor shaft. It constantly monitors the rotational position of the shaft and reports this back to the servo controller.

Common encoder types include optical encoders with lines or slots that break light beams, and magnetic encoders that sense the position of magnets. Either way, the encoder produces a digital square-wave signal where the count and phase indicate the motor shaft's angular location.

This position feedback is compared by the servo controller to the target position value it aims to reach. Any discrepancy between the actual and target positions triggers a correction response. The controller uses a proportional–integral–derivative (PID) algorithm to calculate the size and type of response needed to eliminate the detected position error over time.

The calculated correction is sent from the controller as a signal to the motor's drive unit. This signal dictates how much electrical power in the form of voltage and current should be fed to the motor coils. By carefully regulating these power inputs, the drive can produce precise adjustments in motor speed and torque just large enough to correct the positional deviation.

This closed-loop process of sensing position, calculating a response, and applying correction happens continuously in real-time. The high-speed feedback ability is key to servo motors' talent for maintaining fixed angular orientations with utmost accuracy.

Adjusting Motor Power

Upon receiving the correction signal from the controller, the motor drive gets to work adjusting the servo motor's power input to align the actual position with the target.

A key technology that enables precise power regulation is pulse width modulation (PWM). In this technique, the drive rapidly switches the voltage applied to the motor coils on and off thousands of times per second. It varies the duration that voltage is on within each switching cycle.

By making the "on" time wider or narrower, the effective voltage delivered to the motor can be finely tuned. A wider pulse provides more power, while a narrower pulse provides less. As torque is directly related to supplied current, which depends on voltage, thisPulse width modulation allows the drive to deftly moderate motor speed and torque through micro-adjustments in the duty cycle.

When a positional error is detected, the drive responds by increasing the pulse width slightly to provide more power, or decreasing it to provide less, based on the controller's correction signal. These minuscule power adjustments, precisely applied through PWM, enable the motor to gently speed up or slow down its shaft just enough to reduce position variation.

As the feedback loop repeats many times per second, sloughing off small bits of error each time, the cumulative effect brings the actual position ever closer to aligning with the target. This coordination of sensing, calculating, and incremental power adjustments via pulse-width modulated motor control voltages underlies how servo systems clinch precise holding of commanded angular orientations.

Achieving Precise Positional Accuracy

For servo motors to truly achieve micron-level precision in holding angular setpoints, closed-loop control and feedback are essential but not sufficient on their own. Other factors come into play to minimize residual error over time.

Firstly, PID control algorithms in the controller are tuned to provide just the right balance of proportional, integral and derivative response when correcting errors. Too much of one can cause overshooting or instability. Proper tuning results in smooth, well-damped settling into position.

Mechanically, servo motors are often coupled to gearboxes that translate high-speed, low-torque rotational motion into low-speed, high-torque output needed to hold loads static against external forces. This geared down motion aids the motor in maintaining positional equilibrium.

Additionally, many servo systems employ advanced techniques like acceleration and deceleration profiling. Ramping up and down speed uses the motor's inertia to glide precisely into place while controlling oscillation that could cause over- or under-shoot.This further increases static accuracy.

Managing outside influences like friction, linking elasticity, and external loads also helps the servo keep position more resolutely over time. Components wear with use, so periodic maintenance and parameter adjustments keep performance optimized.

Together, these features allow servo motor positioning accuracy to go beyond just reaching a setpoint quickly. They facilitate truly holding angular orientation indefinitely without drift once the commanded position is attained.

Factors Affecting Ability to Hold Position

While servo control principles enable highly static accuracy, several factors influence a servo system's actual performance in cementedly clutching setpoints:

Motor Selection - More torqueful motors with precision encoders and low cogging/friction allow withstanding larger disturbing loads without shifting off target.

Gearing - Higher gear reduction multiplies torque supplied to the final load, strengthening positional retention. Too much increases inertia and decreases response speed.

Friction - Binding friction detracts from holding strength by adding resistance opposing the constraining torque. Lubrication helps eliminate stiction.

Load Characteristics - Heavier payloads present greater opposition, necessitating sufficient motor sizing. Oscillatory loads exert destabilizing forces.

Linkage Elasticity - Compliance in couplings and belting can transmit motion away from the motor, weakening its immobilizing effect. Rigidity improves stability.

Control Tuning - Gains set too low produce lax holding, while gains set too high cause instability and limit cycle oscillation around the point.

Suppression Methods - Advanced notch filtering and damping adjust for inherent mechanical resonances that could disrupt static positioning if left unchecked.

With care taken in these design aspects, servo systems can achieve extremely tight and lasting latches on commanded angular settings despite disturbances.

In summary, the unique ability of servo systems to securely clamp rotational position opens up vast automation possibilities across many industries. By utilizing continuous closed-loop feedback control, finely tuned power regulation through PWM, optimized mechanical design, and suppression of disturbing forces, servo motor can achieve minute positional accuracy and persistently latch setpoints. This steadfast grasping of angular orientations with micron-level resolution, even under load variations, sets servo technology apart from open-loop motor types. As manufacturing demands ever greater precision, the value of servo motors' talent for upholding targeted shaft placements with certainty will continue growing in importance. Their capacity to securely brace rotational settings empowers highly accurate automated processes across diverse fields.

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