Månadsvis arkiv: april 2016

The Advantages and Disvantages of Servo Motors

Servo motors are somewhat more expensive than steppers — perhaps double the price, or more. They are generally just as accurate, if maintained in a proper state of tune, however they rely on encoders to provide positioning information back to the computer. Thus the complexity of the system is at least doubled, with no accuracy advantage, greater initial cost, and more maintenance issues. The “closed loop” rhetoric that some manufacturers play up sounds convincing to the uninitiated, but provides no benefit over a simpler and more reliable stepper system.

Servo motors are available in larger sizes than stepper motors, and powerful ac servo motor are generally used on heavy machines with gantry carriages in the 500 to 1,000 lb range. They offer no advantage whatsoever on lighter machines, such as Torchmate and its competitors.

Servo motor design

A servo by definition is any device with feedback. Servo motors have coils on their stator which are energized in sequence by a brushless servo motor or amplifier, causing rotation (spinning) of the magnetic rotor. In order for the controller to determine what winding to energize next, it must know the current position of the rotor.This sensing is done using either hall sensors or a dual-function encoder.

Servo motor system components

The block diagram of a servo system is identical to the block diagram of a step motor system,although the technology of the components is different.The servo amplifier is the functional equivalent of the step motor drive. As with step motors, functions are often combined by manufacturers.

Advantages of Servo Motors

For applications where high speed and high torque is needed, servo motors shine. Stepper motors peak around speeds of 2,000 RPM, while servo motors are available many times faster. Servo motors also maintain their torque rating at high speed, up to 90% of the rated torque is available from a servo at high speed. Servo motors are also more effecient than stepper motors with efficiencies between 80-90%. A servo motor can supply roughly twice their rated torque for short periods, providing a well of capacity to draw from when needed. In addition, servo motors are quite, available in AC and DC drive, and do not vibrate or suffer from resonance issues.

Some of the advantages of servo motors over stepper motors are as follows:

High intermittent torque
High torque to inertia ratio
High speeds
Work well for velocity control
Available in all sizes
Quiet

Servo motors are capable of delivering more power than stepper motors, but do require much more complex drive circuitry and positional feedback for accurate positioning. Servo motors are also much more expensive than stepper motors and are often harder to find. Servo motors often require gear boxes, especially for lower speed operation. The requirement for a gearbox and position encoder make servo motor designs more mechanically complex and increase the maintenance requirements for the system. To top it all off, servo motors are more expensive than stepper motors before adding on the cost of a position encoder.

Servos have inherent disadvantages:

• More complex system than step motors
• Motors must be tuned for optimal performance
• More costly system than step motors
• Less torque at lowest speeds compared to step motors
• Non-standard mounting geometries (vary by manufacturer) and shaft diameters
• Less torque than comparably sized step motors at low speeds

Summary

Selecting the best motor for your application depends on a few key design criteria for your system including cost, positional accuracy requirements, torque requirements, drive power availability, and acceleration requirements. Overall, servo motors are best for high speed, high torque applications while stepper motors are better suited for lower acceleration, high holding torque applications.

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Buying a Stepper Motor Driver

When purchasing stepper motor drivers, also called controllers, several factors must be taken into consideration. Buyers should make sure that the motor is compatible with the driver, as there are several different types. The number of wires in the motor determines whether a bipolar or unipolar driver is required. Maximum current input and output of the motor also impact which servo driver to purchase, as do features such as step modes, step frequency, and protection circuitry. There are numerous types of stepper drives available, each with advantages and disadvantages. Choosing the right kind of driver depends on the type of task the stepper motor will be applied to, as well as the step mode requirements. Here recommend you Brand hybrid Stepper Motor by Fasttobuy.com.

 

The Leadshine Stepper drive’s performance comes from its powerful 32-bit DSP processor and associated control algorithms. These achieve smooth performance at low speeds by significantly minimising fluctuations from the desired motor speed. The leadshine am882 can also calculate the natural system frequency and apply a damping function to eliminate resonance. This yields higher speed and better motor performance; it also optimises torque and eliminates mid-range instability. And by cutting stepper motor heating losses, the driver brings energy saving benefits, together with reduced maintenance costs.

System set-up is said to be fast and simple due to the motor auto-tuning and parameter auto-configuration technology. This allows automatic compensation for the unique characteristics of any motor connected to the drive. The motor can be sized from NEMA 17 to NEMA 34 diameter due to wide input voltage coverage and a programmable output current range from 0.5-5.6A. Either two- or four-phase motors can be connected. The drive has a programmable resolution, from full step to 102,400 steps per resolution. The stepper driver’s Multistep function allows this full microstepping resolution to be applied to a standard 200-step motor, so system performance becomes smoother.

Highlights

Suitable to drive size NEMA 17 to NEMA 34 stepper motors

Supply voltage up to +50VDC

Programmable output current range from 0.5-5.6A

Programmable resolution from full step to 102,400 micro steps per resolution

Support PUL/DIR and CW/CCW modes

Over-voltage, over-current and phase-error protection provided as standard

Stepper drives always offer the cheapest solution, so use a stepper wherever appropriate. Remember these major considerations: First, does the system require position confirmation? Second: The wrong stepper drive can cause ringing, resonance, and poor low-speed performance. Third, during high speeds, stepper motors can whine. Because stepper drives have a high pole count, hysteresis and eddy current losses are also common at high speed; for these reasons, a stepper is not recommended for continuous operation above 2,000 rpm. Finally, because full current is needed to produce holding torque, step motors can get hot at a standstill.

Fasttobuy has a large selection stepper drives and controls, available in both new and used condition, and the price range varies significantly across the range.

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