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Selection Guide for Stepper Drives
Date:11/10/2011 9:54:39 | Print | Back

Introduction

 

A stepper motor requires an electrical sequencer and it is called a stepper drive. The stepper drive is one of the key components in a stepping system. When you select a stepper drive for the special application, you can follow the following steps. Firstly, you should choose the drive type and determine the drive operating mode. Secondly, choose right supply voltage and output current according with the application and the motor. In the end, you should consider whether the acceptable control signals of the drive are right for those of your motion controller or not. Of course, the price of the chose drive should be acceptable too.

 

Drive Types

 

The output torque and power from a stepper motor are determined by the operating current, motor size, motor heat sinking, motor winding, and the type of the drive used. You can get much different performances from a given motor by choosing different type stepping drives. In the following section, the performances of some commonly-used drive configurations are compared.

 

Unipolar Constant Voltage Drive

This is the classic low-end drive. It offers the lowest price for the drive electronics-only four transistors are used. 

 

Unipolar L/nR Constant Voltage Drive

This drive is similar to the unipolar constant voltage drive but has external series resistors in series with the motor windings. This drive can be configured with different L/R ratios. L/2R means that the total resistance is equal to two times the motor’s internal resistance. A higher L/R-ratio increases high-stepping-rate output torque, but reduces the system efficiency. 

 

Unipolar Timed Bi-level Drive

This drive uses two voltage levels to increase motor utilization. At every step taken, the voltage across the winding is raised, for a short time, to a higher level compared to the nominal voltage used at stand still. During the remaining time, the nominal voltage is used. This drive can also be configured in the run/stop bi-level mode, where the high voltage is used while the motor is stepped and the low voltage is used at stand still. This drive can also be combined with L/nR-series resistors to give higher flexibility in selecting stand-by holding torque. 

 

Unipolar Constant Current Drive

This drive gives the best performance of the unipolar drives-but it is lower than that of the bipolar chopper drive. The efficiency is reduced as a result of higher resistive losses caused by using only half of the windings at a time. At higher frequencies, power losses caused by leakage inductance and snubbing circuits also appear. 

 

Bipolar Constant Current Drive

The highest output power and motor utilization for a given motor is achieved with the bipolar constant current drive. DC-losses is kept at a minimum due to maximum utilization of the copper in the winding and no power losses from leakage inductance and snubbing circuits since every winding only consists of one part.

During the last 10 years, progress in IC-technology has made it possible to develop fully-integrated bipolar constant-current drives, making this type of drive cost-effective for driving small- and medium-sized motors.
 

 

Bipolar Constant Current Microstepping Drive

This is an improved version of the basic full- and half-step bipolar constant-current drive. Here, the winding currents form a sine/cosine pair. This greatly improves low frequency performances by eliminating overshot movements, ringing, and resonances. Performances at medium and high-stepping rates are close to those of full- and half-step. This drive uses the same power stage as the bipolar constant-current drive, but extra electronics for setting the sine/cosine current levels are used. Microstepping can be used with different microstep lengths. A step length which shorter than 1/32 of a full-step normally does not make any further improvement in the motor’s motion. With most microstepping controllers, it is also possible to run the normal full- and half-step modes. Microstepping can also increase resolution and step accuracy of the stepping systems. 

 

Choosing Drive Type

 

For applications in the low- and medium-power range, several choices exist. If system efficiency is important, then the bipolar constant current drive is the best choice. This drive offers higher flexibility in selecting the motor winding, since both the chopper voltage and the current in the winding can be changed to get the desired pull-out torque curve from the motor. Power supply design gets easier and power supply losses decrease since regulated supply normally is not needed for constant current drives.

If minimum cost for the drive electronics is the most important design criteria, rather than the over all system performances, then the different unipolar drives can be the better choice.
The unipolar L/R-drive offers the lowest cost for the electronics for a given output torque, if the step rate is low. As demand for output power from the stepping increases, more effective drives offer better performance-price ratio. The best motor utilization is achieved with the bipolar constant current drive and this drive is the obvious choice for all high power applications.
All of Leadshine’s stepper drives adopt bipolar constant current chopping technology, while they are still distinguished for their high performance-price ratios.

 

Drive Modes

 

The most common drive modes are full-step, half-step and microstepping.
FULL-STEP MODE: This is the basic stepping driving mode, it offers the simplest control electronics and it is recommended for high and medium frequency operation. At these frequencies, the inertia of the motor and the load smooth out the torque, resulting in less vibration and noise compared to low-speed operation.

HALF-STEP MODE: Half-step gives smoother movement at low step rates compared to full-step and can be used to lower resonances at low speeds. Half-step doubles the system resolution. Observe that for most stepping motors, the step accuracy specification only is valid for 2-phase-on positions. The accuracy is lower and the stop-position hysteresis is larger for 1-phase-on positions.

MICROSTEPPING: The smoothest movement at low frequencies can be achieved with microstepping. If resonance-free movement at low step rates is important, the microstepping drive is the best choice. Microstepping can also be used to increase stop position accuracy beyond the normal motor limits.

Leadshine stepper drives cover all drive modes. However, for Leadshine’s stepping drives are famous for their high performance-price ratios, so all of stepping drives exported to overseas are half-step and microstepping drives for the moment. Please contact Leadshine if you need a full-step drive.

 

Supply Voltage and Output Current

 

The power supply voltage must be within the drive’s allowable operating voltage range. Beyond that, the choice of voltage is dependent on the application and the motor used. The power supply voltage should be between 3 and 25 times the motor’s rated voltage. If it is less than 3 times, the motor may not operate smoothly and motor heating is excessive if it is more than 25 times the motor’s rated voltage.

As an example, if a 4.0A, 2.0 Volt half-coil connected motor is to be used, the power supply voltage should be between the minimum allowable operating voltage of the drive and 50 VDC. If the same motor is series connected, then it may be thought of as a 2.8A, 2.8 Volt motor instead and the power supply voltage can be between the minimum allowable operating voltage of the drive and 70 VDC. If the motor’s voltage is not listed, then multiply the rated current by the resistance/phase to get the motor’s voltage. The drive works best with unregulated power supplies though regulated linear and switching power supplies may also be used. If a linear regulated or a switching supply is to be used, then a large capacitor should be placed across the output terminals.

Higher supply voltage can increase motor torque at higher speeds, thus helpful for avoiding losing steps. However, higher voltage may cause bigger motor vibration at lower speed, and it may also cause over-voltage protection or even drive damage. Therefore, it is suggested to choose only sufficiently high supply voltages for special applications, leaving room for power fluctuation and back-EMF.

For a given motor, higher drive current will make the motor output more torque, but at the same time causes more heating in the motor and drive. Therefore, output current is generally set to be such that the motor will not overheat for long time operation. Since parallel and serial connections of motor coils will significantly change resulting inductance and resistance, therefore it is important to set drive output current depending on motor phase current and connection modes.

Leadshine’s stepper drives cover a broad operating voltage range, from 12VDC to 80VDC or 18VAC to 220VAC. And most of Leadshine’s stepper drives have over-voltage and over-current protection functions. All of Leadshine’s stepper drives use DIP switches to set motor’s operating current, and all of them have automatic idle-current reduction function which means that standstill current can be set to be smaller than the selected dynamic current. Theoretically, this can get smaller motor heating than the original value when the motor stops. Click the following website for more information about Leadshine’s stepper drives (
http://www.leadshine.com ).

 

Types of Control Signal

 

A stepper drive control the motors according to pulse command signals of the motion controller, and these signals include pulse and direction signals. There are two command types: single-pulse control signal (PUL/DIR) and double-pulse control signal (CW/CCW), shown as Figure 1.


(a) PUL/DIR control signal

(b) CW/CCW control signal


Figure 1: PUL/DIR and CW/CCW control signals


In PUL/DIR mode, the PUL signal is for the command pulse chain (position or velocity). The numbers of PUL pulse represent the relative distance?or position? and the frequency of the PUL pulse represents the command for speed or velocity. The DIR signal represents direction command of the positive (+) or negative (-). This mode is the most common used mode. In CW/CCW mode, pulses output from CW pin makes motor move in positive direction, likewise pulse output from CCW pin makes motor move in negative direction.

Some stepper drives just can accept PUL/DIR or CW/CCW control signal, while some others can accept both PUL/DIR and CW/CCW control signals. It is critical to choose the right type of control signal(s) that the stepping drive can accept according to pulse command signals of the motion controller, or else the stepper system won’t work properly.

The control signals can also be divided into two categories: single-ended control signal and differential control signal. Generally speaking, differential control signals can minimize or eliminate electrical noises coupled onto the drive control signals. Recommend use differential (line drive) control signals to increase noise immunity of the system in interference environments if possible.

Most of Leadshine’s stepper drives can accept single-pulse control signal (PUL/DIR) and double-pulse control signal (CW/CCW) control signals, and usually a jumper or DIP switch is used to switch these two modes. And most of Leadshine’s stepping drives can accept differential and single-ended control signals (including open-collector and PNP output). Recommend use differential (line drive) control signals to increase noise immunity of the drive in interference environments if possible. Click the following website for more information about Leadshine’s stepper drives (
http://www.leadshine.com ).

 

Leadshine’s Stepper drives

 

Currently, Leadshine offers two main series of 2-phase microstepping drives, the digital EM series and analog M series. The highper performance EM drives are based on powerful 32-bit DSP control technology. Their features include super-low stepping noise, antiresonance, low-speed ripple smoothing, and low motor heating. The low-cost M drives employ precise analog current control and are characterized by superior high-speed torque, relatively low stepping noise, and low motor heating. Leadshine also supplies 3-phase digital and analog stepper drives. Click the following website for more information about Leadshine’s stepper drives (http://www.leadshine.com ).

Note: Please visit www.leadshine.com for information about our latest drives.

 

References

 

1. Stepper Motors and Stepper Motor drives. http://www.motiongroup.com/steppingmotors_basics.shtml
2. Stepper motor and drive selection. http://isa.umh.es/temas/micros/doc/SteppingMotorSelection.pdf
3. STEP MOTOR BASICS. http://www.geckodrive.com/photos/Step_motor_basics.pdf
4. Drive circuit basics. http://www.solarbotics.net/library/pdflib/pdf/drive.pdf
5. Stepping Motors Fundamentals. By Reston Condit and Dr. Douglas W.Jones. http://www.cs.uiowa.edu/~jones/step/an907a.pdf
6. STEPPER MOTOR DRIVING. By H. SAX. http://www.technologicalarts.com/myfiles/data/1679.pdf