The Advantages and Disvantages of Stepper Motors

A stepper motor is essentially a servo motor that uses a different way of motorisation. Where a servo motor uses a continuous rotation DC motor and integrated controller circuit, stepper motors utilise multiple toothed electromagnets arranged around a central gear to define position.

Stepper motors consists of two varieties; unipolar or bipolar. Bipolar motors are the strongest type of stepper motor and usually have four or eight leads. They have two sets of electromagnetic coils internally, and stepping is achieved by changing the direction of current within those coils. Unipolar motors, identifiable by having 5,6 or even 8 wires, also have two coils, but every one has a centre tap. Unipolar motors can step without having to reverse the direction of current in the coils, making the electronics simpler. However, because the centre tap is used to energise only half of each coil at a time they typically have less torque than bipolar.

Stepper Motors Design

Step motors are called “digital motors” because they move in steps, like the hands on a clock.  When the first coil is energized, the rotor teeth align with the teeth in the first stator winding and hold position. When the second winding is energized, the teeth in the rotor move slightly and align with the second stator winding and hold position.  The total movement in this example is one full step.

There are usually 200 steps per revolution, each step being 1.8°.The step motor was made possible by the development of electronic step   motor controllers. Electronics are required to energize the windings   with proper voltage and current, with the proper phase, in the right sequence, at the right time.  Controllers have evolved to be able to move step motors in as many as 20,000 steps per revolution, providing 100 times finer movement (0.018° per step).



Stepper Advantages:

Stepper motors offer several advantages over servo motors beyond the larger number of poles and easier drive control. The design of the Step motor driver offers a constant holding torque without the need for the motor to be powered.

The torque of a stepper motor at low speeds is greater than a servo motor of the same size. One of the biggest advantages of stepper motors is their relatively inexpensive and availability.

Stepper Limitations:

• Low Efficiency – Unlike DC motors, stepper motor current consumption is independent of load. They draw the most current when they are doing no work at all. Because of this, they tend to run hot.
• Limited High Speed Torque - In general, stepper motors have less torque at high speeds than at low speeds. Some steppers are optimized for better high-speed performance, but they need to be paired with an appropriate driver to achieve that performance.
• No Feedback – Unlike servo motors, most steppers do not have integral feedback for position. Although great precision can be achieved running ‘open loop’. Limit switches or ‘home’ detectors are typically required for safety and/or to establish a reference position.

Step motors can lose sync; that is, lose synchronization with the step pulses from the controller. In other words, step pulses from the controller are converted into power to the windings of the motor, but the motor does not rotate.  This will happen when the torque required to move the load exceeds the torque capability of the motor at the desired speed.

Fasttobuy supply both hybrid Stepper Motor and brushless AC servo motors and drives for machine automation. We do not supply DC brushed servomotors and drives which are older technology. Both steppers and brushless servomotors are similar in construction but the servomotors have feedback devices which enable closed loop operation. There are a lot of varying opinions about the pros and cons of either style of motor, so this can help you decide which to use. It isn't as easy as one being much better than the other. Overall machine functionality is highly determined by the controller and software and is more important than just comparing one style of motor with the other.

The Linear Motor Concept

The idea is easy enough. Take a conventional rotary servo motor and unwrap it. So now what was the stator is now a forcer and the rotor can be a coil or magnet rail. With this design, the load is connected directly to the motor. Direct linear motion is achieved without any rotary to linear transmission devices. Linear motor technology is not new. Step motor and brushed linear motor products have been available for quite some time. 

Brushless technology is becoming more and more popular as applications take advantage of its technology. Brushed linear had the coils in the linear rail and the magnets were in the forcer. Commutation was accomplished by a linear commutation bar that ran the length of the motor with brushes in the forcer. This method was both expensive and limited. The cost of winding feet after feet of linear motor rail was time and material intensive. High-speed operation was limited due to commutation bar and brushes. Linear step motors have both windings and permanent magnets within the forcer. It travels along a rail having an etched tooth structure. While keeping the step motor benefit of open loop operation, the technology does have some limitation in speed and available force.

With brushless servo motor technology, and the supporting electronics to drive them, the above limitations have been eliminated. The forcer is now a set of windings while the stator is a rail of magnets. Commutation is done electronically either by Hall-effect sensors or sinusoidal. Hall effect sensors located within the forcer are activated by the magnets on the rail. The amplifier translates these signals into appropriate phase currents. Sine commutation is accomplished using the linear encoder signals back to the controller. A common technique is the use of Hall-effect initially and then switching to sinusoidal commutation. In any case, the speed of commutation is not the limiting factor. The force generated by the same size motor is greater than brush motor technology because of improved magnet materials.

Linear Motor Benefits:

• High speeds, The maximum speed of a linear motor is limited only by the bus voltage and the speed of the control electronics. Typical speeds for linear motors are 3 meters per second with 1 micron resolution and over 5 meters per second, 200ips, with coarser resolution.
• High Precision: The accuracy, resolution, and repeatability of a linear motor driven device is controlled by the feed back device. With the wide range of linear feedback devices available, resolution and accuracy are primarily limited to budget and control system bandwidth.
• Fast Response: The response rate of a linear motor driven device can be over 100 times that of a mechanical transmission. This means faster accelerations and settling times, thus more throughput.
• Stiffness: Because there is no mechanical linkage, increasing the stiffness is simply a matter of gain and current. The spring rate of a linear motor driven system can be many times that of a ball screw driven device. However it must be noted that this is limited by the motors peak force, the current available and the resolution of the feedback.
• Zero Backlash: Without mechanical transmission components, there is no backlash. Resolution considerations do exist. That is the linear motor must be displaced by 1 feedback count before it will begin to correct its position.
• Maintenance Free Operation: Because the linear motors of today have no contacting parts there is no wear.

Choosing a linear motor

Choosing the right linear motor for an application is not a simple task. Selecting the right technology for the application, force calculations, thermal considerations, bearing loading, commutation methods, etc., must be considered. Within this article, technology will be discussed, not sizing solutions. However, knowing the basic types and the associated advantages and disadvantages will assist in the end solution. Three technologies of brushless motors are discussed. They are; ironcore, aircore (ironless), and slotless.

We offer a variety of NEMA stepper motors and servo motors especially for use with linear actuators. Through our Your Motor Here program we can supply the correct mounting for any motor you specify. We also have stepper drivers/stepper controllers and ac servo motor designed for use with electric linear actuators.

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 stepper drive 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 stepper drive 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.