A stepper motor is an electrically powered motor. Current is passed to the drive shaft, which then rotates in small step-like motions. Stepper motors do not rotate smoothly or continuously. They are designed to power digital devices, which require precise and tightly-controlled motor movements with no feedback. Items containing stepper motors include cars, computers, toys, robotic arms, printers, and robotic model kits.
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 driver to purchase, as do features such as step modes, step frequency, and protection circuitry.
Types of Stepper Drives.
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.
Chopper drivers are among the most common type of stepper drive. Chopper drivers are a type of constant current driver, which means they feed continuous current to the coils. High voltage current is passed simultaneously to each coil until the desired current level in the coil is reached. At this point, the voltage passed to the coils is "chopped." This means that the voltage is greatly reduced, but still maintains the higher levels present in the coils. Chopper drivers are usually equipped with interchangeable resistors, enabling users to accurately alter the voltage. Models with frequencies of 20 KHz or greater are generally the preferred choice, since they are very reliable. However, they are also more expensive than lower frequency models. Low frequency chopper drivers have significantly reduced efficiency and performance, and can produce an unwanted humming or whining sound.
There are two different types of chopper drivers: unipolar and bipolar. Each is compatible with different stepper motors.
Unipolar drivers are compatible with motors with five, six, or eight wires. A unipolar stepper control driver has a double wrapped coil in each phase. This eliminates the need to reverse the direction of the current to switch magnetic polarization. Unipolar drivers are less expensive than bipolar models and are simpler to set up. The main drawback of unipolar drivers is that they are inefficient. A great deal of the input energy is wasted by being converted into heat.
Bipolar drivers are compatible with four, six, and eight wire stepper motors. Bipolar drivers are designed for use with two-phase stepper motors. These drivers have only one wound coil per phase. They are complex devices because, with only one coil per phase, the current direction has to be rapidly and seamlessly reversed to change the polarity of the motor. They are more expensive than unipolar drivers, but they are also considerably more efficient, producing around 40% more torque than a unipolar driver with the same motor and same input.
Inductance and Resistance Drives
Inductance and resistance drives are known as L/R drives. Like chopper drives, these are classed as constant voltage drives. They are less expensive than other drive circuit types, but are considerably less efficient. They work on a resistance and induction circuit, which means that reaching full torque is a long, slow process. Inductance and resistance drives also waste large amounts of energy, which is turned into heat, meaning that the input is far greater than the output.
Bi-level drivers are very efficient and waste very little energy, which means they have a very low risk of overheating. Bi-level drivers operate in a manner similar to chopper drives. The current is passed to the coils very rapidly. When the desired level is reached, the high voltage is replaced with a low voltage to maintain the levels present in the coils. Bi-level drivers are not compatible with micro stepping, only full step and half step modes (see below).
Stepper drivers come in a variety of step modes, which influence the rotation speed, size, and action of the drive shaft. A complete set of steps does not necessarily imply a full rotation of the drive shaft. In some instances, multiple complete step sequences are required to achieve a quarter of a turn of the drive shaft. The following sections discuss several different step modes and their characteristics.
Wave Step Mode
Wave step mode is found on a number of older stepper driver models. Wave step mode involves passing energy to one phase of the motor at a time. This creates fluctuating torque, which is unstable. It is also considerably less efficient than other step modes, because energy is lost in between energizing each phase. Because of the energy loss, less torque is produced than with other step modes.
Full Step Mode
With a full step mode driver, both phases of the motor are energized simultaneously and continuously. This produces full torque for all phases of the motor at all times. Full step motors are very stable, and one complete turn of the drive shaft is equal to one complete step sequence. For example, if there are 50 steps in the step sequence, 50 steps equals one 360-degree rotation of the drive shaft.
Half Step Mode
Half step mode is an alternating energy circuit. First, one coil phase is energized, swiftly followed by both coil phases being energized. This technique doubles the resolution of the step sequence. It creates varying degrees of torque because the single coil phase energy produces half the torque of the double coil phase energy. High end models decrease the torque differential by increasing the amount of current that passes through the single coil phase. This enables the single coil to increase torque output. Half step mode is very stable and doubles the possible resolution and number of step positions. A 50-step sequence stepper motor capable of operating in half step mode to allow for greater resolution has 100 step positions.
Micro Step Mode
Micro stepping mode is only available on high end stepper drive models, and, as such, are more expensive than other kinds. Micro stepping energizes each coil to a set fraction, allowing fractional steps to occur. This creates very high resolution and precise stepping. Micro stepping creates a very smooth step motion, and some models can move at 1/256th of a step.
Buying Stepper Drives and Controls on eBay
eBay has a large selection stepper drives and controls, available in both new and used condition, and the price range varies significantly across the range. Buyers should use specific search terms to get the most appropriate results results, such as "micro stepper drive," "chopper drive," or "unipolar chopper drive." Buyers can search from the Business and Industrial, Consumer Electronics, and Computer, Tablets, and Networking portals, which also provide relevant search results.
Buyers can narrow their search results by choosing to view specific listing types, such as Buy It Now or auction format. Other result narrowing options include viewing only new or only used items, items within a specific price range, or only items from Top Rated Sellers.
Buyers should ensure the drive is in good working order, particularly if it is a secondhand unit.
Buyers should also ensure that the postage costs are acceptable, and should take note of the estimated delivery time. They should also determine whether the seller offers postal insurance for expensive items. Buyers are encouraged to contact sellers and ask any questions not covered in the item description.
Buyers should choose to buy from sellers who have large amounts of positive feedback, or sellers who possess a "Top Rated Seller" seal. A "Top Rated Seller" seal indicates that the seller has a sterling eBay reputation, and has provided many customers with quality products and great service.
To choose the correct stepper drive, buyers must consider their budget, the intended application of the stepper motor, and the required features. Buyers should ascertain which drives are compatible with the motor in question, since some motors will not work with an incorrect drive. The required features are also important considerations. For example, bi-level drives, while very stable and efficient, are not capable of performing micro stepping. This means that despite their reliability, these drives do not offer the same precision and resolution as a chopper drive. For those on a tight budget, secondhand drives in good working order are a good choice. This enables buyers to purchase a high end drive model at an affordable price.