Key: Sort out the contributions of the step motor and driver
By Randy Frank – Design News
When it comes to precision in motion control, many factors contribute to inaccurate steps. An engineer considering a total system must be aware of how the step motor and the driver/controller unit work together in determining its overall accuracy.
A common misunderstanding is that all the inaccuracy in a system is the motor’s fault. From the step motor’s perspective, there are tolerances to meetâ€”both mechanical and electrical. Phase imbalance in inductance is a large factor, as is pole misalignment, rotor misalignment, inconsistent air gap between rotor and stator, stator and rotor tooth relationship, and torque stiffness, to name several. Yet, it isn’t difficult to achieve and maintain control of these parameters.
Motor inductance, for example, is proportional to the number of turns per coil squared, so correct winding of the motor achieves consistent inductance between phases. Most step motor manufacturers use automatic winding machines that maintain this consistency. Maintaining uniform inductance throughout the motor requires a consistent rotor magnet material. Most of the other specifications are mechanical. Manufacturers using reliable, quality parts with good process control for consistent grinding of the rotor and stator achieve step motor accuracy within design limits. For instance, Lin Engineering holds a ±5 percent difference in inductance between the two phases in a two-phase bipolar step motor, according to Belal Azim. This tight tolerance allows a step motor to perform with a ±1.5 arc-min error during 64 microstepping for a 0.9-degree motor, combining accuracy and precision. By meeting these specifications, the step motor will do exactly what it is told.
The next task is to make the driver/controller tell the step motor exactly where to move and what to do without degrading motor precision with an inaccurate driver. The driver, by giving a specified amount of current to each phase, dictates how many microsteps the motor should take. Full-stepping a motor provides the best accuracy because it matches the motor’s mechanical design. The stator and rotor teeth are aligned and the driver outputs maximum current to the phases. As the step divisions become greater, as in higher microstepping, accuracy becomes more difficult to maintain.
Typical drivers are capable of 2X, 4X, 8X, 16X, 32X, 64X, 128X, and 256X microstepping. Dividing the step motor’s full step by the respective number indicates the improved precision from operating in that mode. For example, a 0.9-degree step motor in 64Ã— microstepping mode will move 0.014 degree for each step it takes. For this to happen, the driver must divide the amount of current going to the motor phases in a precise matter.
Examining the phase diagram for quarter stepping (4Ã— microstepping) and its graph of current versus time provides insight into the percentage of current a driver must output to maintain accurate microsteps. From the driver’s perspective, the main component to output the correct amount of current is the integrated circuit (IC) driver. The performance of a driver is limited by the design of the chip. Other factors that can lead to inaccurate performance are the internal MOSFETS, capacitors and resistors, circuitry layout, firmware (software), and inadequate heat dissipation. Even though the driver chip alone can create smooth and precise sinusoidal current waveforms, it is not sufficient to guarantee optimum accuracy.
Every step motor has certain performance characteristics, many of which are dictated by the target applications. Motors made for low-speed performance have high inductance values. Motors designed specifically for high-speed applications have low inductance values. Step motor engineers accommodate different motion profiles by changing the windings in the coils to meet the mathematical formulas associated with speed, torque requirements, current, resistance, and inductance. As a result, one driver will not perform equally with different step motors, and one step motor will not output the same amount of torque with different drivers.
The ultimate solution for getting the smoothest and most accurate system is to match the step motor to a driver. Currently, the closest the industry has come to customization of drivers is adjustable functions for the output current. Some drivers have adjustable trimpots to alter the current waveform, others have adjustable gain, and some have the ability to download specific sine tables to match the motor’s characteristic using a graphical user interface. Trim pots allow the user to manually match the current for the combination without dealing with the technical aspects of the motor and driver. However, there are pitfalls for users looking for the optimum combination.
Five simple steps to matching motor and driver accuracy in a motion control system:
1. Choose the motor (based on application speed and torque requirements).
2.Ensure that specifications of the motor inductance can be limited to a Â±5 percent difference between phases.
3.Choose the driver. If possible, obtain a current waveform diagram of the driver.
4.Determine the availability of special features or options that can provide smoother motion, such as changes in damping (slow and/or fast decay) or adjustable trimpots.
5.Match the motor’s inductance to the driver’s capability. As a general rule, higher inductance motors perform best at low speeds and require drivers that can produce higher damping, or fast decay. Damping helps motors discharge the inductance quicker. Lower-inductance motors perform best at high speeds. These motors will work best with drivers that can produce low damping (slow decay) since they do not need the extra help in discharging the inductance. For any midrange inductance motors, use a driver that will produce a mixture of slow and fast, or mixed, decay.