Steppermotors are a category of electric motors that accomplish motion by pulling therotor to many discrete positions per rotation. This allows for simple speedcontrol by incrementing the desired position at a desired rate, as well assimple position control, by continuing to pull the rotor to the desiredposition once it has been reached.

Toaccomplish these tasks, a drive is required to control the direction andmagnitude of the current in the motor windings. These drives can be voltagecontrolled, where the resistance of the windings and the input voltage controlthe magnitude of the current in the windings, or current controlled, where acircuit senses the current in the windings, and repeatedly disconnects andreconnects the windings from the source voltage in order to limit the currentto a given level. This second type of drive, commonly referred to as a chopperdrive, allows for higher source voltages, increasing the maximum speed of themotor, while ensuring the motor windings are not damaged.

Bothof these drive types have been around for years and have limited utility ontheir own. In order for any but the simplest tasks to be completed, someintelligence is required to tell the drive when to take a step and whichdirection to take the step in. Preferred features for drives include theability to create acceleration and deceleration ramps, as well as a counter tokeep track of the position of the motor. Some system designers choose to createtheir own controller and driver, spending time up front creating a driveoptimized for their application. This is desirable when the project has thebudget and time to invest, but when development time is most important, anoff-the-shelf solution is best.

Inthe past, a common approach has been to program a separate controller, whichthen provided the drive with a signal indicating the direction to take a stepin, as well as pulses when it was time to take a step. This works well for themovement of the motor, but doesn't provide easy access to the drive's otherparameters, such as current during the move, current when not moving, and howlong after a step to switch from one current level to the other.

IntegratedIntelligence

Manycompanies have recently begun taking advantage of the shrinking size ofcomponents and more powerful processors to integrate intelligence into thedrive. This allows for reduced cost and size, as well as control of all thedrive's parameters through the same interface that controls the motor'smovements. It has also opened the door for more advanced features, such asincreasing the current when the motor is accelerating against a load andbacking off the current when at a stable speed, allowing larger loads to bemoved without risk of burning out the motor or drive. These drives aregenerally programmed using a proprietary language, most of which have commonfeatures such as looping, branching based on inputs and settings outputs.

Thefocus of these drives has been to give as much power to the system designer aspossible, allowing complex applications to be handled without additionalcontrollers. The drawback to this is that, for a system designer to begin towork with one of these drives, there is usually an entirely new programminglanguage that must be learned.

Thepurpose of buying an off-the-shelf drive is to save time; investing the timeyou saved by not designing your own controller into learning how to use someoneelse's controller defeats the purpose of using an off-the-shelf drive to beginwith. The best way to save time in the development cycle is to use a drive thatis simple to program and use.

In response to market demand for these types of drives,more companies are beginning to offer such products. An example of this is Haydon Kerk's programmable IDEA drive,available as a stand-alone unit or mounted to the back of the motor. Allprogramming is completed through the use of a graphical user interface withon-screen buttons such as "Extend," "Retract" and "Move To Position." Programsare built sequentially, top to bottom, with the completion of one commandimmediately followed by the command below it.

Whilesimplicity is a major point of interest for such drives, designers also needthe programming power that made the controller a necessary part of theirproject to begin with. A key consideration when looking to use a simple drivewith enough programming power for a variety of application is to ensure thatbasic features are included, such as: position counters, acceleration anddeceleration ramps, conditional branching, inputs and outputs. More advancedfeatures to consider are multiple different interrupt sources with differentpriority levels, and the ability to increase the current for the accelerationor deceleration portion of a movement. These features allow for a high degreeof motor control and interaction with the rest of the system.

Josh Pyne is electrical design engineer at HaydonKerk Motion Solutions.