MINT motion controllers from Baldor can position drives ranging from a mini brushless servo to an 800-hp vector drive or a dual axis linear step motor. Programmable functions include speed, position, and master/slave.

Increasingly, drives must position a part as well as control the speed and direction of shaft rotation. This is especially true for indexing, pick-and-place, and cut-tolength applications.

Traditionally, servo systems (servo drive and motor) have provided position control through adjustable-speed drives. Their main drawback is cost. So many engineers are turning to ac drive technology and ac systems to control position. Their performance is nearly as good as that of servo systems, but at lower cost.

Servo positioners

A typical servo system consists of a dc brush or brushless drive and motor with feedback for closed-loop control. A PLC or PC controller tells the drive the speed and direction plus the elapsed time to move a part from one location to another.

A tachometer-generator (tach) usually supplies motor speed feedback to the controller while an encoder or resolver provides position data for the motor or part being moved. Based on this input, the controller adjusts current to the motor so that the actual and intended positions coincide. In some cases, an encoder supplies both speed and position feedback.

Dc servomotors are usually chosen because they are smaller and lighter than ac versions, and they have low-inertia rotors. As a result, a dc servo can accelerate and decelerate a load faster (up to 4,000 rad/sec2) and position it more accurately than an ac drive. Brushless dc drives are commonly used for precision applications of less than 20 hp.

AC vector drives

VLT 5000 SyncPos control cards from Danfoss mount inside ac adjustable frequency drives ranging from 1 to 300 hp. Each card includes programming software and is used for speed and position control, plus synchronizing. Being inside the drive, it requires no extra space or wiring. Fieldbus options include PROFIBUS, DeviceNet, Modbus Plus, and LonWorks.

Most general-purpose ac adjustable- speed drives don't have position control. However, vector controlled, pulse width modulated (PWM) drives are an exception. These drives use regulators with microprocessors and DSPs that improve drive response and position regulation.

Vector drives with built-in position controllers operate ac induction or permanent-magnet synchronous motors, and they can produce 100% of full torque at zero speed. Like dc servos, these ac drives incorporate both speed and position feedback loops. Therefore, they can also be considered servos.

For applications that require less accuracy than that of a traditional servo system, an ac vector drive with an induction motor and encoder can replace a low end servo system for about half the cost. Moreover, ac induction motors are simple and reliable, which means low maintenance.

Positioning applications that require drives of 20 hp or more increasingly are using newer ac vector drives. Examples include indexing and sorting, pick-and-place, cut-tolength, palletizing, machining (grinding), and packaging equipment.

Inside story

A palletizer machine typically uses an adjustable-speed drive and a PLC with sensors to monitor and control motion. It runs at two speeds in both forward and reverse directions -- high speed to begin movement, and low speed approaching the stop point. A closed-loop position controller eliminates the low-speed and stop sensors. The motion profile can be tailored to achieve a higher peak speed and faster ramp-up and ramp-down.

A vector drive gets its position control capabilities from a built-in control card or intelligent motion controller. Typically, this device can be programmed to set motion profiles plus home and absolute positions based on encoder pulses. The card fits inside a standard drive enclosure, so it requires no extra space or wiring.

Using a built-in unit eliminates the D-to-A and A-to-D signal conversions that take place with standalone devices. Digital signals between the motor control and motion controller eliminate both conversion time and noise (errors).

Option boards for network communications (top), I/O expansion (middle), and ac to dc control interface (bottom) mount inside a GV3000/SE ac adjustable-speed drive from Reliance Electric. Installing either the network communications or I/O expansion card lets you use the drive's inner and outer control loops for a specific application.

Such controllers also ease the burden on system PLCs. For example, some adjustable-speed drives rely on a series of limit switches or proximity sensors to aid in accelerating or decelerating to a stop. These sensors signal changes in motor speed at different points along the motion path (high-speed start, reduced speed when approaching target position, zero speed at target). A PLC monitors all of these sensors as well as encoder positions, and sends signals to the drive. A built-in controller eliminates the need for these sensors, making it possible to use a smaller PLC, and delivers more accurate speed and position control.

Information for this article was provided by R.J. DiMaggio of Baldor Electric Co., Jeff Duncan of Danfoss Inc., Brian Macomber of Reliance Electric, and Bart de Vries of Saftronics Corp.