Motion Control - Drives
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Today’s drives can go much further than simply handing down commutation commands to a motor. The addition of onboard processing power and memory have dramatically increased functionality. Some drives can handle limited motion-control tasks. Other drives can act as master devices to a set of slave drives to create distributed control networks that reduce the need for wiring and control cabinets. In the case of so-called universal drives, the same device can be used to drive stepper motors, brushed motors, or servomotors.
Built-in functionality is another trend. Drives are available with embedded fieldbus capabilities that equip them to support a variety of protocols such as EtherCAT, Ethernet/IP, and CANopen. Safety-rated drives feature built-in redundancy and integrated safety functions like Safe Torque Off and Safe Limited Speed. They can also handle dozens of I/O points, with input from multiple types of sensors.
Stepper motors are 50- to 200-pole motors that can deliver high-resolution positioning for a variety of low-end applications. The drive delivers a series of pulses to energize the windings, causing the rotor to turn. The amount it turns is a function of both drive and windings.
Stepper motors can be driven in various modes such as (in order of increasing steps per revolution) full-step mode, half-stepping mode, or micro-stepping mode. The greater the number of steps per revolution, the greater the resolution¾but only to a point. Frictional forces prevent stepper motors from being reliably positioned to within a single step. The motor can be very accurately commanded to move to a specified angular position, however. This ensures that positional errors are not cumulative.
Stepper motors are typically operated open loop (without feedback). Eliminating feedback devices cuts both cost and complexity. For open-loop stepper motors, the drive does not need to be tuned, which simplifies commissioning. Stepper motors also can be operated closed-loop by adding an encoder or resolver. By introducing position and velocity feedback, closed-loop operation helps prevent the homing issues associated with stalling.
Closed-loop stepper motors provide higher position and velocity accuracy than open-loop stepper motors. Although their performance is still somewhat below that of servo motors, they offer different advantages such as holding torque. They also have lower overall cost, despite the addition of feedback and the need for tuning. For the right application, a closed-loop stepper motor may be the best choice.
Servo drives are available to control single-phase brushed motors and three-phase brushless motors. Combined with the right feedback, servo drives can close the loop on velocity, position, and/or torque, delivering high accuracy, repeatability, and precision. We offer them with connectivity to most popular fieldbus protocols such as Ethercat, Ethernet/IP, and ProfiNET.
Auto tuning options in the latest drives significantly speed commissioning. OEMs no longer have to spend hours manually tuning PID loops. Auto tuning technology also works to maintain machine performance by compensating for vibrational changes caused by wear. Although this is a key functionality, it can also mask developing problems. It’s important that the drive send notification of any changes, so that maintenance can investigate potentially troubled assets.