Servo systems lead the way to implement high-performance motion control

Servos have reigned for many years as the high-performance motion solution for applications such as robotics, machine control and material handling. Servo motor systems deliver precise, responsive and powerful control, but often at the expense of some implementation complexity. Following the “faster, better, cheaper” trend that started in aerospace a few decades ago, today’s servo drives are incorporating built-in features to improve ease of use and make servos work in more applications, while saving material and labor costs.

Servo systems

Servos are a type of electrical motor, usually with a rotating output shaft that can be connected to equipment directly or via a gearbox. As a dc motor, it provides high torque even when moving from zero speed. A key difference between a servo motor system, as compared to typical ac induction or dc motors, is that the servo motor incorporates an encoder used to provide positional feedback. Using this sensor, the servo motor operates as part of a closed-loop system and can thus be driven to particular rotational locations at dictated speeds, often following elaborate movement profiles (Figure 1).

While servo motor sizing and selection is important, the real brain behind the operation is a servo drive. The drive translates logical motion commands into an electrical signal driving the motor, and it interprets the feedback from the motor to ensure precise motion. Some servo drives can perform significant logical control internally, while others may be operated from a supervisory PLC, PC or other controller via a communication link such as Modbus TCP or EtherNet/IP.

Some typical servo system movements can be:

• rotating a filling machine turret at a fixed velocity
• accelerating and decelerating a conveyor to precisely space items on a belt
• moving X/Y/Z portions of a machine gantry from one position to another
• accelerating a robot joint from a standstill to a certain velocity and then decelerating back to a standstill so the joint stops where commanded.

While there are hard-coded approaches for performing these motions, many users would like a simplified approach for creating these moves and for implementing other advanced functionality.

Built-in control

Some of the newest servo drives take advantage of powerful processors and software to enable wizard-like functionality. Graphical user interfaces provide simple setup and programming, while support of standard communication protocols makes it straightforward to integrate servo drives with other systems. Following are a few built-in control features users can lean on to quickly create solutions for many types of common motion and application needs.

• Internal motion: While an external PLC or PC can certainly send commands to a drive, there are many smaller systems where it is better for the servo drive to provide native capability for users to configure logic and mathematical operations. In some cases, the drive can be an all-in-one logic and motion control solution.
• Electronic gearing: This is a feature where many secondary axes can be commanded at speeds that are a ratio of a primary axis. The ratios and even phase relationships can be changed instantly and on the fly so that machine elements move at the proper relational speed without needing gearboxes.
• Electronic camming: Physical cams operate mechanisms over a stroke and duration profile. Servos with electronic camming capability can reproduce these mechanical system movements, while adding the capability to be readily changed to match the needs of machine and product changeovers.
• Rotary and flying shears: These machines cut material on the fly and therefore require precise servo operation to move the cutter to match the material speed.
• Registration: Packaging, cutting and printing machines often take advantage of registration, which is using a detector to identify where moving product is located so a servo-driven portion of the machine can be driven exactly to where it is needed.