Machinery and automated processes often rely on two or more axes of motion coordinated to achieve a common end. As applications get stricter and more elaborate, control techniques have to be refined. Along with the basics of multi-axis synchronization, some system configurations are explained.

Keep your axes sharp

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An “axis of motion” is a one-degree-of- freedom entity. It can be a linear or rotary path. Often, two separate axes must move together to get a desired result. Multi-axis synchronization is therefore required to ensure that relative motions take place effectively.

One example of a synchronized-axis application is an X-Y plotter. The individual axes are able to be move independently, but without the other’s help they can only draw straight lines. To draw any 2D figure precisely, the axes must be coordinated, and a slight mistiming in the actuation of either can ruin the plot.

Relatively simple endeavors such as plotting a diagonal line require that the two axes merely begin moving at the same time. But most applications demand more than just start-stop togetherness. Tracing complex shapes, ensuring moving parts operate without collision, and positioning work pieces are instances where thorough position and velocity control is necessary. Sometimes the location or speed of one axis dictates the motion of another axis, and the accuracy of the combined motions depends on how well the triggering axis is monitored.

Mechanical motivation

Mechanical linkages represent the traditional method of multi-axis control. The usual setup is a single, centralized prime mover with gears and drive trains branching off to activate separate axes. Such arrangements work well if reduction ratios are constant and drive trains are short. With complex layouts, though, the mechanisms become costly and backlash and wear begin to stack up.

If the relation between axes is a repeating pattern, cams are another mechanical answer. The follower traces the cam profile, converting the driving motion into a different output motion. Very complex cam shapes can be designed and manufactured, although not without difficulty and expense, to deliver intricate movement. Cams also are subject to localized stress as well as frictional wear, and their accuracy and repeatability may begin to taper off as such factors take their toll.

Clutches and brakes should not be overlooked either, providing start-stop as well as acceleration and deceleration to cycle and control individual axes. Again, wear and tear is an issue, and the slip between frictional elements can delay the response, causing inaccuracy.

Electronic actuation

It is perhaps obvious that there is a great contrast between electronic and mechanical positioning techniques. Purely electronic systems always use motors on each axis, never channeling a single power source through multiple drive trains, cams, and the like. With fewer moving parts, backlash and surface wear generally leave the picture, and accuracy depends mainly on programming, timing, and electronic control.

For electronic motion systems, a single axis consists of a motor, drive, and controller. The controller takes motion instructions from a host computer or an internal program, turning the code into continuously updated position commands (motion profiles) that flow to the drive. The motor drive adjusts motor current to achieve the commanded position. In a multi-axis system, one controller can handle several motors along with their drives.

The motion control system can be either stepper or servo. Stepper systems are usually less expensive than servo versions, with less speed and power for a given motor size. In stepper systems the drive receives position commands in the form of low-voltage pulses (steps) and adjusts current phase in two sets of motor coils to align the motor shaft. Every new step received corresponds to another increment of shaft rotation. Current is maintained in the motor coils even when the rotor is in the correct position. Step motors commonly have resolutions ranging from 200 steps/rev (full stepping) to 50,000 steps/rev (micro stepping).

Servo systems rely on rotor position feedback, either from an incremental encoder or from a resolver. The actual position and velocity derived from the feedback is compared to that commanded in the motion profile, resulting in a torque command to the drive. The drive controls the motor current amplitude proportional to the torque command. In servomotors the current phase is adjusted according to the actual shaft position, in a process called commutation. The phase adjustment is continuous, producing maximum torque for the given current amplitude. Commutation is done mechanically in brushed motors, and electronically in brushless.

Servo systems are either analog or digital. In analog systems, feedback goes to the controller, whose output is an analog torque command. In digital servo systems the drive interprets steps as position commands, and shaft feedback goes to the drive only. To work best, servo systems should be tuned to match the load. A properly tuned system results in powerful and precise load positioning.

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