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
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 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.
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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In the situations described above, we could easily assume that all axes are under some central control that synchronizes their individual motions. In many applications, however, motion may be externally generated or controlled. Furthermore, it may be important to synchronize other axes to this motion. This type of synchronization, known as “following,” applies to any controlled motion that responds to a separate measured motion. The axis of the measured motion is called the “master,” and the controlled axis is called the “follower.”
A typical following arrangement has other axes moving in direct relation to an externally controlled rotating shaft. This master shaft influences the other shafts according to a given function or functions.
Apart from mechanical, there are electronic means by which following can be put to use. Electronic gearing is achieved by moving a follower axis at a defined ratio to the motion of the master shaft. Electronic cams may be designed around a repeating pattern of changing ratios referenced to the motion of the master shaft. The master motion is usually measured with an encoder.
Ratio is one of the most basic concepts in following. It is the change in an axis position (follower travel) with respect to the change in master travel. A direct analogy is velocity, which is the change in axis position with respect to time. The important difference is that control of the follower is programmed as a function of motion, not time. This locks the relationship between the motion of two or more axes and allows precise synchronization. The ratio may change within a profile, but to completely define the position relationship between master and follower, the master travel over which the ratio changes must be specified. Change in ratio over a known master travel is analogous to acceleration, which is a change in velocity over time.
Coil winding and filter winding are examples of applications benefiting from following. These tasks often use a repetitive changing-ratio pattern. Typically, a rapidly rotating spindle holds a bobbin onto which a coil is wound. A traverse axis moves back and forth along the length of the coil at some ratio to the spindle rotation, guiding the wire as it winds onto the spool. A low ratio of reciprocating motion to spindle rotation will result in a tight coil – a large number of turns per inch and a low helix angle. A higher ratio of traverse motion to spindle rotation will result in a looser coil, with the turns distributed widely along the spool.
For management and control purposes we often sort objects and events into increments. (Time, for example, marches on, but is taken in seconds, minutes, and hours.) The master cycle method divides continuous master motion into meaningful portions. A programmer defines the master cycle length to the controller, which measures master travel according to cycles and positions in a cycle.
A master cycle may correspond to one machine cycle or one finished product. In the winding example above, it would likely be set as the number of spindle rotations occurring during a complete forward and backward stroke of the wire spool (one complete follower cycle). A coil with 20 layers would go through 10 master cycles.
Because a master cycle and product cycle often coincide, it’s important to begin measuring master travel at a spot corresponding to the start of the product cycle. This is usually done by electronically sensing the arrival of a product or moving machine part. In some cases it may not be feasible to physically place the sensor at a concurring location. If so, program the controller to assign the master travel a non-zero initial value according to the physical offset of the sensor.
If a controlled follower axis is in sync with a master axis at a certain ratio, that ratio establishes follower position change with respect to master position change; but it disregards the alignment of master and follower. While the ratio of their rates of displacement may be perfect, their relative positions may be off. In most applications, a moving machine part must match the speed of another moving part exactly – they move at a 1:1 ratio. Proper alignment, or phase, is also mandatory. A familiar example of phase manipulation is the use of a strobe light to adjust automobile engine timing.
A phase shift may be commanded to correct the alignment of master and follower without affecting the ratio of the motion. With regard to visual alignment, the phase shift appears as an advance or delay of the follower.
Motion has two components during such a shift. One of these is a result of following, the other of shifting. The shift is a normal move specified with acceleration and velocity. This is superimposed onto the designated follower motion. With a known amount of alignment correction, a preset shift can be commanded. However, a machine operator must often perform visual alignments. In any case a shift is instated until alignment is correct. The shift is then discontinued without interrupting the component of motion resulting from following.
A drum roll, please
Web processing is an application facilitated by the capabilities of following. An example of web processing is printing onto a continuous sheet of paper. Here, the inked print portion of the print drum must apply the pattern to the paper precisely between registration marks. While the print is applied, the surface speed of the print drum has to match the paper speed exactly or ink will smear. In this particular paper product, the distance between registration marks is shorter than the circumference of the drum. Hence, the drum must speed up during the non-print portion of its rotation, then slow down, ensuring surface and paper speeds are matched on cue to resume priniting. Although the distance between registration marks is nominally even, minor variations require an alignment correction each time a registration mark is detected.
To work this out smoothly, set the paper travel as the master axis, measuring it with an encoder. The print drum (the follower axis) answers to the motion of the paper. The surface travel ratio must be 1:1 during the printing portion of the cycle. During the remainder of the cycle, the ratio is higher, such that the drum travels one revolution for each registration mark. When a registration mark passes the registration sensor, the drum should theoretically be exactly halfway through the non-print portion of the cycle. The actual drum position is captured when the registration mark is sensed, and the alignment is corrected with a superimposed phase shift.
John Rathkey is Senior Design Engineer with Parker-Hannifin, Compumotor Div., Rohnert Park, Calif.