No matter what your motion control application it must be: Predictable Verifiable Controllable In order for this to happen with your application, you must keep the fallowing three things in mind: Accuracy Stiffness Vibration
Accuracy vs. Precision Precision also called Repeatability Accuracy is not the same thing as precision. "Accuracy" represents the ability to get closest to the true point you are expecting. "Precision" measures the ability to repeat a move and get back to the same point (regardless of accuracy). “Precision” can be achieved regardless of Accuracy. I like to the following illustration. A watch dial is graduated in l/5th of a second intervals between each minute mark. Thus the watch is precise, but unbeknownst to the person using the watch to observe the time, the watch is five minutes slow. Reading a time to the nearest 1/5th second with this watch, while being precise, is ridiculous, because the time reading is five whole minutes away from the true time - the watch is inaccurate.
Accuracy & Precision (Repeatability) Accuracy – Difference between expected position and achieved position. Precision (Repeatability) Bi-directional – The error from nominal when repeatedly approaching a position from opposite directions. Precision (Repeatability) Uni-directional – The error from nominal when repeatedly approaching a position from the same direction. Here you will see a graph that helps illustrate this.
Accuracy ResolutionWhile Resolution is not Accuracy…
It is part of what makes up Accuracy
Two parts of Resolution1. Electrical Resolution2. Mechanical Resolution
And then there’s resolution It can be divided into the fallowing points: Resolution is the fineness of position precision that is attainable by a motion system. The precision of repetitive positioning is dependent on the resolution of the linear encoder. In addition, it is also necessary to have sufficient machine rigidity. In the same way, the absolute positioning precision is also fundamentally dependent on the linear encoder. Resolution, Electrical – The smallest increment that can be “commanded” by a servo system (minimum programmable move increment). Precision of the feedback system (scale, reader head, & controller logic) sets this figure. Resolution, Mechanical – The smallest increment that can be “controlled” by a motion system (minimum actual mechanical move increment). Mechanical precision is often coarser than electronic resolution due to: friction; station; deflections; etc.
Mechanical stiffness Motor built-in stiffness - The epoxy structure of an ironless motor has a low inherent stiffness. The motor rigidity is given by the copper coils that are inserted and is dependent on how the coils are physically put together: separated or overlapped. An overlapping configuration would lead to a higher bending stiffness compared to a construction with independent coils whereas the lateral stiffness will be almost identical in both overlapping and separated configuration. The steel structure of an iron core motor make it obviously much stiffer than an ironless solution. The Cylindrical design of the Linear Shaft Motor is makes it the much stiffer than an ironless or iron core solution.. Ability to respond to commanded motion Flux pattern (Magnetic Servo)
Mounting stiffness - By design, the mounting surface of an ironless motor (for example) corresponds generally from 10% to 25% of the active surface of the motor. The attachment surface is located on one side of the motor. Besides allowing almost no thermal conduction, this kind of mounting can be problematic when high position stability or very tight settling times are required. The main reason is that the current that is flowing in the motor phases can excite the transversal natural frequency of the motor, leading to oscillations once in position.
In an iron core and Shaft motor, the mounting surface is generally 100% of the active surface of the motor and centered above the motor. The stiffness of this mounting is therefore much higher. As long as the carriage is stiff enough to withstand the attraction forces, no vibration problem should be foreseen. The Linear Shaft Motor does not have this problem.
The fallowing sketches are presented to illustrate general concepts of common position feedback control schemes and should be viewed as a “food for thought” exercise. In most applications, work is being performed on or near the slide’s mounting surface, either by moving a target on the slide with respect to a fixed tool or by moving the tool on a slide with respect to a fixed target (something moving & something stationary). In either case, motion control should be near the slide. Cost vs. precision generally dictates the feedback method used for positioning systems. This figure illustrates an open-loop stepper-motor driven linear stage. With this approach, a control system sends a string of pulses to the motor which causes it to rotate. For example, 10,000 pulses to a 2,000 pulses/rev motor will cause it to rotate 5 times. Directly coupled to a 5-pitch lead screw, 10,000 pulses will cause the slide to move precisely 1.0 inch (maybe). “Precisely” 1.0 inch will only happen if the stepper motor rotates exactly 1,800° and the lead of the screw is “exactly 0.200” and there is no compliance (zero backlash) in the nut and the coupling is stiff and there is no lost motion caused by stiction in the bearing systems. All of these “If, ands, or buts” can be minimized by using very high priced precision components. Fortunately, there are less costly, alternative approaches to consider.
This figure replaces the stepper motor with a servo motor/rotary encoder combination. Now the rotation precision can be sensed and corrections made if necessary. All of the other “and” parameters listed above are still present though.
This figure replaces the motor-mounted rotary encoder with a linear encoder scale and a slide-mounted reader head. Though this bypasses all the potential error causing parameters mentioned above, position control is only good to the point of reader head and scale interfacing. Work is typically not performed at the encoder but somewhere above the slide’s surface. As shown in an earlier section, angular errors could adversely affect precision at the point of work.
This figure illustrates a method for bypassing error potentials all the way to the point of work. In this illustration, a laser provides position feedback “where” the work is being performed (at point A).
power applications where fine control isn’t needed.
• DC motors provide a range of performance:– DC brush: versatile, “servo” motor, high speed, torque– DC brushless: speed/toque depend on electronics– Stepper: simple control signals, variable speed/accuracy
without gearing, lower power– Direct-drive (torque) motors, expensive, lower torque
• Linear actuation via drives, or voice coils.ELECTROMATE