Even if you could mathematically model systems exactly, the physical world is too messy — has too many variables — to flawlessly produce paths described by clean, exact equations. Nonlinear friction, noise, vibration, and various forms of backlash color all machine motion. But machine accuracy and precision are getting better all the time, making for improved positioning and increasingly defined motion.
“Two things limit precision: Interference from physical and electrical sources over the entire system, and the ability of the design (including manufacturability) and materials to provide highly repeatable results over time,” says Robert Davis of HBM Inc., Marlborough, Mass. To illustrate: Precision ball valves require extremely tight tolerances for a consistent seal when closed. In most industries, tolerances are typically in the thousandths of an inch, if not higher. But improved hardening techniques are now producing valve balls (and, in fact, rollers for other applications as well) with diameter and sphericity tolerances within millionths of an inch. This makes for consistent repeatability — that remains over time.
How is overall ball precision improved? New electronic controls are replacing manual control of grinding pressures during manufacturing. “A pressure control motor utilizes a resistor that senses pressure reduction as the balls decrease in size during the grinding process, and readjusts the grinding plate distance accordingly. This keeps a constant pressure on the balls and allows for more consistency,” explains Joe Beltrami of Hartford Technologies, Rocky Hill, Conn. “This is especially beneficial to high accuracy and precision designs, which perform better with through-hardened elements of high-carbon and martensitic-type stainless steel,” he adds. In contrast, case-hardened materials (of low-carbon steels and softer materials — normal austenitic stainless steel or brass) can deform and don't hold tolerances as well.
Accuracy — the closeness with which a system can come to a target mark — is different for system stops than it is during motion. For example, when it comes to controlling torque, spring-loaded clutches are more accurate than friction-style slip clutches. Oil shear brakes resist high temperatures for better accuracy under cycling, but low inertia is paramount to quick-response stopping accuracy.
Sending motion over any distance reduces accuracy, especially when transmitting through various linkages. “The power transmission chain, particularly when powered by a rotary motor connected to couplings, gearboxes, belts, and screws can have significant negative impact on system accuracy, even if there is a sensor on the load downstream,” says Curt Wilson, engineering and research V.P. at Delta Tau Data Systems, Inc., Chatsworth, Calif. Like a game of telephone, each time power transmission signals are handed off, error is introduced. The same goes for electronic components. “Minimizing low, mV-level signal paths by moving signal conditioning close to signal sources can significantly increase measurement quality,” says Davis of sensors detecting accuracy. “In fact, modern component sizes have made this particular technique readily available, since reliable circuits can now be built in extremely small footprints.” In the same way, mechanical linkages degrade performance less when they're small and close to main system action, with handoffs kept to a minimum. “For example, to reduce tolerance stack, some drives integrate a ballscrew, carriage, and pillow-block bearing supports in one rigid unit,” says Clint Hayes of Bosch Rexroth Corp., Hoffman Estates, Ill.
What if all but one component is high precision? “We've seen situations where designers have selected a certain level precision to incorporate in a component, only to find that some mating part doesn't allow for the same level of precision,” says Beltrami. He stresses that one solution is in material science itself: New, diverse materials — engineered plastics, ceramics, and glass — are increasingly strong and corrosion resistant, which translates into better precision. Picking lighter materials for power-train components (couplings, in particular) also makes the job of correcting servo-loop gains easier. “The first thing designers should do for maximized accuracy and precision is to contact suppliers to help with the selection of component materials to interface in their design,” Beltrami advises.
Another approach is to use a direct-drive system. “High-precision systems increasingly employ linear motors for linear motion that greatly simplify the transmission chain and dramatically reduce the errors and control difficulties introduced,” explains Wilson. Along the same lines are integrated rack-and-pinion systems.
Rack-and-pinion systems reduce nodes of tolerance stack, but are less expensive — and improved rack production and test engineering has made for units with positioning accuracy of 12 µm or better. Servomotor and transmission technology, and calculation and simulation of the complete system (including, for example, the type of connection between system elements) also improve their accuracy. “These actuators consist of a high-precision rack-and-pinion unit and a rotary actuator which moves the load,” explains Miriam Bilstein of Alpha Gear Drives, Inc., Bartlett, Ill. “They're appropriate for applications such as high-end machine building — machine tools, for instance.”
The friction factor
You'll never completely rid a moving system of friction, and that's fine — as long as it's constant. (Controls can correct for consistent phenomenon.) The real concern is unpredictability. Mechanical friction, particularly what is known as stick-slip friction, can make it very difficult to position precisely. “For this reason, high-precision systems often employ hydrostatic or aerostatic bearings that either eliminate friction or at least eliminate the difference between static and running friction,” says Wilson. One caveat: Aerostatic bearings can be pricey. Sleeve bearings, though their friction is higher, maintain a consistent value even after long operation.
“Ballscrews move load (generating push or pull action) from point to point with repeatability and precise positioning,” says Hayes. In these traditional units, the issue of inconsistent friction can be addressed in a few ways. More durable balls of harder material are one solution: “The main limiting factor with regard to precision and accuracy in balls, pins, and bearings is the material,” reiterates Beltrami. Another solution is proper lubrication. Maintaining consistent contact (and preventing backlash) can be done with preloading, but designers must keep in mind here that preloading increases accuracy-degrading twisting and wear.
All mechanical devices have a critical speed — the speed at which they reach first order frequency and vibrate excessively. Ballscrews are a prime example. As their unsupported length increases, permissible rotational speed decreases. For optimum accuracy and precision, end supports and increased material diameter compensate for speed. Engineers have developed makeshift devices to improve critical speed with engineered solutions, providing temporary or varying support units that decrease the unsupported length of the ballscrews. Before, this was left up to machine builders to design, develop, and implement — a considerable investment in engineering time and additional materials.
“Consider an application requiring a ballscrew with a length of 6,000 mm between supports, a diameter of 40 mm and a lead (linear travel per one rotation) of 40 mm. A standard ballscrew without radial support except at the end of the screw could achieve a linear velocity of only 5.5 to 6 m/min. By comparison, an equivalent drive unit with 3 pairs of screw supports would achieve a linear velocity of 48 m/min.,” explains Hayes.
Thanks to these speed capabilities, ballscrews can be used in applications which previously could only be fulfilled by rack and pinion and belt and pulley drives — which provide greater positional accuracy and more repeatable (precise) motion, Hayes adds.
In contrast, compliance in timing belts can't be avoided. Instead, the approach for higher accuracy is to make settling time as quick as possible — usually with higher tensile strengths.
Accuracy is the ability to produce true, consistent, and repeatable results. For motors, this means delivering smooth, routine, and error-free motion. Feedback mechanisms, controllers, and drives work behind the scenes to ensure a motor maintains highly accurate position and velocity.
At the motor-drive level, accuracy ultimately comes down to precisely controlling current for positioning. “One way this is achieved is through drives that use special commutation algorithms, such as phase advance techniques,” says Ed Novak, Trio Motion Technology, Pittsburgh, Pa. In addition, feedback devices serve up motor commutation and position information to the drive. “By using an encoder and a stepmotor, for example, a motion system can monitor position accuracy and account for missed steps,” says Mindy Lin, Lin Engineering Inc., Santa Clara, Calif.
Position and current transducers are options too. “Position transducers deliver very high linearity and accuracy,” says Rob Schmidt, Rockwell Automation: Kinetix Motion Control Business, Mequon, Wis. “Current transducers, on the other hand, measure motor current, exhibiting low offset, low gain error, and high linearity.”
Although feedback devices with high resolution deliver high accuracy, it's generally the controller that verifies position after a move is completed, and if necessary, corrects it. “True servo systems constantly monitor position and velocity and make ‘on-the-fly’ corrections,” comments Mike Rogen, Maxon Motors Inc., Burlingame, Calif.
Some controllers and drives also allow users to configure system parameters, such as motor resistance and inductance, potentially increasing precision even further. “With these, designers should use feedforward terms (if available) and take time manually adjusting servo loop gains,” advises Rick Dye, Ormec Systems Corp., Rochester, N.Y.
For engineers not so inclined, real-time autotuning, available on some controllers, will automatically adjust servo gains according to the machine. “The addition of position smoothing functions in the amplifier — different modes accommodate different situations — let motors respond more uniformly to sudden position commands,” says Sunny Ainapure, Mitsubishi Electric Automation Inc., Vernon Hills, Ill.
Lastly, whether the goal is position or velocity accuracy, the ideal approach is to shoot for a resolution finer than what's required. “Higher resolution leads to higher servo gains, which improves tracking and disturbance rejection,” explains Curt Wilson, Delta Tau Data Systems Inc., Chatsworth, Calif.
Pushing the limits
Every motion component limits a control system, and overall accuracy, differently. Resolvers, for instance, can cause position variances by several arc minutes over one revolution, due to cyclic error. “At times, this position error varies with temperature,” explains Rick Dye, Ormec Systems Corp., Rochester, N.Y. “When used with a tracking converter, resolvers often create position error during acceleration.”
With encoders, their own assembly mostly limits accuracy. “Any eccentricity in the disc or optics' mounting causes errors,” says Rob Schmidt, Rockwell Automation: Kinetix Motion Control Business, Mequon, Wis. As motor speed and encoder resolution decrease, the position feedback's discrete steps, or quantization, become pronounced. To minimize limitations in positioning applications, Mike Rogen, Maxon Motors Inc., Burlingame, Calif. suggests employing encoders with complementary signals. “In addition, proper cabling and shielding can prevent electrical noise, and ultimately, positioning errors,” he says. Environmental conditions also pose limitations. According to Mindy Lin, Lin Engineering Inc., Santa Clara, Calif., “Wind, dust, and rain can harm motors not enclosed in the completed system.”
Back at the component level, controllers must maintain high resolution and speed. Quantization noise (from limited resolution) and implementation delays most limit control-loop performance. Resolution can be limited at the input (feedback) and output (servo command). “Key delays include sampling (about one-half cycle behind) and transporting — moving data in and out of the controller. Serial encoders and ‘smart’ amplifiers can actually increase these delays,” says Curt Wilson, Delta Tau Data Systems Inc., Chatsworth, Calif.
Often overlooked is how the drive internally converts the controller's commands. “Most new digital drives convert an analog 10-Vdc command to a digital value, which is less precise and lowers accuracy,” says Ed Novak, Trio Motion Technology, Pittsburgh, Pa. High-speed digital control networks remove analog commands from the controller and push all control loops to the drives for improved system accuracy.
How then, do drives help motors achieve high accuracy? It turns out that various algorithms, as well as a driver's update frequency are quite influential. “As such, drives with feedforward permit quicker move times than trial and error manual tuning methods,” says Rogen. “And, high pulse width modulation and control-loop update rates enable high current-loop bandwidth,” says Dye. “Furthermore, linearly modulating the power transistors can eliminate electromagnetic noise from switching,” explains Wilson.
Drives programmed with 32-bit floating-point representation of position and velocity deliver accurate motor control. “Today's encoder resolutions are less than 32 bits,” states Schmidt. “As accuracy requirements become more stringent, 64-bit representation may be necessary.” To suppress quantization and raise dynamic accuracy, drives can even alter current to the motor via high order and notch filters.
Drives also improve accuracy for stepmotors through pole-damping technology (PDT), which creates smooth motion. “PDT helps tweak current sine waves to alleviate the ‘jerk’ a motor experiences while moving toward its full step ‘on’ position,” explains Lin. Additionally, an on-board trimpot helps stepmotors achieve more accurate speed by altering the current waveform as it exits the motor and enters the coils.
Ultimately, drives control phase currents for accurate motion. “When properly matched and designed together, a drive compensates for motor variations and inaccuracies,” says Novak. Examples include calibrating commutation currents from winding tolerances, correcting commutation angles as a function of speed, and feedback alignment.