Accuracy ingredients

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 onboard 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.