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Direct-driven ball-nut key to high performance
By Mike Everman, CEO, Bell-Everman, Inc. -- Design News, June 3, 2002
COMPANY: Bell-Everman, Inc.
LOCATION: Santa Barbara, CA
ENGINEERING SERVICE PROVIDED: Motion stages for high performance pick-and-place, metrology, and genomics
PROJECT DESCRIPTION: A circuit board inspection OEM had a difficult set of requirements for a two and three axis motion system, most importantly low volume cost, high repeatability, and settling times below 20 msec with near continuous 2g accelerations of a heavy payload. Their existing design was of a fairly conventional two axis stacked type, light duty re-circulating linear bearings, a folded-back brushless servo-motor with rotary encoder and a belt pass to a ball-screw shaft. While the existing system had acceptable performance for many years, it was clearly not going to meet the new, more demanding specifications for accuracy, repeatability, speed, and stability.
ENGINEERING CHALLENGES: Known factors were that the customer would need linear encoders in the system for position feedback, a highly responsive and stiff drive method, and more rigid mechanics. Linear motors were first considered as the prime mover, but the preferred system was of a stacked linear stage configuration for economy (as opposed to a YYX gantry style). The base stage of this multi-axis stack would require a very large linear motor to move its load (150 lb) at 2g accelerations with an 80% duty cycle, further complicated by the mass of the second and third axis linear motor magnet track. Also, the budget would nearly be exhausted by the linear motors and associated shock absorbers alone. Further, the envelope required that a ballscrew driveline be folded back in a similar fashion to the existing system with a belt pass or some other approach in order to reduce the length. In addition, the efficiency of sinusoidal commutation as opposed to "trapezoidal," Hall-effect commutation would be required to get the most out of the selected motor.
TOOLS AND COMPONENTS EMPLOYED: Bell-Everman, Inc. presented the customer an adaptation of its Lowboy™ linear stage chassis with the new ServoNut™ integrated servo motor driven ball-nut actuator. The basic Lowboy stage consists of two linear bearing rails, optical tape and read-head digital linear encoder with 1 micron (0.00004 inch) resolution, optical gap limit sensors, and internal "energy chain" cable tracks to manage utilities out to the moving carriage. Key to the price/performance success of the forcer is the consistent preload and balance of its precision rolled ballscrew from Bosch Rexroth, and the high performance of its custom-wound frameless motor sets from Hathaway/Emoteq. Position feedback is provided by Heidenhain LIDA 400 tape-scale linear encoders, and linear bearings are by NSK.
Everman further introduced the customer to SST 1500 digital amplifiers from Teknic, Inc. In order to reach the cost and performance required of the mechanics and electronics, the system would need the efficiency of sinusoidal commutation and vector torque control, an extremely tight servo-loop for quick settling (owing to its 10 kHz servo update rate and less than 50 micro-second total servo phase delay), and the jerk-limiting of a "Regressive Auto-Splining™" feature.
SOLUTION: The requirement for high rms forces and drive stiffness made the decision for a ServoNut screw drive an easy one, however static screw/driven nut designs are nothing particularly new. Typical of this "moving motor" genre, it employs a belt driven ball-nut, rotary encoder and precision ground ballscrew. We found that optimizing the motor windings of a direct-driven ball-nut made extremely low settling times possible(&5 msec), with less service points by eliminating the belt. The addition of linear encoder feedback allowed the use of a lesser-cost ballscrew, and it was fortunate that a good quality pre-loadable rolled screw product could be found. If there were to be any stiffness dead-band in the ball nut, a completely different control scheme would have to be used. This involved the addition of a rotary encoder to the motor, and a centralized controller capable of "dual loop" operation (in which the rotary encoder is used for velocity control, and the linear encoder is used for position). The selected control scheme brought high efficiency and, therefore, greater than normal rms (continuous) torque capability because of the amplifier's ability to control motor torque as a vector, as opposed to the more common and less efficient scalar current control methods. Higher efficiency allowed the use of smaller, less expensive NEMA 23 motor sets in an application that normally would have required a NEMA 34 frame.
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