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All aboard for motion control

All aboard for motion control

Chicago--In the evolution of automation, motion control has moved from fixed mechanical systems through relay and electromechanical control, centralized programmable control, networked manufacturing systems, distributed processing and I/O, to today's open distributed-control model. Result: Abundant access to flashy modern amenities such as CD and mini-disk players, cellular phones, laptop and palm computers, advanced medical testing, and biotechnology. In large part, motion control suppliers deserve much of the credit. Striving to increase automation efficiency, suppliers increasingly face more of the integration challenges, so that engineers don't have to. Consequently, designers deal with fewer components and subsystems as the brushless-motor world shifts toward complete motion-control solutions.

For example, fractional horsepower motors with on-board electronics have been around for a while. However, such devices have had trouble keeping up with the advanced filtering, thermal protection, and connectivity advances of larger external motion controllers. Integrated motion pioneer Animatics Corp. (Santa Clara, CA), for instance, has had to come up with nine variants of its Micro Adapter to address connectivity challenges interfacing various combinations of inputs and outputs to the miniature controller embedded in its SmartMotor(TM). Another company, Intelligent Motion Systems Inc. (Marlborough, CT) has invested its resources in developing a patented heat sink fan-clip assembly to keep its powerful, yet compact, IM483H and IM805H hybrid microstepping drivers cool.

Even so, the integration effort continues at booth 1945 at the National Design Engineering Show as MicroMo Electronics Inc. (Clearwater, FL) unveils its new miniature 3564-BC (brushless motor with controller). Traditional external controllers, explains Steve O'Neil, MicroMo's VP of advanced research and planning, are burdened with complex connection and compatibility problems with different motor types. "Brushless servomotors require Hall sensors for commutation and the external controller requires an external encoder for accurate positioning and speed control. Combined, these two factors drive up the price of traditional systems, make them more complex, and consequently less reliable. In addition, long cables between external encoders and a controller are susceptible to electromagnetic fields, something that users of integrated motion control systems just don't have to deal with."

Due to the traditional six-pulse commutation scheme, most brushless motors exhibit torque variance at very low speeds, while higher speeds can create "noise" problems. According to O'Neil, the integrated motion control solution addresses these issues, yielding the following benefits:

  • Low torque variance at low speeds

  • More added functions such as velocity profiles, thermal protection, etc.

  • Flexible communication interface

  • Ease of use and connection

  • Low cost

  • Increased reliability and performance

Sine-wave current. Faulhaber GmbH, MicroMo's Germany-based sister company, uses a skew-wound coreless coil, powerful samarium cobalt magnets, and a layered-metal casing to improve magnetic properties of its brushless motors. A 35-mm diameter, 64-mm length motor, for example, delivers smooth power up to 70W without the "cogging" effect common to iron-core induction motors. "The coreless design's low inertial moment provides excellent dynamic performance," O'Neil adds, "and its back-EMF signal is a perfect sine wave for highly efficient motion control with very low torque variance. The 3564's sine-wave commutation optimizes motor compatibility."

A single chip microcomputer aids in miniaturization. Chips capable of motor/electronics integration became available just a few years ago. But the 8-bit technology of these devices would be overtaxed in this application, says O'Neil, requiring additional components to handle peripheral systems. That's why the 3564 uses a 16-bit microcomputer. It not only has a processor with fast division/multiplication commands, program memory, and RAM, but it includes peripheral capabilities such as a serial interface, quadrature pulse for the encoder signal, and a 10-bit AD converter, all in a single chip. In general, fewer discrete components means more reliable electronics.

  • Medical instruments

  • Test and measurement equipment

  • Semiconductor manufacturing

A special driver allows use of N-channel MOSFETs in the upper branch of the half-bridge circuit, to minimize electrical losses. "Due to higher electron mobility in N-channel MOSFETs," says O'Neil, "they have a lower ON-resistance than P-channel MOSFETs, and are the best choice." Producing 24V motor voltage and 5V for control electronics, the 3564's on-board switching controller minimizes operating current in the main controller by reducing losses. The biggest challenge, according to O'Neil, was circuit layout. The design required a two-layer integrated controller circuit to isolate the delicate sensor signals being processed in the microcontroller from the high-frequency PWM current pulses of the power MOSFETs.

While resolvers are normally used with sine-wave commutation, they are expensive and take up too much space for this type of application, explains O'Neil. Instead, Faulhaber uses linear Hall sensors positioned around the cylindrical magnet that constitutes the brushless motor's rotor, for precise positioning. These Hall sensors register an almost perfect sine wave signal except at the minimal and maximal sine-wave amplitudes. With the help of simulation software, engineers created an arcsine function table, stored in the controller that compensates for any deviation from the ideal by compensating for zero error and amplitude error in the controller.

Two of three Hall signals, calculated through a weighting function to an angular value, provide the rotor's position, and reduce the remaining errors that lead to pulses in velocity values. "With the correct field vector for every rotor position," says O'Neil, "the motor performs much like a dc motor with a large number of commutator segments."

Speed and position control. Iron-core motors require current control because current follows voltage jumps very quickly in motors with low-inductance air-gap coils. In contrast, Faulhaber's motor design eliminates the need for current control altogether. "Consequently," O'Neil explains, "the optimal solution to velocity control of a coreless motor, which has good linear properties, is a PI controller." Digital filtering means no offset- or amplification-error as in analog solutions, because the PI (Proportional Integral) controller has no filtering deviations. The result is accurate compliance with the command velocity. To set command position without deviation, a position control layered over the velocity controller uses a simple P (Proportional) controller. This position output value then becomes the command value for the velocity controller.

The RS-232 port makes it possible to control the drive from a PC. The controller can be configured as a velocity control or a position control. In speed control mode, in addition to being able to program with the ASCII command at the RS-232 port, the command velocity can be regulated at the analog input with a potentiometer, or with a PWM signal. This means that the drive can also be controlled independently of the host PC. The speed is then simply proportional to a given voltage. With configurations, filter settings, and other parameters saved to the on-board EEPROM, the drive becomes a self-configuring autonomous unit as the parameters are recalled at the next power-on.

Integrating electronics makes a real thermal protection barrier for the motor itself possible. A temperature sensor on the circuit board measures the motor casing's temperature. The controller performs differential calculations on thermal (heat dissipation) and current flow models in the motor, and outputs an accurate value for the temperature of the motor coil. When the maximum operating temperature set point is reached, the drive automatically shuts down and reactivates only after the temperature has fallen below the maximum value, and the velocity is zero. Additional protection, provided by the I2 current limiting, sets the peak current to prevent short duration overloads. The continuous operation value can also be set, and the unit is protected from over voltage.

Additional details...Contact Steve O'Neil, MicroMo Electronics Inc., 14881 Evergreen Ave., Clearwater, FL 33762-3008; Tel: (800) 807-9166; Fax: (727) 573-5918; E-mail: [email protected].

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