Spindle Control

What is Spindle Control?

Spindle Control means controlling high speed motors (usually Brushless DC motors) in applications such as machine tool cutting, drilling, centrifuges, cryo-pumps, bar code readers, rotating marking and scanning systems, spin coating, and more. What connects all these applications is the need to drive the motor at high rotation speeds sometimes exceeding 50,000 RPM, and with high efficiency and minimal motor heating.

Design Considerations for Spindle Control

The motion control challenge involved with high speed spindle control begins with the unusual nature of typical spindle motors themselves. To achieve their high rotation rate these motors typically have very low coil resistances and electrical time constants. So when driven by a PWM (Pulse Width Modulation) switching amplifier a central consideration is that the bridge circuitry operate efficiently at high PWM rates.

Typical PWM rates for high speed spindles are 80 kHz or higher. If driven with a lower PWM rate the current ripple due to the switching waveforms will generate excess heat in the motor. Switching amplifier high PWM rates are best-accommodated by SiC (Silicon Carbide) MOSFETs due to their very low switching losses. Nevertheless Silicon MOSFETs remain popular and have improved in performance over time.

A second important consideration in high speed spindle control with Brushless DC motors is use of FOC (Field Oriented Control). FOC improves the available torque output for a given motor at high speed and reduces heat generation in the motor. FOC is recommended for all spindle control applications where current (torque) control is needed for the application. This would apply for spindles that drive a high inertia load such as centrifuges or cryo-pumps, or where the load may create variable levels of resistive torque such as routers and drills as they cut into the target material.

An alternate control method can be used for spindles which operates the motor in voltage made and uses a scheme called third-leg floating to drive the motor coils. Third leg floating helps minimize the speed-reducing impact of back-EMF and has the advantage of being simple to implement. This scheme does not provide current/torque control and so typically relies on the operator to avoid stalling the motor resulting in an over current situation. A common application is dental and surgical drills/cutters where the doctor/practitioner listens to the motor speed and applies more or less pressure to moderate the applied torque.

Most spindle controllers also provide a velocity control and profiling function. While traditional position PID servo loops can be used for velocity control (if you control position with time you also control velocity with time) use of a dedicated velocity PI servo loop may have benefits such as higher velocity tracking accuracy.

spindle-control-application-image

Vital Spindle Control Techniques

  • High PWM (Pulse Width Modulation) frequency
  • Use of SiC MOSFETS
  • FOC (Field Oriented Control)
  • Third-leg floating drive
  • PI velocity servo loop
  • i2t current foldback
  • Shunt control of DC power supply
  • Direct motor temperature sensor input

Regardless of the control method used, safety techniques to avoid overheating the motor are important in the spindle control application. One important method is called i2t current foldback. This is a technique that measures how much current (and therefore heat) is being injected into the motor above and beyond what it can naturally radiate or conduct away. A more accurate approach is for the controller to input a thermistor input signal located in the motor. In both cases the controller shuts down or limits current output if the motor heats up excessively.

Finally, another safety feature that may be important for spindle operation is management of the motor drive voltage (HV) via a shunt function. Shunt control 'dumps' excess voltage that may occur when the motor decelerates rapidly into a resistor. Motors spinning at high rates have large rotational inertias and can therefore act as a generator during deceleration. Depending on the design of the power supply used a shunt function may be needed to avoid a potentially damaging overvoltage conditions on the HV supply.


Application Architecture & Approach

The diagram below shows the calculation flow for the FOC (Field Oriented Control) technique used with Brushless DC motors, also known as Synchronous AC motors. FOC is an important technique used in spindle control to achieve the greatest efficiency and torque output at high spin rates. 

SpindleControlDiagram

 


Spindle Control Solutions from PMD

Since 1994 Performance Motion Devices products have been used in a range of spindle control applications including spin coating, centrifuges, bar code scanners, machine tools cutters, grinders, surgical drills, electric aircraft propulsion, centrifugal pumps, marine propulsion, and more. PMD’s Juno Velocity and Torque Control ICs are ideally suited for high performance spindle control applications, as are PMD's ION/CME N-Series Digital Drives providing up to 1 KW of output in a compact PCB-mounted module format.


Machines That Frequently Use Spindle Control

  • Drills
  • Routers
  • Machine Tool Cutters
  • Centrifuges
  • Bar Code Scanners
  • Cyro Pumps
  • Centrifugal Pumps
  • Winding Equipment
  • Surgical Drills
  • Surgical Cutters
  • Drum Scanners
  • Laser Scanners
  • High Speed Cameras
  • Spin Coaters
  • Motor Winding Machines
  • Marine Propulsion
  • Turbo Pumps
  • Electric Aircraft Propulsion
  • Gyroscopes
  • Ventilators

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