How Do Calnetix Active Magnetic Bearings Transform Efficiency and Reliability?

Position sensors are critical components of any active magnetic bearing. Their role is to continuously provide the controller with accurate up-to-date information about the rotor position unaffected by the external factors. They must be immune to external factors such as speed, temperature, dust, working fluids, external magnetic and electrical fields, etc.

Significant progress has been made in developing self-sensing AMBs, that combine position measurement and force generation functions into one physical component. However, such combination units have fundamental performance limitations, likely limiting their use to low-cost, low-speed machines. A conventional bearing arrangement utilizing dedicated radial and axial sensors will likely remain dominant.

Different types of position sensors have different property combinations, making some more suitable for specific applications than others. In industrial-use environment, it is typically preferable to have one technology that covers as many applications as possible. When considering the wide breadth of available technologies, such as optical, capacitive, and electromagnetic position sensors, electromagnetic position sensors are the most universal solution.  

Calnetix utilizes four types of sensors, depending on the application specifics:

• Radial Reluctance Sensors
• Axial Reluctance Sensors
• Constant -Flux Axial Edge Sensors
• CBR Eddy-Current Sensors

All sensors include the sensitive element (sensor head) integrated into the machine, where the displacement needs to be measured. Conversely, the sensor electronics are typically located in the Magnetic Bearing Controller (MBC).

A speed sensor provides the active magnetic bearing with information about the rotational speed and angular position of the rotor. This information is required to adjust  the control parameters as speed changes to enable the rotor unbalance cancellation. Calnetix uses two types of dedicated speed sensors. Both sensors utilize a source of a DC magnetic field and a  Hall-effect element that measures that field. They also contain additional rotor features, such as  a notch which modulates the magnetic field.

• Smart Speed Sensors - The smart speed sensors utilize a built-in magnet as the magnetic field source, two Hall-effect sensors for the differential field measurement, and an internal logic controller that  looks for periodic patterns  in magnetic field variations over time, interpreted as speed. Since these sensors are designed to look for any periodic pattern of the magnetic field, regardless of the field intensity levels, they do not need any calibration and are very easy to use. On the downside, they do not offer any flexibility in sensitivity adjustments, which may be needed in some applications.
• Programmable Speed Sensors - These sensors utilize two programmable Hall-effect sensors and an external magnetic field source. The parameters of the programmable Hall-effect sensors can be adjusted to address specifics of a particular application. These sensors are often integrated into the “dead poles” of the radial actuators to take advantage of the available actuator bias magnetic field. This significantly reduces axial length of the machine, allowing for higher rotordynamic margins.

Backup bearings support the rotating assembly when the magnetic bearing system is deactivated. They also provide rotor support in the event of an overload of the magnetic bearings or a failure of one of the components. During normal operation, the backup bearings are inactive, and have a small radial and axial clearance to the rotating assembly. This radial clearance is, usually on the order of 0.003”-0.009”.

Calnetix’s most common backup bearing configuration is two Duplex stainless-steel pairs of angular contact ball bearings preloaded face-to-face (DF). This mounting configuration requires the bearing outer races to be clamped – either rigidly or via a spring – on the outer race of the bearing. Hybrid bearings are available as backup bearing options, typically fabricated from conventional SAE 52100 steel races and SiN3 balls. Backup bearings for high-speed active magnetic bearings are usually cageless and use either a grease film, dry film lubricant or a light grease fill.

Calnetix Technologies has developed and deployed advanced Magnetic Bearing Controllers (MBCs) that contain all of the elements necessary to operate magnetic bearing systems.  

The company currently produces two  MBC platforms in the Insight™ series and one in Concurrence™ series.

For more information, visit the Magnetic Bearing Controllers product page.

The Calnetix Service Graphical User Interface (GUI) is used for system setup, monitoring, diagnostics and system identification. The upper half of the front panel, shown below, displays orbit plots from the two radial bearing sensors as well as a time history plot from the axial bearing sensor. Other operating variables shown on the GUI include spin speed, static current, and synchronous displacement and current. Status variables, such as levitation and fault status, are also indicated on the GUI. Finally, a System Information window on the right gives version information for the GUI, firmware, parameter file and machine serial number specifier.

More detailed information displays and functionalities can be accessed through the action buttons: Digital Signal Processors (DSP) Actions, View Logs, Fault Summary, View/Set Parameters and Measurements.

Such additional functionality includes the following: 

• Transfer function measurement- plant transfer function, compensator transfer function, open loop transfer function, closed loop transfer function and sensitivity transfer function
• Logs- fault, event, ESD data, machine specific calibration and system data 
• Scope- internal high frequency measurements for key operating signals, such as rotor motion and coil current (actuator forces)
• Continuous data acquisition- long-term recording of high-frequency data, such as position, current and speed for disk storage