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, such as speed, temperature, dust, working fluids, external magnetic and electrical fields, etc. Even though significant progress has been made in developing self-sensing AMBs, wherein  the functions of the position measurement and the force generation are combined in one physical component, their fundamental performance limitations will likely limit their use to low-cost, low-speed machines. A conventional bearing arrangement utilizing dedicated radial and axial sensors will remain dominant.

Different types of position sensors have different combinations of properties, making some of them more suitable for a given application than the others. However, in the industrial environment, it is typically preferable to have one technology that would cover as many applications as possible. From a wide variety of the available technologies, including optical, capacitive and electromagnetic position sensors, the latter appears to be 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) located in the machine, where the displacement needs to be measured, and sensor electronics, typically located in the Magnetic Bearing Controller (MBC).

A speed sensor provides the active magnetic bearing with information about the rotational speed and the angular position of the rotor needed for adjusting the control parameters as speed changes and enabling the rotor unbalance cancellation. Calnetix uses two types of dedicated speed sensors. Both sensors utilize a source of a DC magnetic field, a Hall-effect element measuring that field and a feature on the rotor, such as a notch modulating the magnetic field.

• Smart Speed Sensors - The smart speed sensors utilize a built-in magnet as a source of the magnetic field, two Hall-effect sensors for the differential field measurement and an internal logic that looks for a periodical pattern in magnetic field variations over time, which is then interpreted as speed. Since these sensors are designed to look for any periodic pattern of the magnetic field, regardless of the 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 source of the magnetic field. 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 already available actuator bias magnetic field. This significantly reduces axial length of the machine and allows for greater rotordynamic margins.

Backup bearings provide support for the rotating assembly when the magnetic bearing system is deactivated, and provide rotor support in the event of an overload of the magnetic bearings or a failure of some component of the magnetic bearings. In normal operation, the backup bearings are not active as they will have a small radial and axial clearance to the rotating assembly. This clearance is at most one-half of the magnetic bearing air gap and typically considerably smaller.

The most common backup bearing configuration used by Calnetix is two duplex pairs of angular contact ball bearings preloaded face-to-face (DF). This mounting requires the bearing outer races to be clamped – either hard or through a spring – on the outer race. Hybrid bearings with conventional SAE 52100 steel races and SiN3 balls are standard. Backup bearings for high-speed applications 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 different MBC platforms in the Insight™ series and one in Concurrence™ series.

For more information, see 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 and a time history plot from the axial bearing sensor. Other operating variables shown include spin speed, static current, and synchronous displacement and current. Status variables, such as levitation and fault status, are indicated as well. A system Information window on the right gives version information for the GUI, firmware, parameter file and machine serial number specifier.

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

• 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