Multi-Turn Position Sensor Provides true zero-power turn-on capabilities

Stephen Bradshaw, Christian Nau and Enda Nicol, Analog Devices

This article outlines the current methods used to enable multi-turn true-on (TPO) sensing capabilities and introduces a new simplified solution that is set to change both the industrial and automotive position sensing markets. The simplified system will allow designers, with or without experience in designing magnetic systems, to replace bulky and expensive solutions that exist.

Position sensors and encoders are ubiquitous in automotive and industrial applications where it is vital that the position of the system is known at all times. However, existing position sensors and encoders can only provide single-rotation or 360° TPO position information. Systems that require TPO position information over multiple turns or a wider measurement range typically include a backup power supply to track and store the multiple turns of the sensor for a single turn after an unexpected loss of power, or to track multiple turns of motion along shutdown or power-down time. Alternatively, a gear reduction mechanism can be added to the system to reduce multiple turns to a single turn and in combination with a single turn sensor to find TPO position information over multiple turns. These solutions are expensive and bulky, and in the case of a battery backup system, a regular maintenance contract is required.

Rotary and linear encoders are key devices used in applications where the system designer must ensure that the position of the mechanical system is always known for closed-loop control, even after power is lost as part of the normal duty cycle or accidentally. The challenge for system designers is to ensure that the TPO position is available even after a power loss. If system state is lost, then a long and often complex procedure is required to reset the system to a known state.

Current solutions

Modern factories are becoming increasingly dependent on robots and cobots to reduce cycle times, increase productivity and improve efficiency. One of the major costs and inefficiencies associated with standard robots, cobots, and other automated assembly equipment is the resulting downtime required to reroute and initialize power after a sudden loss of power during operation. This resulting downtime and loss of productivity is both costly and inefficient. Although this problem can be solved with spare batteries, memory and sensors in one turn, these solutions have their limitations. Battery packs have a limited life and maintenance/service contracts are required to manage battery replacement. In certain environments where there is a risk of explosion, the maximum energy that can be stored in the battery is limited. The reduction in energy storage results in a shorter maintenance cycle where batteries need to be replaced more often.

An alternative to battery backup is to use Wiegand cable modules to harvest power. These modules use specially treated wire where the magnetic coercivity of the outer shell is much higher than the coercivity of the inner core. The different coercivities create voltage spikes at the output of the device as the magnetic field rotates. The spikes can be used to power external circuits and write the number of turns into a ferroelectric random access memory (FRAM). The magnetic multi-turn memory, which was developed by Analog Devices, does not require an external power supply to record the number of turns of an external magnetic field. This results in reduced system size and cost.

Multi-turn sensor technology

At the core of the magnetic multi-turn sensor is a spiral of giant magnetoresistive (GMR) material composed of multiple nanowires of GMR elements. The principle of operation of the sensor is based on shape anisotropy and generation of domain walls in a domain wall generator in the presence of an external magnetic field. As the external magnetic field rotates, the domain walls propagate through the narrow spiral paths (nanowires) attached to the domain wall generator, as shown in Figure 1.

As the domain walls move through the helical leg structures, the state of each element of the helical legs changes. Since the elements are manufactured from GMR material, the condition of each one can be determined by measuring their resistance. The sensor relies only on the external magnetic field and no additional backup power or energy harvesting technique is required for the RPM sensing operation. When power is re-applied to the sensor, readout of the spin count status is available without requiring additional user action or system reset.

A combined technological solution that simplifies system design

The top-level block diagram of the ADMT4000, shown in Figure 2, combines the previously described GMR multi-turn sensor with a high-precision AMR angle sensor and an integrated signal conditioning IC to provide a solution capable of recording 46 turns or 16,560° of movement with a typical accuracy of ±0.25°. An integrated signal conditioning IC enables further system enhancements to maintain harmonic calibration, which can eliminate errors due to magnetic and mechanical tolerances from the application. ADMT4000 provides absolute 46 revolutions (0° to 16.560°) digital output via SPI or SENT interface. The ADMT4000 is positioned against a dipole magnet mounted to the rotating shaft as shown in Figure 3.

ADI is preparing a magnetic reference design that will allow users with little or no magnetic design capabilities to easily adopt the ADMT4000 into their application. In addition to the magnet core design, this reference design will provide immunity and resistance to stray magnetic fields, allowing customers to deploy the sensor in harsh environments. Stray fields can be generated from many sources where current is carried along a wire, especially when used in close proximity to electric motors or actuators. The ADMT4000’s capabilities are valuable in many industrial applications, including tracking the position of robot and cobot arm joints in the event of a power failure or during a power outage (see Figure 4). Other industrial applications include absolute and TPO tracking of xy tables in industrial automation, machine tool, or medical equipment applications (shown in Figure 5). Other rotary to linear applications include, but are not limited to, counting the revolutions of spools, drums, pulleys, pulleys, hoists, winches, and elevators (Figure 6) when powered or tracking motion when power is off or during of power failure

Additionally, the TPO position sensing provided by the ADMT4000 is of significant value to automotive applications including, but not limited to, transmission actuators (Figure 5), electric power steering (EPS), including power steering (Figure 7), parking lock actuators, other general purpose actuators and seat belt retractors (Figure 8).

The ADMT4000’s size, price, and operating temperature range allow it to be used in a wide range of applications, including safety-critical applications in the automotive and industrial space. Automotive safety-critical applications are compliant with ISO 26262 and a specified Automotive Safety Integrity Level (ASIL). The ADMT4000 will be supplied as ASIL-QM or ASIL-B(D) to suit applications that have and do not need the extended ASIL or SIL functionality.

Conclusion

The ADMT4000 and the first integrated TPO multi-turn position sensor are set to significantly reduce system design complexity and effort, ultimately resulting in smaller, lighter and cheaper solutions. The ease of use of the ADMT4000 will allow designers with and without magnetic design capability to add new and improved functionality to current applications and open the door to many new applications.

To learn more about the ADMT4000 and the magnetic reference design, please contact your local ADI sales team who will be happy to discuss your requirements and applications, or visit analog.com/magnetics.