SBAA530 September 2021 TMAG5170 , TMAG5170-Q1 , TMAG5173-Q1
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Achieving the highest accuracy possible often requires running a system at a lower speed, which decreases system performance. For applications requiring fast, real-time control – highly accurate measurements at high speeds – decreased system performance is not an option. One application with these requirements is a motor transport system, also known as a linear mover. A linear mover consists of a smart track system with built-in microcontrollers (MCUs) that enable independent positioning of each mover. This system configuration provides multiple advantages in automation control, including high transfer speeds, accurate positioning and the ability to move products forward and backward on the line to increase manufacturing efficiency.
Linear movers use an array of sensors to track the absolute position of a magnet mounted on the mover. As Figure 1-1 shows, the sensors are placed equidistant along the X-axis. Calculating the angle from the X and Z components of the magnetic fields using the adjacent linear 3D Hall-effect position sensors resolves the absolute position of the mover.
For linear mover systems that require accuracy and high speeds, a high-performance linear 3D Hall-effect position sensor can reduce errors. Internally, these error sources occur because of sensitivity, offset, linearity, noise produced by temperature variations and input-referred magnetic noise that couples into each axis. Externally induced errors come from many sources: mechanical vibration, unintentional spatial variations and tolerances between the magnet and the sensor, fast-changing magnetic fields, and off-axis angle measurements.
An important aspect of a high-performance linear 3D Hall-effect position sensor is an onboard angle coordinate rotation digital computer (CORDIC) calculator that performs iterative trigonometric approximations in a two-dimensional space to calculate both angle and magnitude, achieving a resolution of 0.25 degrees. Performing this calculation on the device eliminates the need to post-process the magnetic information.
Fast-changing magnetic fields are problematic for linear 3D Hall-effect position sensors that do not have independent internal signal paths, which include single-axis measurement, gain adjustment, filtering, and data conversion. To reduce errors, 3D Hall-effect position sensors with a single data converter and a feature called pseudo-simultaneous sampling provide the best solution.
Figure 1-2 illustrates the ideal circumstance where sampling of Z and X1’ occurs at the same instance in time. With changing magnetic fields and a single signal path for taking measurements; however, this is not possible. With pseudo-simultaneous sampling, an average of the X component just before and just after X1‘ (labeled X1 and X2, respectively) is used as the X-axis value that matches up to Z. Assuming that changes in the B-fields are relatively linear over small intervals, the results are similar to the sampling of both axes at the same time.
Linear 3D Hall-effect position sensors typically house sensor technologies that enable the sensing of magnetic fields both in-plane and perpendicular to the package. Because these are two distinct sensors, the magnetic noise for each is different. Therefore, it is advantageous to equalize the noise floor by averaging samples of each axis. A linear 3D Hall-effect position sensor that can average as many as 32 samples reduces the noise floor substantially.
A high-accuracy 3D Hall-effect position sensor that can take precise measurements at high speeds – such as the TMAG5170 – is very useful for this application. The TMAG5170 delivers ultra-high accuracy thanks to a maximum full-scale sensitivity error of ±2.6%, low sensitivity mismatch error (per axis), and low drift overtemperature. Such precision could eliminate the need for system-level calibration, which decreases total system cost. Additionally, the TMAG5170 delivers sample rates as high as 20 kSPS, which is fast enough for many linear mover applications. For more information on the impact of sample rate, see the Angle Measurement with Multi-Axis Hall-Effect Sensors application report.
Ensuring sufficient measurement accuracy and speed while consuming as little power as possible is also important for systems that monitor movement or position. This is essential for battery-powered or low-power systems, where the power consumption of the magnetic sensor could use a significant portion of a system’s total power. One example of this is a remote monitor for a factory floor that communicates with a programmable logic controller through a 4- to 20-mA loop.
Other battery- or low-powered application examples are door and window sensors used in building or home security systems. A door sensor with angle detection can detect an open-and-close event and measure how far the door has opened. It is possible to implement this capability with a ring magnet and a linear 3D Hall-effect position sensor embedded in a traditional door hinge (see Figure 2-1). For low-power applications, linear 3D Hall-effect position sensors should be placed in a low-power duty-cycle mode running at low frequencies of 5 Hz or less.
An added benefit to using a linear 3D Hall-effect position sensor in this application is that only two axes of sensitivity monitor the angle, enabling the use of the third axis for detecting malicious magnetic tampering or disabling, such as placing a magnet over the sensor to trick the system into believing that the door is still closed.
The TMAG5170 offers various power options (see Table 2-1) to optimize system power consumption. Depending on application needs, the sensor can be set to full active conversion, standby, duty cycle, sleep or deep sleep.
Power Modes | Operation |
---|---|
Active conversation | Produces a continuous stream of data. Used when fully analyzing or processing magnetic fields. Typical current consumption is in the low, single-digit mA range. |
Standby | Device is in a low-power state and will start a measurement upon a prompt by the MCU. Current consumption is < 1 mA. |
Duty cycle | The device is placed in a sleep mode and wakes up at specified intervals to take measurements. Current consumption is 1.3 µA at 1 Hz. |
Sleep | Similar to duty-cycle mode, except that the device is waiting for an MCU prompt to take a measurement, rather than doing so on its own. |
Deep sleep | The device is essentially powered down, consuming 5 nA of current. |