Incremental encoders are used on knobs, wheels, motors, and flow meters to measure relative rotary movement. By attaching a ring magnet to the rotating component and placing the TMAG511x nearby, the sensor will generate voltage pulses as the magnet turns. The TMAG511x integrates two sensors and two signal chains. This means each channel can go up to the maximum speed independently from each other.
When the magnet rotates, the TMAG5110 will generate alternate pulses on each output. One input will be the result of what is sensed from one specific axis, while the other output will sense from another specific axis. This is also referred as Radial and Tangential magnetic flux in Table 9-1. Those two signals are the result of two different components of the same magnetic field resulting in the two signals being 90° from one another. Also called quadrature output, this type of signal is ideal to measure a rotational count as well as a change in direction of the ring magnet.
The TMAG5111 directly generates the speed and direction outputs. This eliminates the need for external processing.
The maximum rotational speed that can be measured is limited by the sensor bandwidth and the magnetic strength of the magnet.
Generally, the bandwidth must be faster than two times the number of poles per second. In this design example, the maximum speed is 22500 RPM, which involves a rotation of 3000 poles per second when using an 8-pole magnet. The TMAG511x sensing bandwidth is typically 40 kHz, which is more than thirteen times the pole frequency.
The strength of the magnet also has an impact on how fast the magnet can turn. A weaker magnet with a maximum strength very close to the threshold value will limit the maximum speed by limiting the amount of time where this field will be higher than the BOP. The time spent above the BOP value will be longer for a magnet with stronger field.
When the magnet strength is significantly higher than BOP, Equation 5 can be used to calculate the allowed speed.