AS5040 10-BIT PROGRAMMABLE MAGNETIC ROTARY ENCODER
Using the SSI interface for absolute data transmission,
an additional delay must be considered, caused by the
asynchronous sampling (t= 0…1/fs) and the time it takes
the external control unit to read and process the data.
15.4.1 Angular Error Caused by Propagation Delay
A rotating magnet will therefore cause an angular error
caused by the output delay.
This error increases linearly with speed:
esampling = rpm ∗ 6 * prop.delay
where: esampling = angular error [°]
rpm
= rotating speed [rpm]
prop.delay = propagation delay [seconds]
Note: since the propagation delay is known, it can be
automatically compensated by the control unit that is
processing the data from the AS5040, thus reducing the
angular error caused by speed.
15.5 Internal Timing Tolerance
The AS5040 does not require an external ceramic
resonator or quartz. All internal clock timings for the
AS5040 are generated by an on-chip RC oscillator. This
oscillator is factory trimmed to ±5% accuracy at room
temperature (±10% over full temperature range). This
tolerance influences the ADC sampling rate and the
pulse width of the PWM output:
ƒ Absolute output; SSI interface:
A new angular value is updated every 100µs (typ.)
ƒ Incremental outputs:
the incremental outputs are updated every
100µs (typ.)
ƒ PWM output:
A new angular value is updated every 100µs (typ.).
The PWM pulse timings Ton and Toff also have the
same tolerance as the internal oscillator.
If only the PWM pulse width Ton is used to measure
the angle, the resulting value also has this timing
tolerance.
However, this tolerance can be cancelled by
measuring both Ton and Toff and calculating the
angle from the duty cycle (see section 6):
( ) Position = ton ⋅1025 −1
ton + toff
15.6 Temperature
15.6.1 Magnetic Temperature Coefficient
One of the major benefits of the AS5040 compared to
linear Hall sensors is that it is much less sensitive to
temperature. While linear Hall sensors require a
compensation of the magnet’s temperature coefficient,
the AS5040 automatically compensates for the varying
magnetic field strength over temperature. The magnet’s
temperature drift does not need to be considered, as the
AS5040 operates with magnetic field strengths from
±45…±75mT.
Example:
A NdFeB magnet has a field strength of
75mT @ –40°C and a temperature coefficient of
-0.12% per Kelvin. The temperature change is from
–40° to +125° = 165K.
The magnetic field change is: 165 x -0.12% = -19.8%,
which corresponds to
75mT at –40°C and 60mT at 125°C .
The AS5040 can compensate for this temperature related
field strength change automatically, no user adjustment
is required.
15.6.2 Accuracy over Temperature
The influence of temperature in the absolute accuracy is
very low. While the accuracy is ≤ ±0.5° at room
temperature, it may increase to ≤±0.9° due to increasing
noise at high temperatures.
15.6.3 Timing Tolerance over Temperature
The internal RC oscillator is factory trimmed to ±5%.
Over temperature, this tolerance may increase to ±10%.
Generally, the timing tolerance has no influence in the
accuracy or resolution of the system, as it is used mainly
for internal clock generation.
The only concern to the user is the width of the PWM
output pulse, which relates directly to the timing
tolerance of the internal oscillator. This influence
however can be cancelled by measuring the complete
PWM duty cycle (see 15.5).
Revision 1.6, 03-Oct-06
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