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tuning_gains [2018/09/22 22:56] (current)
73.128.99.196 created
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 +The basic autopilot uses an enhanced PID filter to form a feedback loop.   ​Various gains can be adjusted to improve performance and vary depending on the boat, seastate, and [[rudder drive motor]]
  
 +The gains are as follows:
 +  - P - proportional - heading error
 +  - I - integral -  based on the accumulated error
 +  - D - derivative - rate of turn
 +  - DD - derivative'​ - rate of rate of turn
 +  - PR - proportional root - square root of heading error
 +  - D2 - derivative^2 - rate of turn squared
 +  - FF - feed forward - change in heading command
 +
 +It is recommended to use the opencpn plugin, or openplotter control for tuning the gains because visual feedback is provided.
 +
 +To get started retuning from scratch (or on a new boat) set all of the gains to zero, except the P and D gains. ​  It is possible to have a fully usable (but less efficient) autopilot using only these two gains.
 +
 +Set the P gain to a low value (say .003) and the D gain to .01.   ​Typically on larger boats, you will need higher values, but it really depends on how fast the drive motor turns the rudder.
 +
 +
 +The hard over time is how long it takes to turn the rudder from end stop to end stop.  This is typically 30 degrees for each side.   If a smaller motor is geared down more, and takes, say 16 seconds, then these gains should be doubled to P=.006 and D = .02 as a starting point.
 +
 +
 +From here, adjust just these two gains seeing how the boat reacts before adding other gains. ​ Once you are comfortable adjusting these gains consider:
 +
 +P - proportional gain
 +This value should normally be set low.  If it is set too high, the boat will constantly turn across the desired heading. ​ If it is too low, the boat may fail to maintain course. ​  As it is increased a higher D gain is needed to compensate (prevent overshoot)
 +
 +D - derivative gain
 +This is the gyro gain, and the main driving gain of the autopilot. ​ Most of the corrections should be as a result of this gain.   Once the best value is found it can typically work in a range of conditions, however, in light air, it can be reduced (along with reducing other gains) to significantly reduce power consumption.
 +
 +
 +PR - proportional root gain
 +This gain can be really useful preventing oscillation especially upwind. ​  To use it, increase it until it takes effect, and gradually back off on the P gain.   You will still need some P gain, but it may be less than half of before if a sufficient PR gain is used.
 +
 +
 +DD - derivative'​ gain
 +This gain is useful to improve reaction time.   It can allow for corrections sooner than they would occur from the D gain alone. ​  To use it, gradually increase this value up to 1.5x the D gain value without changing other gains, and notice the results.
 +
 +
 +FF - feed forward gain
 +This gain is only useful when making course changes. ​ For holding heading it has no effect. ​ Following a route can cause course changes. ​  It can be very useful in improving the response time since a low P value is normally desirable.
 +
 +
 +I - integral gain
 +This gain does not need to be used to hold a course, however it can compensate if the actual course held is different from the commanded course. ​  If following routes, and the boat tends to follow along a line parallel to the route, this will compensate for that error. ​ It is best to start at zero, and very carefully increase it until the results are improved. ​  If the value is too high, it will simply increase power consumption.
 +
 +
 +D2 - derivative squared gain
 +This gain is not very well proven, but the intention is to compensate for large yaw rates from wave action. ​  ​Typically set it to zero unless you want to experiment.
 +
 +
 +Hints:
 +
 +upwind - less D gain, more P (or PR) gain
 +downwind - more D gain, and possibly add DD gain
 +light wind - less gains - save power
 +strong wind - more gains - needed to operate correctly
 +
 +For sailing in protected waters, steering a less straight course is a tuning error, and will only increase power consumption.
 +
 +If you can tolerate less straight steering it may save power in waves. ​ Generally you just want to keep the sails pulling, and the average course that you desire. ​ This was always the goal with a wind vane anyway, and can save power consumption as well as wear on the motor.
tuning_gains.txt ยท Last modified: 2018/09/22 22:56 by 73.128.99.196