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POPID controller integral term causing extreme instability
 

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    <p>I have a PID controller running on a robot that is designed to make the robot steer onto a compass heading. The PID correction is recalculated/applied at a rate of 20Hz.</p> <p>Although the PID controller works well in PD mode (IE, with the integral term zero'd out) even the slightest amount of integral will force the output unstable in such a way that the steering actuator is pushed to either the left or right extreme.</p> <p>Code:</p> <pre><code> private static void DoPID(object o) { // Bring the LED up to signify frame start BoardLED.Write(true); // Get IMU heading float currentHeading = (float)RazorIMU.Yaw; // We just got the IMU heading, so we need to calculate the time from the last correction to the heading read // *immediately*. The units don't so much matter, but we are converting Ticks to milliseconds int deltaTime = (int)((LastCorrectionTime - DateTime.Now.Ticks) / 10000); // Calculate error // (let's just assume CurrentHeading really is the current GPS heading, OK?) float error = (TargetHeading - currentHeading); LCD.Lines[0].Text = "Heading: "+ currentHeading.ToString("F2"); // We calculated the error, but we need to make sure the error is set so that we will be correcting in the // direction of least work. For example, if we are flying a heading of 2 degrees and the error is a few degrees // to the left of that ( IE, somewhere around 360) there will be a large error and the rover will try to turn all // the way around to correct, when it could just turn to the right a few degrees. // In short, we are adjusting for the fact that a compass heading wraps around in a circle instead of continuing // infinity on a line if (error &lt; -180) error = error + 360; else if (error &gt; 180) error = error - 360; // Add the error calculated in this frame to the running total SteadyError = SteadyError + (error * deltaTime); // We need to allow for a certain amount of tolerance. // If the abs(error) is less than the set amount, we will // set error to 0, effectively telling the equation that the // rover is perfectly on course. if (MyAbs(error) &lt; AllowError) error = 0; LCD.Lines[2].Text = "Error: " + error.ToString("F2"); // Calculate proportional term float proportional = Kp * error; // Calculate integral term float integral = Ki * (SteadyError * deltaTime); // Calculate derivative term float derivative = Kd * ((error - PrevError) / deltaTime); // Add them all together to get the correction delta // Set the steering servo to the correction Steering.Degree = 90 + proportional + integral + derivative; // We have applied the correction, so we need to *immediately* record the // absolute time for generation of deltaTime in the next frame LastCorrectionTime = DateTime.Now.Ticks; // At this point, the current PID frame is finished // ------------------------------------------------------------ // Now, we need to setup for the next PID frame and close out // The "current" error is now the previous error // (Remember, we are done with the current frame, so in // relative terms, the previous frame IS the "current" frame) PrevError = error; // Done BoardLED.Write(false); } </code></pre> <p>Does anyone have any idea why this is happening or how to fix it? </p>
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    1. CO120-character-long lines? 80 (79) please.
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    2. COWhat are you running this on? PID is a real-time application but C# on .Net Micro is not real-time capable, and most targets do not have an FPU, so the floating point implementation may be il advised too.
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