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runge_kutta5_integrator.cc
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runge_kutta5_integrator.cc
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#include "drake/systems/analysis/runge_kutta5_integrator.h"
namespace drake {
namespace systems {
/*
* RK5-specific initialization function.
* @throws std::logic_error if *neither* the initial step size target nor the
* maximum step size has been set before calling.
*/
template <typename T>
void RungeKutta5Integrator<T>::DoInitialize() {
using std::isnan;
// TODO(drum) Verify the integrator-specific accuracy settings below.
const double kDefaultAccuracy = 1e-5;
const double kLoosestAccuracy = 1e-3;
const double kMaxStepFraction = 0.1; // Fraction of max step size for
// less aggressive first step.
// Set an artificial step size target, if not set already.
if (isnan(this->get_initial_step_size_target())) {
// Verify that maximum step size has been set.
if (isnan(this->get_maximum_step_size()))
throw std::logic_error(
"Neither initial step size target nor maximum "
"step size has been set!");
this->request_initial_step_size_target(this->get_maximum_step_size() *
kMaxStepFraction);
}
// Sets the working accuracy to a good value.
double working_accuracy = this->get_target_accuracy();
// If the user asks for accuracy that is looser than the loosest this
// integrator can provide, use the integrator's loosest accuracy setting
// instead.
if (working_accuracy > kLoosestAccuracy)
working_accuracy = kLoosestAccuracy;
else if (isnan(working_accuracy))
working_accuracy = kDefaultAccuracy;
this->set_accuracy_in_use(working_accuracy);
}
template <typename T>
bool RungeKutta5Integrator<T>::DoStep(const T& h) {
using std::abs;
Context<T>& context = *this->get_mutable_context();
const T t0 = context.get_time();
const T t1 = t0 + h;
// CAUTION: This is performance-sensitive inner loop code that uses dangerous
// long-lived references into state and cache to avoid unnecessary copying and
// cache invalidation. Be careful not to insert calls to methods that could
// invalidate any of these references before they are used.
// We use Butcher tableau notation with labels for each coefficient:
/*
0 (c1) |
1/5 (c2) | 1/5 (a21)
3/10 (c3) | 3/40 (a31) 9/40 (a32)
4/5 (c4) | 44/45 (a41) -56/15 (a42) 32/9 (a43)
8/9 (c5) | 19372/6561 (a51) −25360/2187 (a52) 64448/6561 (a53) −212/729 (a54)
1 (c6) | 9017/3168 (a61) −355/33 (a62) 46732/5247 (a63) 49/176 (a64) −5103/18656 (a65)
1 (c7) | 35/384 (a71) 0 (a72) 500/1113 (a73) 125/192 (a74) −2187/6784 (a75) 11/84 (a76)
------------------------------------------------------------------------------------------------------------------------------------
35/384 (b1) 0 (b2) 500/1113 (b3) 125/192 (b4) −2187/6784 (b5) 11/84 (b6) 0 (b7)
5179/57600 (d1) 0 (d2) 7571/16695 (d3) 393/640 (d4) −92097/339200 (d5) 187/2100 (d6) 1/40 (d7)
*/
// Save the continuous state at t₀.
context.get_continuous_state_vector().CopyToPreSizedVector(&save_xc0_);
// Evaluate the derivative at t₀, xc₀ and copy the result into a temporary.
derivs1_->get_mutable_vector().SetFrom(
this->EvalTimeDerivatives(context).get_vector());
const VectorBase<T>& k1 = derivs1_->get_vector();
// Cache: k1 references a *copy* of the derivative result so is immune
// to subsequent evaluations.
// Compute the first intermediate state and derivative (i.e., Stage 2).
// This call marks t- and xc-dependent cache entries out of date, including
// the derivative cache entry. Note that xc is a live reference into the
// context -- subsequent changes through that reference are unobservable so
// will require manual out-of-date notifications.
const double c2 = 1.0 / 5;
const double a21 = 1.0 / 5;
VectorBase<T>& xc =
context.SetTimeAndGetMutableContinuousStateVector(t0 + c2 * h);
xc.PlusEqScaled(a21 * h, k1);
// Evaluate the derivative (denoted k2) at t₀ + c2 * h, xc₀ + a21 * h * k1.
derivs2_->get_mutable_vector().SetFrom(
this->EvalTimeDerivatives(context).get_vector());
const VectorBase<T>& k2 = derivs2_->get_vector();
// Cache: k2 references a *copy* of the derivative result so is immune
// to subsequent evaluations.
// Compute the second intermediate state and derivative (i.e., Stage 3).
const double c3 = 3.0 / 10;
const double a31 = 3.0 / 40;
const double a32 = 9.0 / 40;
// This call marks t- and xc-dependent cache entries out of date, including
// the derivative cache entry. (We already have the xc reference but must
// issue the out-of-date notification here since we're about to change it.)
context.SetTimeAndNoteContinuousStateChange(t0 + c3 * h);
// Evaluate the derivative (denoted k3) at t₀ + c3 * h,
// xc₀ + a31 * h * k1 + a32 * h * k2.
xc.SetFromVector(save_xc0_); // Restore xc ← xc₀.
xc.PlusEqScaled({{a31 * h, k1}, {a32 * h, k2}});
derivs3_->get_mutable_vector().SetFrom(
this->EvalTimeDerivatives(context).get_vector());
const VectorBase<T>& k3 = derivs3_->get_vector();
// Compute the third intermediate state and derivative (i.e., Stage 4).
const double c4 = 4.0 / 5;
const double a41 = 44.0 / 45;
const double a42 = -56.0 / 15;
const double a43 = 32.0 / 9;
// This call marks t- and xc-dependent cache entries out of date, including
// the derivative cache entry. (We already have the xc reference but must
// issue the out-of-date notification here since we're about to change it.)
context.SetTimeAndNoteContinuousStateChange(t0 + c4 * h);
// Evaluate the derivative (denoted k4) at t₀ + c4 * h,
// xc₀ + a41 * h * k1 + a42 * h * k2 + a43 * h * k3.
xc.SetFromVector(save_xc0_);
xc.PlusEqScaled({{a41 * h, k1}, {a42 * h, k2}, {a43 * h, k3}});
derivs4_->get_mutable_vector().SetFrom(
this->EvalTimeDerivatives(context).get_vector());
const VectorBase<T>& k4 = derivs4_->get_vector();
// Compute the fourth intermediate state and derivative (i.e., Stage 5).
const double c5 = 8.0 / 9;
const double a51 = 19372.0 / 6561;
const double a52 = -25360.0 / 2187;
const double a53 = 64448.0 / 6561;
const double a54 = -212.0 / 729;
// This call marks t- and xc-dependent cache entries out of date, including
// the derivative cache entry. (We already have the xc reference but must
// issue the out-of-date notification here since we're about to change it.)
context.SetTimeAndNoteContinuousStateChange(t0 + c5 * h);
// Evaluate the derivative (denoted k5) at t₀ + c5 * h,
// xc₀ + a51 * h * k1 + a52 * h * k2 + a53 * h * k3 + a54 * h * k4.
xc.SetFromVector(save_xc0_); // Restore xc ← xc₀.
xc.PlusEqScaled({{a51 * h, k1}, {a52 * h, k2}, {a53 * h, k3}, {a54 * h, k4}});
derivs5_->get_mutable_vector().SetFrom(
this->EvalTimeDerivatives(context).get_vector());
const VectorBase<T>& k5 = derivs5_->get_vector();
// Compute the fifth intermediate state and derivative (i.e., Stage 6).
const double a61 = 9017.0 / 3168;
const double a62 = -355.0 / 33;
const double a63 = 46732.0 / 5247;
const double a64 = 49.0 / 176;
const double a65 = -5103.0 / 18656;
// This call marks t- and xc-dependent cache entries out of date, including
// the derivative cache entry. (We already have the xc reference but must
// issue the out-of-date notification here since we're about to change it.)
context.SetTimeAndNoteContinuousStateChange(t1);
// Evaluate the derivative (denoted k6) at t₀ + c6 * h,
// xc₀ + a61 * h * k1 + a62 * h * k2 + a63 * h * k3 + a64 * h * k4 +
// a65 * h * k5.
xc.SetFromVector(save_xc0_);
xc.PlusEqScaled({{a61 * h, k1},
{a62 * h, k2},
{a63 * h, k3},
{a64 * h, k4},
{a65 * h, k5}});
derivs6_->get_mutable_vector().SetFrom(
this->EvalTimeDerivatives(context).get_vector());
const VectorBase<T>& k6 = derivs6_->get_vector();
// Cache: we're about to write through the xc reference again, so need to
// mark xc-dependent cache entries out of date; time doesn't change here.
context.NoteContinuousStateChange();
// Compute the propagated solution (we're able to do this because b1 = a71,
// b2 = a72, b3 = a73, b4 = a74, b5 = a75, and b6 = a76).
// Note that a72 is 0.0, so we leave that term out below.
const double a71 = 35.0 / 384;
const double a73 = 500.0 / 1113;
const double a74 = 125.0 / 192;
const double a75 = -2187.0 / 6784;
const double a76 = 11.0 / 84;
// This call marks t- and xc-dependent cache entries out of date, including
// the derivative cache entry. (We already have the xc reference but must
// issue the out-of-date notification here since we're about to change it.)
// Note that we use the simplification t1 = t0 + h * c7 = t0 + h * 1.
context.SetTimeAndNoteContinuousStateChange(t1);
// Evaluate the derivative (denoted k7) at t₀ + c7 * h,
// xc₀ + a71 * h * k1 + a72 * h * k2 + a73 * h * k3 + a74 * h * k4 +
// a75 * h * k5 + a76 * h * k6.
xc.SetFromVector(save_xc0_);
xc.PlusEqScaled({{a71 * h, k1},
{a73 * h, k3},
{a74 * h, k4},
{a75 * h, k5},
{a76 * h, k6}});
const ContinuousState<T>& derivs7 = this->EvalTimeDerivatives(context);
const VectorBase<T>& k7 = derivs7.get_vector();
// WARNING: k7 is a live reference into the cache. Be careful of adding
// code below that modifies the context until after k7 is used below. In fact,
// it is best not to modify the context from here on out, as modifying the
// context will effectively destroy the FSAL benefit that this integrator
// provides.
// Calculate the 4th-order solution that will be used for the error
// estimate and then the error estimate itself. The part of this
// formula that uses the "a" coefficients (re)-computes the fifth order
// solution. The part of this formula that uses the "d" coefficients computes
// the fourth order solution. The subtraction (and negation) operations
// yield the error estimate.
// Note: d2 is 0.0; it has been removed from the formula below.
const double d1 = 5179.0 / 57600;
const double d3 = 7571.0 / 16695;
const double d4 = 393.0 / 640;
const double d5 = -92097.0 / 339200;
const double d6 = 187.0 / 2100;
const double d7 = 1.0 / 40;
err_est_vec_->SetZero();
err_est_vec_->PlusEqScaled({{(a71 - d1) * h, k1},
{(a73 - d3) * h, k3},
{(a74 - d4) * h, k4},
{(a75 - d5) * h, k5},
{(a76 - d6) * h, k6},
{(-d7) * h, k7}});
// If the size of the system has changed, the error estimate will no longer
// be sized correctly. Verify that the error estimate is the correct size.
DRAKE_DEMAND(this->get_error_estimate()->size() == xc.size());
this->get_mutable_error_estimate()->SetFromVector(
err_est_vec_->CopyToVector().cwiseAbs());
// RK5 always succeeds in taking its desired step.
return true;
}
} // namespace systems
} // namespace drake
DRAKE_DEFINE_CLASS_TEMPLATE_INSTANTIATIONS_ON_DEFAULT_NONSYMBOLIC_SCALARS(
class drake::systems::RungeKutta5Integrator)