Create y2022 localizer

TODO:
* Update Roborio's localizer to listen to LocalizerOutput messages.
* Actually listen to camera messages.
* Correct the interface between this and the pico-reading process for
  encoder values.

Change-Id: If1c7b0cca60581780fbe39fe2f78fa6b4a6933e3
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/y2022/control_loops/localizer/localizer_test.cc b/y2022/control_loops/localizer/localizer_test.cc
new file mode 100644
index 0000000..5857bda
--- /dev/null
+++ b/y2022/control_loops/localizer/localizer_test.cc
@@ -0,0 +1,525 @@
+#include "y2022/control_loops/localizer/localizer.h"
+
+#include "aos/events/simulated_event_loop.h"
+#include "gtest/gtest.h"
+#include "frc971/control_loops/drivetrain/drivetrain_test_lib.h"
+
+namespace frc971::controls::testing {
+typedef ModelBasedLocalizer::ModelState ModelState;
+typedef ModelBasedLocalizer::AccelState AccelState;
+typedef ModelBasedLocalizer::ModelInput ModelInput;
+typedef ModelBasedLocalizer::AccelInput AccelInput;
+namespace {
+constexpr size_t kX = ModelBasedLocalizer::kX;
+constexpr size_t kY = ModelBasedLocalizer::kY;
+constexpr size_t kTheta = ModelBasedLocalizer::kTheta;
+constexpr size_t kVelocityX = ModelBasedLocalizer::kVelocityX;
+constexpr size_t kVelocityY = ModelBasedLocalizer::kVelocityY;
+constexpr size_t kAccelX = ModelBasedLocalizer::kAccelX;
+constexpr size_t kAccelY = ModelBasedLocalizer::kAccelY;
+constexpr size_t kThetaRate = ModelBasedLocalizer::kThetaRate;
+constexpr size_t kLeftEncoder = ModelBasedLocalizer::kLeftEncoder;
+constexpr size_t kLeftVelocity = ModelBasedLocalizer::kLeftVelocity;
+constexpr size_t kLeftVoltageError = ModelBasedLocalizer::kLeftVoltageError;
+constexpr size_t kRightEncoder = ModelBasedLocalizer::kRightEncoder;
+constexpr size_t kRightVelocity = ModelBasedLocalizer::kRightVelocity;
+constexpr size_t kRightVoltageError = ModelBasedLocalizer::kRightVoltageError;
+constexpr size_t kLeftVoltage = ModelBasedLocalizer::kLeftVoltage;
+constexpr size_t kRightVoltage = ModelBasedLocalizer::kRightVoltage;
+}
+
+class LocalizerTest : public ::testing::Test {
+ protected:
+  LocalizerTest()
+      : dt_config_(
+            control_loops::drivetrain::testing::GetTestDrivetrainConfig()),
+        localizer_(dt_config_) {}
+  ModelState CallDiffModel(const ModelState &state, const ModelInput &U) {
+    return localizer_.DiffModel(state, U);
+  }
+
+  AccelState CallDiffAccel(const AccelState &state, const AccelInput &U) {
+    return localizer_.DiffAccel(state, U);
+  }
+
+  const control_loops::drivetrain::DrivetrainConfig<double> dt_config_;
+  ModelBasedLocalizer localizer_;
+
+};
+
+TEST_F(LocalizerTest, AccelIntegrationTest) {
+  AccelState state;
+  state.setZero();
+  AccelInput input;
+  input.setZero();
+
+  EXPECT_EQ(0.0, CallDiffAccel(state, input).norm());
+  // Non-zero x/y/theta states should still result in a zero derivative.
+  state(kX) = 1.0;
+  state(kY) = 1.0;
+  state(kTheta) = 1.0;
+  EXPECT_EQ(0.0, CallDiffAccel(state, input).norm());
+
+  state.setZero();
+  state(kVelocityX) = 1.0;
+  state(kVelocityY) = 2.0;
+  EXPECT_EQ((AccelState() << 1.0, 2.0, 0.0, 0.0, 0.0).finished(),
+            CallDiffAccel(state, input));
+  // Derivatives should be independent of theta.
+  state(kTheta) = M_PI / 2.0;
+  EXPECT_EQ((AccelState() << 1.0, 2.0, 0.0, 0.0, 0.0).finished(),
+            CallDiffAccel(state, input));
+
+  state.setZero();
+  input(kAccelX) = 1.0;
+  input(kAccelY) = 2.0;
+  input(kThetaRate) = 3.0;
+  EXPECT_EQ((AccelState() << 0.0, 0.0, 3.0, 1.0, 2.0).finished(),
+            CallDiffAccel(state, input));
+  state(kTheta) = M_PI / 2.0;
+  EXPECT_EQ((AccelState() << 0.0, 0.0, 3.0, 1.0, 2.0).finished(),
+            CallDiffAccel(state, input));
+}
+
+TEST_F(LocalizerTest, ModelIntegrationTest) {
+  ModelState state;
+  state.setZero();
+  ModelInput input;
+  input.setZero();
+  ModelState diff;
+
+  EXPECT_EQ(0.0, CallDiffModel(state, input).norm());
+  // Non-zero x/y/theta/encoder states should still result in a zero derivative.
+  state(kX) = 1.0;
+  state(kY) = 1.0;
+  state(kTheta) = 1.0;
+  state(kLeftEncoder) = 1.0;
+  state(kRightEncoder) = 1.0;
+  EXPECT_EQ(0.0, CallDiffModel(state, input).norm());
+
+  state.setZero();
+  state(kLeftVelocity) = 1.0;
+  state(kRightVelocity) = 1.0;
+  diff = CallDiffModel(state, input);
+  const ModelState mask_velocities =
+      (ModelState() << 1.0, 1.0, 1.0, 1.0, 0.0, 1.0, 1.0, 0.0, 1.0).finished();
+  EXPECT_EQ(
+      (ModelState() << 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0).finished(),
+      diff.cwiseProduct(mask_velocities));
+  EXPECT_EQ(diff(kLeftVelocity), diff(kRightVelocity));
+  EXPECT_GT(0.0, diff(kLeftVelocity));
+  state(kTheta) = M_PI / 2.0;
+  diff = CallDiffModel(state, input);
+  EXPECT_NEAR(0.0,
+              ((ModelState() << 0.0, 1.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0)
+                   .finished() -
+               diff.cwiseProduct(mask_velocities))
+                  .norm(),
+              1e-12);
+  EXPECT_EQ(diff(kLeftVelocity), diff(kRightVelocity));
+  EXPECT_GT(0.0, diff(kLeftVelocity));
+
+  state.setZero();
+  state(kLeftVelocity) = -1.0;
+  state(kRightVelocity) = 1.0;
+  diff = CallDiffModel(state, input);
+  EXPECT_EQ((ModelState() << 0.0, 0.0, 1.0 / dt_config_.robot_radius, -1.0, 0.0,
+             0.0, 1.0, 0.0, 0.0)
+                .finished(),
+            diff.cwiseProduct(mask_velocities));
+  EXPECT_EQ(-diff(kLeftVelocity), diff(kRightVelocity));
+  EXPECT_LT(0.0, diff(kLeftVelocity));
+
+  state.setZero();
+  input(kLeftVoltage) = 5.0;
+  input(kRightVoltage) = 6.0;
+  diff = CallDiffModel(state, input);
+  EXPECT_EQ(0, diff(kX));
+  EXPECT_EQ(0, diff(kY));
+  EXPECT_EQ(0, diff(kTheta));
+  EXPECT_EQ(0, diff(kLeftEncoder));
+  EXPECT_EQ(0, diff(kRightEncoder));
+  EXPECT_EQ(0, diff(kLeftVoltageError));
+  EXPECT_EQ(0, diff(kRightVoltageError));
+  EXPECT_LT(0, diff(kLeftVelocity));
+  EXPECT_LT(0, diff(kRightVelocity));
+  EXPECT_LT(diff(kLeftVelocity), diff(kRightVelocity));
+
+  state.setZero();
+  state(kLeftVoltageError) = -1.0;
+  state(kRightVoltageError) = -2.0;
+  input(kLeftVoltage) = 1.0;
+  input(kRightVoltage) = 2.0;
+  EXPECT_EQ(ModelState::Zero(), CallDiffModel(state, input));
+}
+
+// Test that the HandleReset does indeed reset the state of the localizer.
+TEST_F(LocalizerTest, LocalizerReset) {
+  aos::monotonic_clock::time_point t = aos::monotonic_clock::epoch();
+  localizer_.HandleReset(t, {1.0, 2.0, 3.0});
+  EXPECT_EQ((Eigen::Vector3d{1.0, 2.0, 3.0}), localizer_.xytheta());
+  localizer_.HandleReset(t, {4.0, 5.0, 6.0});
+  EXPECT_EQ((Eigen::Vector3d{4.0, 5.0, 6.0}), localizer_.xytheta());
+}
+
+// Test that if we are moving only by accelerometer readings (and just assuming
+// zero voltage/encoders) that we initially don't believe it but then latch into
+// following the accelerometer.
+// Note: this test is somewhat sensitive to the exact tuning values used for the
+// filter.
+TEST_F(LocalizerTest, AccelOnly) {
+  const aos::monotonic_clock::time_point start = aos::monotonic_clock::epoch();
+  const std::chrono::microseconds kDt{500};
+  aos::monotonic_clock::time_point t = start - std::chrono::milliseconds(1000);
+  Eigen::Vector3d gyro{0.0, 0.0, 0.0};
+  const Eigen::Vector2d encoders{0.0, 0.0};
+  const Eigen::Vector2d voltages{0.0, 0.0};
+  Eigen::Vector3d accel{1.0, 0.2, 9.80665};
+  Eigen::Vector3d accel_gs = accel / 9.80665;
+  while (t < start) {
+    // Spin to fill up the buffer.
+    localizer_.HandleImu(t, gyro, Eigen::Vector3d::UnitZ(), encoders, voltages);
+    t += kDt;
+  }
+  while (t < start + std::chrono::milliseconds(100)) {
+    localizer_.HandleImu(t, gyro, accel_gs, encoders, voltages);
+    EXPECT_EQ(Eigen::Vector3d::Zero(), localizer_.xytheta());
+    t += kDt;
+  }
+  while (t < start + std::chrono::milliseconds(500)) {
+    // Too lazy to hard-code when the transition happens.
+    localizer_.HandleImu(t, gyro, accel_gs, encoders, voltages);
+    t += kDt;
+  }
+  while (t < start + std::chrono::milliseconds(1000)) {
+    SCOPED_TRACE(t);
+    localizer_.HandleImu(t, gyro, accel_gs, encoders, voltages);
+    const Eigen::Vector3d xytheta = localizer_.xytheta();
+    t += kDt;
+    EXPECT_NEAR(
+        0.5 * accel(0) * std::pow(aos::time::DurationInSeconds(t - start), 2),
+        xytheta(0), 1e-4);
+    EXPECT_NEAR(
+        0.5 * accel(1) * std::pow(aos::time::DurationInSeconds(t - start), 2),
+        xytheta(1), 1e-4);
+    EXPECT_EQ(0.0, xytheta(2));
+  }
+
+  ASSERT_NEAR(1.0, localizer_.accel_state()(kVelocityX), 1e-10);
+  ASSERT_NEAR(0.2, localizer_.accel_state()(kVelocityY), 1e-10);
+
+  // Start going in a cirlce, and confirm that we
+  // handle things correctly. We rotate the accelerometer readings by 90 degrees
+  // and then leave them constant, which should make it look like we are going
+  // around in a circle.
+  accel = Eigen::Vector3d{-accel(1), accel(0), 9.80665};
+  accel_gs = accel / 9.80665;
+  // v^2 / r = a
+  // w * r = v
+  // v^2 / v * w = a
+  // w = a / v
+  const double omega = accel.topRows<2>().norm() /
+                       std::hypot(localizer_.accel_state()(kVelocityX),
+                                  localizer_.accel_state()(kVelocityY));
+  gyro << 0.0, 0.0, omega;
+  // Due to the magic of math, omega works out to be 1.0 after having run at the
+  // acceleration for one second.
+  ASSERT_NEAR(1.0, omega, 1e-10);
+  // Yes, we could save some operations here, but let's be at least somewhat
+  // clear about what we're doing...
+  const double radius = accel.topRows<2>().norm() / (omega * omega);
+  const Eigen::Vector2d center = localizer_.xytheta().topRows<2>() +
+                                 accel.topRows<2>().normalized() * radius;
+  const double initial_theta = std::atan2(-accel(1), -accel(0));
+
+  std::chrono::microseconds one_revolution_time(
+      static_cast<int>(2 * M_PI / omega * 1e6));
+
+  aos::monotonic_clock::time_point circle_start = t;
+
+  while (t < circle_start + one_revolution_time) {
+    SCOPED_TRACE(t);
+    localizer_.HandleImu(t, gyro, accel_gs, encoders, voltages);
+    t += kDt;
+    const double t_circle = aos::time::DurationInSeconds(t - circle_start);
+    ASSERT_NEAR(t_circle * omega, localizer_.xytheta()(2), 1e-5);
+    const double theta_circle = t_circle * omega + initial_theta;
+    const Eigen::Vector2d offset =
+        radius *
+        Eigen::Vector2d{std::cos(theta_circle), std::sin(theta_circle)};
+    const Eigen::Vector2d expected = center + offset;
+    const Eigen::Vector2d estimated = localizer_.xytheta().topRows<2>();
+    const Eigen::Vector2d implied_offset = estimated - center;
+    const double implied_theta =
+        std::atan2(implied_offset.y(), implied_offset.x());
+    VLOG(1) << "center: " << center.transpose() << " radius " << radius
+            << "\nlocalizer " << localizer_.xytheta().transpose()
+            << " t_circle " << t_circle << " omega " << omega << " theta "
+            << theta_circle << "\noffset " << offset.transpose()
+            << "\nexpected " << expected.transpose() << "\nimplied offset "
+            << implied_offset << " implied_theta " << implied_theta << "\nvel "
+            << localizer_.accel_state()(kVelocityX) << ", "
+            << localizer_.accel_state()(kVelocityY);
+    ASSERT_NEAR(0.0, (expected - localizer_.xytheta().topRows<2>()).norm(),
+                1e-2);
+  }
+
+  // Set accelerometer back to zero and confirm that we recover (the
+  // implementation decays the accelerometer speeds to zero when still, so
+  // should recover).
+  while (t <
+         circle_start + one_revolution_time + std::chrono::milliseconds(3000)) {
+    localizer_.HandleImu(t, Eigen::Vector3d::Zero(), Eigen::Vector3d::UnitZ(),
+                         encoders, voltages);
+    t += kDt;
+  }
+  const Eigen::Vector3d final_pos = localizer_.xytheta();
+  localizer_.HandleImu(t, Eigen::Vector3d::Zero(), Eigen::Vector3d::UnitZ(),
+                       encoders, voltages);
+  ASSERT_NEAR(0.0, (final_pos - localizer_.xytheta()).norm(), 1e-10);
+}
+
+using control_loops::drivetrain::Output;
+
+class EventLoopLocalizerTest : public ::testing::Test {
+ protected:
+  EventLoopLocalizerTest()
+      : configuration_(aos::configuration::ReadConfig("y2022/config.json")),
+        event_loop_factory_(&configuration_.message()),
+        roborio_node_(
+            aos::configuration::GetNode(&configuration_.message(), "roborio")),
+        imu_node_(
+            aos::configuration::GetNode(&configuration_.message(), "imu")),
+        dt_config_(
+            control_loops::drivetrain::testing::GetTestDrivetrainConfig()),
+        localizer_event_loop_(
+            event_loop_factory_.MakeEventLoop("localizer", imu_node_)),
+        localizer_(localizer_event_loop_.get(), dt_config_),
+        drivetrain_plant_event_loop_(event_loop_factory_.MakeEventLoop(
+            "drivetrain_plant", roborio_node_)),
+        drivetrain_plant_imu_event_loop_(
+            event_loop_factory_.MakeEventLoop("drivetrain_plant", imu_node_)),
+        drivetrain_plant_(drivetrain_plant_event_loop_.get(),
+                          drivetrain_plant_imu_event_loop_.get(), dt_config_,
+                          std::chrono::microseconds(500)),
+        roborio_test_event_loop_(
+            event_loop_factory_.MakeEventLoop("test", roborio_node_)),
+        imu_test_event_loop_(
+            event_loop_factory_.MakeEventLoop("test", imu_node_)),
+        logger_test_event_loop_(
+            event_loop_factory_.GetNodeEventLoopFactory("logger")
+                ->MakeEventLoop("test")),
+        output_sender_(
+            roborio_test_event_loop_->MakeSender<Output>("/drivetrain")),
+        output_fetcher_(roborio_test_event_loop_->MakeFetcher<LocalizerOutput>(
+            "/localizer")),
+        status_fetcher_(
+            imu_test_event_loop_->MakeFetcher<LocalizerStatus>("/localizer")) {
+    aos::TimerHandler *timer = roborio_test_event_loop_->AddTimer([this]() {
+      auto builder = output_sender_.MakeBuilder();
+      auto output_builder = builder.MakeBuilder<Output>();
+      output_builder.add_left_voltage(output_voltages_(0));
+      output_builder.add_right_voltage(output_voltages_(1));
+      builder.CheckOk(builder.Send(output_builder.Finish()));
+    });
+    roborio_test_event_loop_->OnRun([timer, this]() {
+      timer->Setup(roborio_test_event_loop_->monotonic_now(),
+                   std::chrono::milliseconds(5));
+    });
+    // Get things zeroed.
+    event_loop_factory_.RunFor(std::chrono::seconds(10));
+    CHECK(status_fetcher_.Fetch());
+    CHECK(status_fetcher_->zeroed());
+  }
+
+  aos::FlatbufferDetachedBuffer<aos::Configuration> configuration_;
+  aos::SimulatedEventLoopFactory event_loop_factory_;
+  const aos::Node *const roborio_node_;
+  const aos::Node *const imu_node_;
+  const control_loops::drivetrain::DrivetrainConfig<double> dt_config_;
+  std::unique_ptr<aos::EventLoop> localizer_event_loop_;
+  EventLoopLocalizer localizer_;
+
+  std::unique_ptr<aos::EventLoop> drivetrain_plant_event_loop_;
+  std::unique_ptr<aos::EventLoop> drivetrain_plant_imu_event_loop_;
+  control_loops::drivetrain::testing::DrivetrainSimulation drivetrain_plant_;
+
+  std::unique_ptr<aos::EventLoop> roborio_test_event_loop_;
+  std::unique_ptr<aos::EventLoop> imu_test_event_loop_;
+  std::unique_ptr<aos::EventLoop> logger_test_event_loop_;
+
+  aos::Sender<Output> output_sender_;
+  aos::Fetcher<LocalizerOutput> output_fetcher_;
+  aos::Fetcher<LocalizerStatus> status_fetcher_;
+
+  Eigen::Vector2d output_voltages_ = Eigen::Vector2d::Zero();
+};
+
+TEST_F(EventLoopLocalizerTest, Nominal) {
+  output_voltages_ << 1.0, 1.0;
+  event_loop_factory_.RunFor(std::chrono::seconds(2));
+  drivetrain_plant_.set_accel_sin_magnitude(0.01);
+  CHECK(output_fetcher_.Fetch());
+  CHECK(status_fetcher_.Fetch());
+  // The two can be different because they may've been sent at different times.
+  ASSERT_NEAR(output_fetcher_->x(), status_fetcher_->model_based()->x(), 1e-6);
+  ASSERT_NEAR(output_fetcher_->y(), status_fetcher_->model_based()->y(), 1e-6);
+  ASSERT_NEAR(output_fetcher_->theta(), status_fetcher_->model_based()->theta(),
+              1e-6);
+  ASSERT_LT(0.1, output_fetcher_->x());
+  ASSERT_NEAR(0.0, output_fetcher_->y(), 1e-10);
+  ASSERT_NEAR(0.0, output_fetcher_->theta(), 1e-10);
+  ASSERT_TRUE(status_fetcher_->has_model_based());
+  ASSERT_TRUE(status_fetcher_->model_based()->using_model());
+  ASSERT_LT(0.1, status_fetcher_->model_based()->accel_state()->velocity_x());
+  ASSERT_NEAR(0.0, status_fetcher_->model_based()->accel_state()->velocity_y(),
+              1e-10);
+  ASSERT_NEAR(
+      0.0, status_fetcher_->model_based()->model_state()->left_voltage_error(),
+      1e-1);
+  ASSERT_NEAR(
+      0.0, status_fetcher_->model_based()->model_state()->right_voltage_error(),
+      1e-1);
+}
+
+TEST_F(EventLoopLocalizerTest, Reverse) {
+  output_voltages_ << -4.0, -4.0;
+  drivetrain_plant_.set_accel_sin_magnitude(0.01);
+  event_loop_factory_.RunFor(std::chrono::seconds(2));
+  CHECK(output_fetcher_.Fetch());
+  CHECK(status_fetcher_.Fetch());
+  // The two can be different because they may've been sent at different times.
+  ASSERT_NEAR(output_fetcher_->x(), status_fetcher_->model_based()->x(), 1e-6);
+  ASSERT_NEAR(output_fetcher_->y(), status_fetcher_->model_based()->y(), 1e-6);
+  ASSERT_NEAR(output_fetcher_->theta(), status_fetcher_->model_based()->theta(),
+              1e-6);
+  ASSERT_GT(-0.1, output_fetcher_->x());
+  ASSERT_NEAR(0.0, output_fetcher_->y(), 1e-10);
+  ASSERT_NEAR(0.0, output_fetcher_->theta(), 1e-10);
+  ASSERT_TRUE(status_fetcher_->has_model_based());
+  ASSERT_TRUE(status_fetcher_->model_based()->using_model());
+  ASSERT_GT(-0.1, status_fetcher_->model_based()->accel_state()->velocity_x());
+  ASSERT_NEAR(0.0, status_fetcher_->model_based()->accel_state()->velocity_y(),
+              1e-10);
+  ASSERT_NEAR(
+      0.0, status_fetcher_->model_based()->model_state()->left_voltage_error(),
+      1e-1);
+  ASSERT_NEAR(
+      0.0, status_fetcher_->model_based()->model_state()->right_voltage_error(),
+      1e-1);
+}
+
+TEST_F(EventLoopLocalizerTest, SpinInPlace) {
+  output_voltages_ << 4.0, -4.0;
+  event_loop_factory_.RunFor(std::chrono::seconds(2));
+  CHECK(output_fetcher_.Fetch());
+  CHECK(status_fetcher_.Fetch());
+  // The two can be different because they may've been sent at different times.
+  ASSERT_NEAR(output_fetcher_->x(), status_fetcher_->model_based()->x(), 1e-6);
+  ASSERT_NEAR(output_fetcher_->y(), status_fetcher_->model_based()->y(), 1e-6);
+  ASSERT_NEAR(output_fetcher_->theta(), status_fetcher_->model_based()->theta(),
+              1e-1);
+  ASSERT_NEAR(0.0, output_fetcher_->x(), 1e-10);
+  ASSERT_NEAR(0.0, output_fetcher_->y(), 1e-10);
+  ASSERT_LT(0.1, std::abs(output_fetcher_->theta()));
+  ASSERT_TRUE(status_fetcher_->has_model_based());
+  ASSERT_TRUE(status_fetcher_->model_based()->using_model());
+  ASSERT_NEAR(0.0, status_fetcher_->model_based()->accel_state()->velocity_x(),
+              1e-10);
+  ASSERT_NEAR(0.0, status_fetcher_->model_based()->accel_state()->velocity_y(),
+              1e-10);
+  ASSERT_NEAR(-status_fetcher_->model_based()->model_state()->left_velocity(),
+              status_fetcher_->model_based()->model_state()->right_velocity(),
+              1e-3);
+  ASSERT_NEAR(
+      0.0, status_fetcher_->model_based()->model_state()->left_voltage_error(),
+      1e-1);
+  ASSERT_NEAR(
+      0.0, status_fetcher_->model_based()->model_state()->right_voltage_error(),
+      1e-1);
+  ASSERT_NEAR(0.0, status_fetcher_->model_based()->residual(), 1e-3);
+}
+
+TEST_F(EventLoopLocalizerTest, Curve) {
+  output_voltages_ << 2.0, 4.0;
+  event_loop_factory_.RunFor(std::chrono::seconds(2));
+  CHECK(output_fetcher_.Fetch());
+  CHECK(status_fetcher_.Fetch());
+  // The two can be different because they may've been sent at different times.
+  ASSERT_NEAR(output_fetcher_->x(), status_fetcher_->model_based()->x(), 1e-2);
+  ASSERT_NEAR(output_fetcher_->y(), status_fetcher_->model_based()->y(), 1e-2);
+  ASSERT_NEAR(output_fetcher_->theta(), status_fetcher_->model_based()->theta(),
+              1e-1);
+  ASSERT_LT(0.1, output_fetcher_->x());
+  ASSERT_LT(0.1, output_fetcher_->y());
+  ASSERT_LT(0.1, std::abs(output_fetcher_->theta()));
+  ASSERT_TRUE(status_fetcher_->has_model_based());
+  ASSERT_TRUE(status_fetcher_->model_based()->using_model());
+  ASSERT_LT(0.0, status_fetcher_->model_based()->accel_state()->velocity_x());
+  ASSERT_LT(0.0, status_fetcher_->model_based()->accel_state()->velocity_y());
+  ASSERT_NEAR(
+      0.0, status_fetcher_->model_based()->model_state()->left_voltage_error(),
+      1e-1);
+  ASSERT_NEAR(
+      0.0, status_fetcher_->model_based()->model_state()->right_voltage_error(),
+      1e-1);
+  ASSERT_NEAR(0.0, status_fetcher_->model_based()->residual(), 1e-1)
+      << aos::FlatbufferToJson(status_fetcher_.get(), {.multi_line = true});
+}
+
+// Tests that small amounts of voltage error are handled by the model-based
+// half of the localizer.
+TEST_F(EventLoopLocalizerTest, VoltageError) {
+  output_voltages_ << 0.0, 0.0;
+  drivetrain_plant_.set_left_voltage_offset(2.0);
+  drivetrain_plant_.set_right_voltage_offset(2.0);
+  drivetrain_plant_.set_accel_sin_magnitude(0.01);
+
+  event_loop_factory_.RunFor(std::chrono::seconds(2));
+  CHECK(output_fetcher_.Fetch());
+  CHECK(status_fetcher_.Fetch());
+  // Should still be using the model, but have a non-trivial residual.
+  ASSERT_TRUE(status_fetcher_->model_based()->using_model());
+  ASSERT_LT(0.1, status_fetcher_->model_based()->residual())
+      << aos::FlatbufferToJson(status_fetcher_.get(), {.multi_line = true});
+
+  // Afer running for a while, voltage error terms should converge and result in
+  // low residuals.
+  event_loop_factory_.RunFor(std::chrono::seconds(10));
+  CHECK(output_fetcher_.Fetch());
+  CHECK(status_fetcher_.Fetch());
+  ASSERT_TRUE(status_fetcher_->model_based()->using_model());
+  ASSERT_NEAR(
+      2.0, status_fetcher_->model_based()->model_state()->left_voltage_error(),
+      0.1)
+      << aos::FlatbufferToJson(status_fetcher_.get(), {.multi_line = true});
+  ASSERT_NEAR(
+      2.0, status_fetcher_->model_based()->model_state()->right_voltage_error(),
+      0.1)
+      << aos::FlatbufferToJson(status_fetcher_.get(), {.multi_line = true});
+  ASSERT_GT(0.01, status_fetcher_->model_based()->residual())
+      << aos::FlatbufferToJson(status_fetcher_.get(), {.multi_line = true});
+}
+
+// Tests that large amounts of voltage error force us into the
+// acceleration-based localizer.
+TEST_F(EventLoopLocalizerTest, HighVoltageError) {
+  output_voltages_ << 0.0, 0.0;
+  drivetrain_plant_.set_left_voltage_offset(200.0);
+  drivetrain_plant_.set_right_voltage_offset(200.0);
+  drivetrain_plant_.set_accel_sin_magnitude(0.01);
+
+  event_loop_factory_.RunFor(std::chrono::seconds(2));
+  CHECK(output_fetcher_.Fetch());
+  CHECK(status_fetcher_.Fetch());
+  // Should still be using the model, but have a non-trivial residual.
+  ASSERT_FALSE(status_fetcher_->model_based()->using_model());
+  ASSERT_LT(0.1, status_fetcher_->model_based()->residual())
+      << aos::FlatbufferToJson(status_fetcher_.get(), {.multi_line = true});
+  ASSERT_NEAR(drivetrain_plant_.state()(0),
+              status_fetcher_->model_based()->x(), 1.0);
+  ASSERT_NEAR(drivetrain_plant_.state()(1),
+              status_fetcher_->model_based()->y(), 1e-6);
+}
+
+}  // namespace frc91::controls::testing