addes comments for SignalLogger

drake/systems/primitives/signal_logger.h

@@ -14,6 +14,9 @@ namespace systems {
 
 /// A sink block which logs its input to memory.  This data is then retrievable
 /// (e.g. after a simulation) via a handful of accessor methods.
+/// This class essentially holds a large Eigen matrix for data storage, where
+/// each column corresponds to a data point. This system saves a data point and
+/// the context time whenever its Publish() method is called.
 ///
 /// @tparam T The vector element type, which must be a valid Eigen scalar.
 ///
@@ -28,7 +31,8 @@ class SignalLogger : public LeafSystem<T> {
   DRAKE_NO_COPY_NO_MOVE_NO_ASSIGN(SignalLogger)
 
   /// Construct the signal logger system.
-  /// @param input_size Dimension of the (single) input port.
+  /// @param input_size Dimension of the (single) input port. This corresponds
+  /// to the number of rows of the data matrix.
   /// @param batch_allocation_size Storage is (re)allocated in blocks of
   /// input_size-by-batch_allocation_size.
   explicit SignalLogger(int input_size, int batch_allocation_size = 1000);

drake/systems/primitives/test/signal_logger_test.cc

@@ -37,19 +37,22 @@ GTEST_TEST(TestSignalLogger, LinearSystemTest) {
   context->get_mutable_continuous_state_vector()->SetAtIndex(0, 1.0);
 
   simulator.Initialize();
+  // The Simulator internally calls Publish(), which triggers data logging.
   simulator.StepTo(3);
 
+  // Gets the time stamps when each data point is saved.
   const auto& t = logger->sample_times();
   EXPECT_EQ(t.size(), simulator.get_num_publishes());
 
-  // Now check the data (against the known solution to the diff eq).
+  // Gets the logged data.
   const auto& x = logger->data();
   EXPECT_EQ(x.cols(), t.size());
 
-  const Eigen::MatrixXd desired_x = exp(-t.transpose().array());
+  // Now check the data (against the known solution to the diff eq).
+  const Eigen::MatrixXd expected_x = exp(-t.transpose().array());
 
   double tol = 1e-6;  // Not bad for numerical integration!
-  EXPECT_TRUE(CompareMatrices(desired_x, x, tol));
+  EXPECT_TRUE(CompareMatrices(expected_x, x, tol));
 }
 
 }  // namespace