Added water boiler example

examples/water_boiler.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python
+
+import time
+import matplotlib.pyplot as plt
+from simple_pid import PID
+
+
+class WaterBoiler:
+    """
+    Simple simulation of a water boiler which can heat up water
+    and where the heat dissipates slowly over time
+    """
+    def __init__(self):
+        self.water_temp = 20
+
+    def update(self, boiler_power, dt):
+        if boiler_power > 0:
+            # boiler can only produce heat, not cold
+            self.water_temp += 1*boiler_power*dt
+
+        # some heat dissipation
+        self.water_temp -= 0.02*dt
+        return self.water_temp
+
+
+if __name__ == '__main__':
+    boiler = WaterBoiler()
+    water_temp = boiler.water_temp
+
+    pid = PID(5, 0.01, 0.1, setpoint=water_temp)
+    pid.output_limits = (0, 100)
+
+    start_time = time.time()
+    last_time = start_time
+
+    # keep track of values for plotting
+    setpoint, y, x = [], [], []
+
+    while time.time() - start_time < 10:
+        current_time = time.time()
+        dt = current_time - last_time
+
+        power = pid(water_temp)
+        water_temp = boiler.update(power, dt)
+
+        x += [current_time-start_time]
+        y += [water_temp]
+        setpoint += [pid.setpoint]
+
+        if current_time - start_time > 1:
+            pid.setpoint = 100
+
+        last_time = current_time         
+
+    plt.plot(x, y, label='measured')
+    plt.plot(x, setpoint, label='target')
+    plt.xlabel('time')
+    plt.ylabel('temperature')
+    plt.legend()
+    plt.show()