Vermont Energy Control Systems

Practical monitoring and control for the real world

Analog Sensors

A picture of a sensor plugged into a Vesta

'Analog Sensors' refer to sensors that are physically plugged into the 'Analog Sensor Input' connectors on the right side of the Vesta. There are many types of analog sensors, but the process of connecting, configuring, and calibrating is the same for all sensor types. The Vesta controller can accommodate any sensor type on any channel.

In this example we'll use the TS-1058 temperature sensor. These sensors work over a range from about -20°F to 212°F. They are inexpensive and in the Vesta controller they provide a temperature resolution of better than a tenth of a degree.

Connecting Sensors

The Vesta can accommodate a total of 8 analog sensors (16 for the Pro model). Sensors use telephone-style (RJ11) connectors. On the Vesta controller, sensor connections are arranged in groups of four. Sensors may be plugged directly into these connectors.

There is also a fifth connector in each group. This is a standard Ethernet style (RJ45) connector that allows use of a sensor breakout box. A breakout box can be used if you have a group of sensors at a remote location - in a nearby building, or on the roof, for instance. Rather than running four long individual sensor cables to the remote location, you can run a single Ethernet-style cable to the remote location, and then use a sensor breakout box to connect the individual sensors. For each row of four sensors on the Vesta, either the individual connectors OR the breakout box may be used - not both.

Sensors are configured using the 'Configure I/O' tab on the Vesta controller web interface. This tab displays all of the possible inputs and outputs for the Vesta controller, and allows you to specify which inputs and outputs you will use in your application. Sensor inputs are identified as Analog Inputs on this page.

Because there are so many inputs and outputs, the Vesta controller allows you to specify which ones you're going to use. Only selected inputs and outputs are visible outside the 'Configure I/O' page. In the Vesta controller, the process of selecting a physical I/O involves three steps:

  1. Identify the connector number on the outside of the controller where your sensor or other device is plugged in. The sensor inputs are on the right side of the Vesta controller and labeled with channel numbers from 1 to 16 (by default - different models may have different numbers of inputs). The picture above shows a sensor plugged into channel 1.

  2. Find the corresponding line in the 'Configure I/O' page. In our example, the first line on the 'Configure I/O' page corresponds to Analog Input 1. To activate a channel, simply use the 'Type' pulldown to designate the type of sensor that's plugged into it. In this example, analog input 1 has a TS-1058 temperature sensor connected to it.

    A screenshot of the user configuring an Analog Sensor on the Configure I/O tab

    Figure 11.3: Configuring an Analog Sensor

    For each sensor, make sure that the correct sensor type is selected. The most commonly used sensor in the Vesta controller is the TS-1058.

  3. Finally, go to the 'Elements & Rules' tab and give your sensor a meaningful name. Here, we'll choose 'Room Temperature':

    A screenshot of the user naming an Analog Sensor

    Figure 11.4: Renaming an Analog Sensor

    As each sensor is added, check to see that a reasonable temperature is displayed. Holding the sensor between your fingers should result in a quick rise in displayed temperature.

Calibration

The Vesta controller allows calibration of both gain and offset for each sensor. In most cases, calibration is not necessary. In the simplest case, you can specify a positive or negative offset to each channel to match the sensor reading to a know good value. Offsets are simply added to the measured temperature.

A screenshot of calibrating an analog sensor on the Configure I/O tab

Figure 11.5: Calibrating an Analog Sensor

In the example above, we've added an offset of -.2 degrees to the Room Temperature sensor.

To get the highest possible accuracy, gain calibration can be done as well. To perform gain calibration, you need to establish two known temperatures that are as far apart as possible while remaining within the sensor's measurement range. The easiest way to do this is to use an ice bath and boiling water. Immerse the sensors in an agitated (stirred) bath of water packed with crushed ice. This will be very close to 0°C (32°F). Water at a vigorous boil will be very close to 100°C (212°F). If you're at a high altitude, you'll have to correct for altitude effects.

Use the sensor calibration page to calculate gain and offset. Follow the instructions in the spreadsheet - enter the actual low and high temperatures (use a reference thermometer if you have one, otherwise use freezing and boiling). Enter the readings for each sensor at low and high temperatures. The blue cells in the spreadsheet will then give you the values for gain and offset for each sensor. Enter those values in the Physical I/O tab.