WEEK THREE: 4 May to 10 May 2015

Task: Determining the Different Type of Sensors

As there are different parameters that we need to obtain from SunSPEC 4, different sensors are needed. 

Current Sensors

Measurements taken from the following hardware:

• 3x string current after Solar MPPT (Maximum Power Point Tracking) 
• 1x total current from batteries (in 34S 36P)
• 2x current from the 2 motors 

Requirements for the current sensors:

1. Non-invasive so that the circuit need not to be broken
2. Hall effect sensors that measures DC Current
3. Must be compatible with the microcontroller that we are using.
4. Accurate / Sensitive enough for data logging

There are multiple power sources and motors for the SunSPEC 4. Therefore, we are required to have multiple current sensors for data tracking, in order to calculate the power consumption of the solar car. The following are the considerations we took for selecting our current sensors.

1. Bi-directional. The sensors for certain parts of the car must be bi-directional. For example, in the case of the motor, current will be flowing to the motor when driving and current will be flowing out of the motor when implementing the regenerative braking.

2. Non-invasive. For the ease of removing the sensors when troubleshooting and also to minimize the possibility of hot spots due to bad contacts, we have decided to use a non-invasive hall effect sensors.

3. Size. The current sensor must also be of a certain size, in order for the wire to be able pass through the sensor for measurements to be taken.

4. Current rating. The rating of the current that we require would also be considered, as we would not want the sensor rating to be lesser than the rated currents of our systems in the solar car.

Coding for current sensors:

void setup() {
Serial.begin(9600); //initiate Arduino serial display }

void loop() { //to be repeated over and over again

float average = 0; //set average to pin A0

for(int i = 0; i < 1000; i++) {
average = average + (.0264 * analogRead(A0) -13.51); //calibrated current calculation
delay(1);  }
Serial.print(average);
Serial.println("mA"); }

We looked through a few non invasive current sensors which includes Panucatt CS-45AL Hall effect sensor, LEM HO series current sensors and AMPLOC current sensors. Although we favoured Panucatt Sensor the most, it has been out of stock since 2012. The LEM sensors on the other hand consumes too much power. Thus, we decided to continue using the AMPLOC current sensors.

Amploc current sensors were used in SunSPEC3.  We found that the readings obtained in SunSPEC3 was rather accurate which made decision-making easy for the telemetry team. Figure 1b shows the dimension of Amploc sensors. The size of the sensor is able to fit all the wire sizes, measuring the current of motor, MPPT, battery and others.

Shortlisted current sensors: AMPLOC AMP Series (Datasheet)

Voltage Sensing Circuits

Measurements taken from the following hardware:

• 1x system voltage
• 1x battery voltage
• 1x solar voltage

Requirements for the voltage sensing circuit:

1. Able to sense up to 150V
2. To step down until 5V for microcontroller boards to function properly
3. Sensitivity:

For voltage sensing, our team would have to come up with a circuit to measure the voltage and step down into a range of 5V to be input into the microcontroller. 

Temperature Sensors

Measurements taken from the following hardware:

• 4x battery temperatures (to be placed at hot spots in the battery box)
• 3x solar panel temperature (to be placed underneath the solar panels)

Requirements for temperature sensors:

1. Big temperature range
2. Durable
3. Compatible with the microcontroller that we chose.  

Coding for temperature sensor

float temp;
int tempPin = 0;

void setup() {
Serial.begin(9600); }

void loop() {
temp = analogRead(tempPin)/9.31;
Serial.print("TEMPRATURE = ");
Serial.print(temp);
Serial.print("*C");
Serial.println();
delay(500); }

Temperature sensors are easily obtained from various stores. We shortlisted LM35 as it is cheap and compatible with any microcontroller. The reading from LM35 is also rather accurate at 10mV/°C. The pins are also easy to configure. Measurement of temperature ranges up to 300°C. 

Shortlisted temperature sensors: LM35 (Datasheet)