By: Sherman Xa (Mission, Systems, and Testing Engineer)
The purpose of this testing is to experiment the power consumption of each of the individual components of the Micro Rover, so the team is able decide on a valuable power supply that will fit the needs and requirements of the mission profile.
Last semester, the Mini RoSco had a total possible run time of 11.85 minutes. Their testing that showed how they came to this run time can be found here.
To find our power consumption and decide on what rating power supply we needed, we took a preliminary power budget report on all the components that we planned to use and what the power draw was for each component. Our preliminary design called for the use of an Arduino Uno, which will consume current to 5 I/O pins, which delivers 40 mA, and a 3.3 V pin, which has a consumption of 50 mA. Next, for our motors, we were to be using two 120:1 Plastic Gearmotor from Pololu, which have a current rating of 70 mA at free run and a stall torque current of 800 mA. For our pan mirror system, we are planning to use a HD-1440A servo, which runs at a rated voltage of 4.8 V at a free-run current draw of 110 mA and a stall torque current of 320 mA. Finally, with the use of Arxterra, we are going with the route of Bluetooth communication. As a result, we were adding an HC-05 Bluetooth module, which has a pairing current draw of 35 mA and a connected current draw of 8 mA, running at a rated voltage of 5 V. After calculating these preliminary current draws, we estimated a maximum current consumption of 2213 mA. The resulting preliminary total run time of 650 mAh, 800 mAh, and 1600 mAh batteries are 17.6 minutes, 21.6 minutes and 43.4 minutes respectively. The results can be seen in the table below.
However, after much testing and alternating the design, we determined to switch the Arduino Uno and plastic gearmotor to an Arduino Micro and Pololu micro metal gearmotors due to sizing issues which were discussed in a separate blog post. Also, for future designing, we will be using an SN754410 motor driver, which will supply sufficient current to our motors and a LM7805 voltage regulator to help protect our components.
For measuring our actual amperage draw of all our components, we used California State University, College of Engineering’s power supply and digital meters located in ECS 314 to test and measure all current draws. For our micro metal motors, we supplied a voltage of 8.6 V and measured it at free-run and stall. For our HC-06, we bread-boarded a prototype PCB and measured the Bluetooth module when it is connected and sending commands and when it is trying to connect to our Android device. As for the HD-1440A servo, we measured it at a rated voltage of 5 V and tested it during no load, operating, and at a stall. As for the motor driver and voltage regulator, I tested the components on our prototype PCB at no load to the servo or motor and when both the motor and servo are operating at full capacity. For the motor driver, I measured the current draw from the VS, which is the logic power supply and the VSS, which is the power supply to the motor driver. The resulting data chart can be found below. The resulting current draw, at worst case scenario, maximum current draw is 2301.64 mA, which gave us 16.9 minutes, 20.8 minutes and 41.7 minutes for the 650 mAh, 800 mAh, and 1600 mAh battery respectively. All these data can be found below.
As a result of this, we decided on the 800 mAH to give us a maximum duration of 20.8 minutes, which is more than the allotted time we need to complete the mission. As a result, we ordered an 8.4 V, 800 mAh NiMH battery from BatterySpace. However, upon receiving the shipment, we discovered they had shipped us the wrong battery, and did not have enough time to receive the replacement. As a result, we canceled the order, and bought 7 NiMH rechargeable batteries and purchased a 4 and 3 block battery holders, which will give us 8.4 V and 850 mAh, when connected in series, which will give us similar results to what we originally wanted in our design.