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Kubo: Safety System Electrical Experiments
Back to Kubo: Safety System
The following document shows the experiments and calculations run to determine current draw for Kubo's safety system. The script used to run the calculations and generate the graphs is located in gravl_experiments/SafetyRelayCalcs.
This experiment determines the current required to activate a single ice cube Relay. The relay's coil was connected to a power supply at an input voltage representing Vbatt, and the power supply current limit was increased until the relay audibly clicked shut. The relay used had a coil resistance of 90 ohms.
| Vbatt (V) | Imin (mA) | Irest (mA) |
|---|---|---|
| 11.5 | 90 | 120 |
| 12.0 | 80 | 130 |
| 12.5 | 90 | 130 |
| 13.0 | 90 | 130 |
This calculation demonstrates the calculated current drawn through two relays in series, as compared to the lowest current limit indicated in Experiment A.
images/200127_gravl_relay-series-current-plot.png
This experiment verifies Calculation B. Two relay coils in series were connected to a power supply at an input voltage representing Vbatt, and the power supply current limit was increased until both relays audibly clicked. The experiment validates the calculations shown above.
The following image displays the circuit diagram:
images/200127_gravl_relay-series-diagram.png
| Vbatt (V) | Imin (mA) | Irest (mA) |
|---|---|---|
| 11.5 | ~ | 70 |
| 12.0 | ~ | 70 |
| 12.5 | ~ | 80 |
| 13.0 | ~ | 80 |
This calculation demonstrates the calculated current drawn through two relays in parallel, as compared to the lowest current limit indicated in Experiment A.
images/200127_gravl_relay-parallel-current-plot.png
This experiment verifies Calculation C. Two relay coils in parallel were connected to a power supply at an input voltage representing Vbatt, and the power supply current limit was increased until both relays audibly clicked. The experiment demonstrates that the total predicted current draw shown in Calculation C is higher than the experimental values, but the margins by which the individual relay currents are higher than the activation minumum of 80 - 90mA are significant enough to ignore this difference.
The following image displays the circuit diagram:
images/200127_gravl_relay-parallel-diagram.png
| Vbatt (V) | Imin (mA) | Irest (mA) |
|---|---|---|
| 11.5 | 200 | 260 |
| 12.0 | 200 | 260 |
| 12.5 | 200 | 270 |
| 13.0 | 200 | 280 |
For Calculation D, it was determined that adding 20 Ohms of resistance in series with the parallel pair of relays will keep the current in each relay above the minimum switching current. The resistance of the estop circuit should be beneath this threshold, preferably by a large factor of safety.
images/200131_gravl_relay-parallel-current-plot2.png
| Resistance | Current | |
|---|---|---|
| R3 = 0 Ohms | 43 Ohms | 270 mA |
| R3 = 10 Ohms | 53 Ohms | 230 mA |
The final current of 230 mA is close to the total current shown in calculation D. It is on the higher side, which provides the circuit a factor of safety, since too low will fail to activate the relays.