THJ Media Blog

SRHS Ecomarathon theory of operation notes Posted on February 06, 2020 08:25:47.

SRHS Ecomarathon Vehicle
Theory of Operation Notes
Chris Jones 2/6/2020

1. With battery pack X1 removed from vehicle, make sure main switch S1 and On switch S5 are both open, and Internal Kill switch S3 and External Kill Switch S4 are both closed.

2. Connect battery pack to vehicle.

3. Close switch S1. Wait TBD seconds until voltage across C1 is greater than 90% of the battery voltage. The precharge resistor R1 slowly charges up C1 to avoid excessive inrush current that could damage it. Max inrush current = 51V/5000 ohms= 10 mA (too low?), 51V^2/5000 ohms = 0.5W through resistor so use 1W. Ripple current requirement TBD. Capacitor ripple current spec TBD. This also provides a power path to the input of the DCDC converter, but the voltage drop through the precharge resistor prevents the DCDC input voltage good circuit from enabling the rest of the circuit so there will be no 12V DCDC output yet.

4. Hold down momentary Start switch S2. This temporarily provides full battery pack voltage to DCDC converter X2 which will produce 12V.

5. While continuing to hold down Start switch, close On switch S5. This energizes the power relay S6 which also provides full pack voltage to the DCDC converter. This also provides power to the deadman switch on the accelerator potbox X3, but the switch is normally open so the motor controller won’t start.

6. Release the Start switch. The power to the DCDC will remain on because its output is being backfed in to the power relay coil.

7. If at any time the motor controller needs to disabled, during an emergency or for storage, press the internal or external kill switch. This will depower the relay coil and the motor controller deadman switch will not have power input.

8. To drive, press the accelerator. This first engages the dead man switch which powers the motor controller board X4. This charges up C2 charge reservoir cap for the 555 timer U1 in TBD seconds, as well as charges up the Tau time constant cap C3 to 90% in 22 us = 2.2 x R4 100 ohms x C3 0.1 uF. Max inrush current 12V / R4 100 ohms = 120 mA. C3 max ripple current TBD.

9. The 555 timer chip powers up in to a random output state. The output of the motor control board controls the power FET Q1. When the output is high the FET turns on and connects the brushed DC motor X5 negative terminal to the negative terminal of the battery which causes current to flow and the motor to turn. The motor speed is controlled by Pulse Width Modulation, where the 555 timer takes the throttle hotbox resistance input and varies the amount of time Q1 is energized during a ~500 us period, running at ~2 kHz. Full throttle achieves 98% duty cycle, and no throttle is very close to 0% duty cycle. Here are the steps:

a. 555 main output pin 3 randomly turns on. Assume for this example it is high. 12V turns on Q1 at the gate which turns on at RC = 5150 pF gate capacitance x 33 ohms R5 gate resistance = 170 nS. The motor then starts to turn as the magnetic field builds up in the rotor coils driven through the brushes and commutator timed to repel the permanent stator magnets. This does happen instantly since the inductance in the coils resists change in current. It draws from C1 which is a low inductance path away so it does’t droop; if C1 wasn’t there the stiff voltage battery far away with lots of inductance in between would cause overshoot and ringing that could damage Q1.

b. DISC discharge output pin 7, the open collector inverse of output pin 3, is pulled low. This makes accelerator pot low diode D4 conduct and high diode D2 does not conduct. This connects the Tau capacitor C3 to DISC through the low side of the potbox resistor R2. When the accelerator is not depressed very far, this a very low resistance and C3 will be discharged quickly. As C3 voltage falls, it connected to both the threshold and trigger inputs of the 555. The threshold comparator flips from on to off as this voltage drops below 8V = 2/3 of 12V per the 3xR voltage divider in the 555 which resets the RS flip flop whose output drops to 0V. The output of the inverter is inverted twice so the Q1 gate voltage falls to 0 and turns off. The motor coil inductance resists current change and tries to generate a negative voltage spike, but D1 starts conducting at -0.7V and dissipates the current through heat until the magnetic field collapses. If D1 wasn’t there Q1 voltage would go very negative and be destroyed.

c. The is then inverted once and fed to DISC then inverted again and fed to the main output of the 555 which turns of the current to the motor. Then with discharge high D3 conducts and D4 doesn’t. This connects 12V to C3 via the 5K 10A limiting resistor R3 and the high side resistance of the hotbox resistor P2 which is almost 5K. C3 charges up with a time constant of RC = 10K x .1u = 1 ms, about 68% of the way up. When C3 voltage gets to 4V the trigger comparator goes high and tries to set the RS flip flop but it is still being held in reset by the threshold comparator. But once the C3 voltage gets above 8V the reset is removed and the RS flip flop turns on and repeats the cycle. So this should run at about 1 kHz. WHY ARE WE SEEING 2 KHZ?

Note: needs smooth acceleration control. A relatively simple op amp RC delay circuit between the throttle and 555 could be added, but it would also slow deceleration which could be a safety hazard. A strategically placed discharge diode could fix this, but the cap could still be charged too long after deenergizing and burn out the 555 or op amp when the deadman opens. This may be possible but no pre-existing robust design was found, so it would take some time designing, building and troubleshooting, which could add risk to ther schedule. Hopefully some combination of adding load and mechanical and electrical improvements will suffice this year. It appears that the best way to address smooth acceleration is to replace the 555 with a microcontroller. An auto-accelerate button on the dash and no potbox could be best for maximum effiency. A future microcontroller based swappable motor control board could be made next year, with an eye on eventually going to BLDC.

ADD MID AND FULL THROTTLE DESCRIPTIONS HERE

DISCUSS WHEN 555 POWERS UP OFF INSTEAD OF ON

ADD LOW, MID AND HIGH SCOPE TRACES AND REFER TO DUTY CYCLE AND VOLTAGE MARKERS, DESCRIBE BACK EMF SEEN

10. When done driving:

a. Turn On switch to off position. This causes a negative voltage spike across the S6 relay coil terminals that is suppressed by D2.

b. Turn Main switch to off position. C1 will slowly discharge through input of DCDC. MEASURE TIME AND DOCUMENT SO SAFETY IS UNDERSTOOD; ADD BLEED RESISTOR ACROSS C1 IF NECESSARY

c. Disconnect battery from vehicle to charge.