INERTIA DYNO PROJECT
I
Dyno Board
This is the inertia dyno board. It contains a pic 16f877a running at 20mhz. Next to it is a programmable interval timer driving by a 1mhz clock ttl clock
on the far right is an opto isolator used for measuring engine speed via the negative of the coil. On the left is an max232 used for serial communication with the pc.
A pulse from the drum causes a CCP interrupt. The value of the internal timer Tmr1 is stored. The period of the drum is then calculated from the times.
An alternative is to start and stop the external timer. The internal timer allows fine resolution since each clock tick is 200ns.
The pulses from the coil causes an interrupt on PORTB pin 0. This is used to start and stop another counter in the external timer.
The engine RPM and drum rpm are passed to the external software every revolution.
A drum and engine interrupts causes the LEDs to flash.If the engine is running and the drum is spinning and they are not flashing some is definitely wrong.
This is the prototype drum. This looks pretty small but weighs 60lbs...It is lodge in two base mount bearings.
The moment of inertia is 1/2mrR
With the ends shaved off I recon this now weigh 55lbs.
0.5 * 24.9 * 0.0762 * 0.0762 = .0722kgm2
Angular velocity...Let say I can accelerate this drum from 1000rpm to 7000rpm in 10seconds.
That is 6000rpm in 10 seconds. 100 revolution a second in 10 seconds. 1 rev = 6.28 radians.
628 radians per second in 10 seconds.
62.8 radians per second per second..
Torque = moment of inertia * Angular Acceleration
.0722kgm2 * 62.8 = 4.5nm
Power = Torque by Angular Velocity.
= 4.5 * 62.8 = 281.7watts
Testing
Internal Timer with 6 sensor slots.
The internal timer TMr1 when operation at 20mhz the maximum period it will time is 13ms.
This is equivalent to an RPM of 4615. With 6 sensors the slowest speed of the drum is 769rpm.
External Timer with 6 sensor slots.
The internal timerof the 8254 programable interval timer when operation at 1mhz the maximum period it will time is 65ms.
This is equivalent to an RPM of 915rpm. With 6 sensors the slowest speed of the drum is 152rpm.
Angular velocity
RadsPerTick = 6.28 / DrumTicks
Not that 2 pi radian is one revolution. If I am making a pulse every revolution and counting the ticks the above calculation gives radians per tick..
Radians per tick is important because it lets you calculate radians per second as this in angular velocity.
With the external timer each tick is 1us. So to find angular velocity we need to multiply by 1000000 as 1000,000 us is a second.
The previous radspertick and the current radspertick are subtracted and multiplied by 1000000 to obtain the change in angular velocity. This change would have taken place over the
time for one rotation. This change over a period of time is the angualr accelleration
Sensor Wheel
The below is a test of How the system is measuring rpm. There are six holes . One of the holes seem to be out of aligment or the sensor is wobbling.
The machine shop will be visited.
The six hole sensor clearly shown The holes need to be accurately drilled otherwise the differences in time between them will show up us changes in rpm even though the rpm is not changing.
The machine person will use a more accurate machine insuring that they are perfectly spaced at 60 degrees.The above rpm ramp shows that wobble and incorrect spacing as it repeats it self in a constant fashion.
The picture below show the Hall effect sensor with built in magnet . The absence of the metal will cause a pulse. The sensor is supplied with five volts . The output is an open collector circuit.
Tied to the 5 volt source. Once triggered it will turn on the transistor and force the output to zero.The pulses go to the ccp1 port which captures the time of a internal timer tmr1 at 200ns ticks or an external timer at 1us ticks. Spinning the wheel slowly show when the microcontroller has entered the interrupt subroutine.The flashes each time the senor passes the hole on the sensor drum
The GUI interface
We are currently focusing on measuring drum rpm accurately.The guage on the right is the drum rpm guage.
The sensor plate has been completed and the holes were drilled accurately by a precision machine.A hole was drilled in the plate and a boss fitted. There is no wobbling
.The below graph show the drum decelerating.
Due to inaccurracy in the distance between holes the six hole sensor wheel has been abandoned. I simple hold was drilled on the drum and the sensor detects it presence every rotation.
The slowest speed it can now detect is 917 rpm but the system has adequate time to send data to the pc. Below is a plot of the drum decellerating from 2400rpm. It is much smoother with no oscillation present.
Replacing the rotating disc enable more space on the shaft for the drill or power source to rotate the drum.The sensor is clear shown and in position to be triggered by the absence of the
metal surface(hole).
Other changes have bee made to the GUI as well.The torque and rpm guages are removed for enhanced speed.Units have been added to the torque and power and the torque is
formatted to two decimal places.Panels have been used to group functions together.
First dyno run with lot of curves.Very choppy torque and power curves.
More changes have beeN made to the GUI .The torque guage is back and replaces engine RPM. Power is being displaced as label.
Second dyno run curves smooth abit.Achieved by averaging drum ticks.I will consider a bipolar hall effect switch and better clock for timer.
Dyno Brake
That would work, but a far more simpler way would be to couple a hydraulic pump to your engine, keeping in mind the maximum RPM of the engine and pump respectively and gearing accordingly. Use a pump capable of handling a little over 10 hp. Pipe the outgoing flow past a pressure gauge and to a needle valve. Out of the needle valve to a flow meter, and return it to the reservoir. With the needle valve open, you will get flow without pressure, this will relate directly to RPM. A little experimenting will yield a conversion factor to calculate RPM from the flow. Closing the needle valve will result in pressure being created. Now you have all of the physical parameters necessary to calculate power, force, distance, time. The equation to calculate power will be:
HP = GPM x PSI x .000583
If you know the pump displacement, the equation for torque would be:
Torque(ft-lb) = PSI x pump displacement(in cubic inches) /24pi
Keep in mind, that these only work for positive displacement pumps. An old worn pump with internal leakage will directly affect this system. Also, there will be some error due to heat loss in the fluid. It all depends on how accurate you want to be. Spend more money, you can get more accuracy. A piston pump will be more accurate than a vane or geroter pump.