Frequency to Voltage Converter Used as RPM Gage

Justin Dasah David Hunter Ellia Lim

Introduction

-In the robotics field there is a need to control vehicles using small internal combustion engines.

-The drawback of small internal combustion engines is that they come with very few sensors.

-In order to properly control the engine the microcontroller must monitor various things such as coolant temperature, oil pressure, and engine RPM.

• Oil pressure and coolant temperature sensors are easy to read, and can be connected directly to the microcontroller since they are low voltage, low current, and relatively clean.

• RPM however is a more difficult problem since it must be read from the engine coil.

The Problem

• The problem with this is that the coil voltage is large, and the output is extremely noisy which may damage expensive components.

• Theoretically, the standard circuit that is implemented in an RPM gauge will work. However, in practice the noise and voltage spikes coming from the engine coil will cause damage to the circuitry and will also give inaccurate voltage readings.

•Standard circuit implemented in an RPM gauge

-Output from the engine coil of a standard 6 cylinder engine

- As can be seen we have a noisy waveform, resembling a square wave, with a large voltage spike reaching as high as 315V. This would surely damage any microcontroller.

The Solution

• In order to eliminate the large voltage spike of 315V, we installed a TVS diode (Transient Voltage Suppression diode) in parallel with the input.

• We also used an operational amplifier as a comparator, to clean up the signal and generate a useable square wave between 0 and 12 Volts

TVS Diodes

• A TVS diode is a gadget that protects electronic devices from over voltages.

• The device works by drawing away excessive current when the induced voltage exceeds the zener breakdown potential.

TVS Diodes

• It is represented by two opposing Zener diodes connected in parallel with the circuit to be protected.

TVS Diodes

• TVS diodes are preferred to other voltage spike protection devices (such as varistors or gas discharge tubes) due to their rapid response time.

• This characteristic makes TVS diodes useful for protection against massive voltage spikes created by such things as motor arcing.

TVS Diodes

• The TVS diode that we chose was the World Products 1.5KE30A.

• This diode suppresses any voltage greater than 41.4V

• This was chosen to discard the large voltage spike without cutting of the remainder of the signal.

TVS Diodes

• TVS Diodes are used to protect computer and data processor circuits, power supplies, airframe avionics and controls, telecom circuits, and many other applications.

• These devices are designed to protect against transient voltages generated by lightning, electro-static discharge (ESD), and inductive switching.

TVS Diodes

1.5KE30A

Op-Amp Comparator

• A comparator is a device that compares two voltages or currents and switches its output to indicate which is larger.

Op-Amp Comparator Circuit

Non-Inverting Terminal Input

Final Circuit

Results

Frequency (Hz) RPM Voltage (V)

70 4200 6.20

65 3900 6.06

60 3600 5.65

55 3300 5.20

50 3000 4.72

45 2700 4.26

40 2400 3.80

35 2100 3.38

30 1800 2.99

25 1500 2.54

20 1200 2.07

15 900 1.61

10 600 1.16

5 300 0.85

0 0 0

1 Hz = 60 RPM

Results

0 10 20 30 40 50 60 700

1

2

3

4

5

6

7

Frequency (Hz)

Vol

tage

(V

)

Frequency vs. Voltage

0 500 1000 1500 2000 2500 3000 3500 4000 45000

1

2

3

4

5

6

7

RPM

Vol

tage

(V

)

Voltage vs. RPM

Real World Applications

• Include turbines/wind turbines

• There are already frequency-to-voltage converters in vehicle-monitoring applications that evaluate the response times of anti-lock braking systems

• Control of robotic vehicles

Future Improvements

• Include more filtering before operation amplifier stage.

• Use Schmitt trigger instead of comparator.

• Design to operate at larger frequencies

Thank You