The Project Aims to Build a Dynamic Water Pump Controller

7/28/2019 The Project Aims to Build a Dynamic Water Pump Controller

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The project aims to build a dynamic water pump controller, that monitors a

tank of water and supply line continuously and keeps the tank topped up.

Selection of Pressure Sensor: The sensor I have selected is a differential

pressure sensor. A differential pressure sensor has generally two ports and are

used to measure pressure differences. In this case one port will be left open to

atmosphere, while the other port would be connected to the drain/supply line.

The advantage is that such an arrangement eliminates variation in pressure

due to changes in atmospheric pressure.

Now lets delve into the mathematics of pressure .Static Pressure exerted by a still column of water is given by the equation

Pressure(P)= fluid density (?) x gravitational acceleration(g) x

height of fluid column(h).

Hence pressure exerted by a water tank of 2 meters height =

P=1,000 kg/m3 x 9.81 m/s2 x 2 = 19.62 kPa (kiloPascals)


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Now above is a very basic equation valid only for a sensor placed at the bottom

of a tank. In real world scenario it is quite likely that the sensor will be

connected at the end of a pipe. Lets consider the following case.

A 2 meter high filled water tank is placed on the roof of a building

30 ft high. The sensor is placed at the bottom of the tank on a 1in

dia pipe.

In such a case above equation are no longer valid as the head changes. I am

still lingering in the realm of static pressure and have not catered for dynamic

pressure.

The new water head= 2 mtr + 30 ft =11.14 mtr.

Hence pressure exerted = 1,000 kg/m3 x 9.81 m/s2 x11.14 = 109.2834 kPa(kiloPascals)

Hence I have selected MPX2100 from Freescale Semi for the sensing pressure

from the main tank.

Selection of Micro-controller: This is a no-brainer. Since the contest is

sponsored by guys at Texas Instruments and they have shipped me a MSP430

development board Launchpad. I already have 3 launchpad boards and this

one adds one more to their company.

Amplifier Design: The MPX2xxx series differential pressure sensors can be

approximated to a resistor (Wheatstone) bridge. Hence the output is a


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differential voltage. The span of the differential voltage demands a sensitive

instrumentation amplifier. Ti guys have been kind to ship me samples of their

instrumentation amplifiers. To keep the power supply or the circuit simple, I

would be designing my circuit around a single supply instrumentation

amplifier.

Characteristics of MPX2010 and MPX2100:

Excitation

Voltage Min Span (mV) Max Span (mV) Min Offset (mV) Max Offset (mV)

10 V 38.5 40 -1 1

5 V (scaled) 19.25 20 -0.5 0.5

9 V (scaled) 34.65 36 -0.9 0.9

12 V (scaled) 45.6 48 -1.2 1.2

15 V (scaled) 57.75 60 -1.5 1.5

MPX2100DP

Excitation

Voltage Min Span (mV) Max Span (mV) Min Offset (mV) Max Offset (mV)

10 V 24 26 -1 1

5 V (scaled) 12 13 -0.5 0.5

9 V (scaled) 21.6 23.4 -0.9 0.9

12 V (scaled) 28.8 31.2 -1.2 1.2

15 V (scaled) 36 39 -1.5 1.5

MPX2010DP

Offset and Span considerations:

What is Span? Span, put simply is the the max output of the sensor minus


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the min input.

What is Offset?Offset is the output of the sensor without any applied

stimulus. For our sensor according to data-sheet Offset is defined as the

output voltage at the minimumrated pressure.

Considering the analog input characteristics of MSP430 devices, the desired

offset is 0.5V and the desired span is 3V. 3V is the maximum analog input

voltage for MSP430G2x53 devices.

Gain Calculation:

Maximum Gain (GMax) = Desired Span(V) ?Sensors Minimum Span

Maximum Gain (GMin)= Desired Span(V) ?Sensors Minimum Span

Excitation

Voltage MPX 2010 GMax MPX2010 GMin MPX 2100 GMax MPX 2100 GMin

10 V 125 115 78 52

5 V (scaled) 250 231 156 104

9 V (scaled) 139 128 87 58

12 V (scaled) 104 96 65 53

15 V (scaled) 83 77 52 35

Gain Calculations

As I mentioned earlier, to keep the overall circuit simple and costs low, I

would be using a single supply instrumentation amplifier from Texas

Instruments. I would be using INA122 from TI for the project.

For INA122, Gain (G) = 5 + (200K ?RG). RG is a gain setting resistor.

Now lets calculate values of R G for different gain settings

RG=200k?(G-5)

Excitation

Voltage 10V 5V 9V 12V 15V


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MPX2010 1.67K? 820? 1.49K? 2.02K? 2.6K?

MPX2100 2.74K? 1.32K? 2.44K? 3.33K? 4.3K?

RG Values

Excitation

Voltage 10V 5V 9V 12V 15V

MPX2010 1.6 K? 820 ? 1.5 K? 2 K? 2.7 K?

MPX2100 2.7 K? 1.3 K? 2.4 K? 3.3 K? 4.3 K?

RG Values approximated to nearest std. resistor values

Schematic Diagram for sensor-amplifier interface

Man Machine Interface I would be using a 204 HD44780 LCD display

and a detented rotary encoder for making the man machine interface. This

interface This I anticipate would take bulk of my coding time. There are two

major obstacles that I foresee in front of me. The first is the the number of


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pins available on microcontroller to interface the LCD.

The LCD in 4 bit mode requires a minimum of 6 pins to

interface with a MCU and the rotary encoder will use 3. Thus ,using 9 pins

from 16 available is going to cause an acute shortage of pins for any additional

use. However yesterday I successfully interfaced the MCU using a shift

register 74HC164. This has had 2 major positive impacts. The code overhead

required for LCD routines has decreased significantly and also has caused

reduction in number of pins from 6 to 3.


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First obstacle ..overcome!

The second obstacle is wring the routine for the menu itself. This is going to be

a long and buggy process as I would require the MCU to be state aware andthus write a state machine for menu function.


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The Project Aims to Build a Dynamic Water Pump Controller