Microcontroller based Pump Controller

Updates:-

1. 28 Jun 11: I cant help but thank the kind guys at Texas Instruments. I cant believe my eyes, they have shipped me a sample of their instrumentation amplifier. Thats would help me simplify my input block. Thanks u guys… You rock.
2. 01 Jul 11: Received Sample. Waiting for Farnell guys to ship my pressure sensors…
3. 07 Jul 11 :  Received Pressure sensors.

 

 

This project will describe building of a Pump control system. The requirement primarily arose to satiate the requirements of my parents, who have to manually switch on the pump every day morning and switch it off and continuously monitor the tank for overflow. So I have taken up the challenge onto myself...

However I would be ungrateful, I do not acknowledge the inputs I received after reading Kayne Richens blog post.

 

This blog is progressive ; hence will get updated as things progress.....

Lets me see how do things unfold...


Basic Design Objectives:-

1. Should be cheap and affordable.
2. Should be as automated as possible requiring minimal user supervision.
3. Should have a user-friendly MMI.
4. Prevent water and electricity wastage.

Design Challenges:-

1. Erratic water supply timings.
2. Non scheduled Electrical Load shedding .
3. High head of water tank.
4. Intelligent control system.

Preliminary System Design:-

System Blocks Functions:-

Input Block:-

Input Description	Sensor Type	Micro-controller Input
Sense Tank Capacity	Differential Pressure Sensor	Analog
Sense Electricity Availability	Optocoupler	Digital/Interrupt
Sense Time	RTC	Digital
Sense Input Water Supply Availability	Differential Pressure Sensor	Analog

Output Block:-

Output Description	Transducer Type	Micro-controller Output
Motor On/Off	Solid State Relay	Digital
Valve Solenoid On/Off	Relay	Digital
Alarm	Buzzer	Analog

 

 

Mathematics for Selecting Differential Pressure Sensor:-

 

Why Differential Pressure Sensor?

 

A differential pressure sensor will eliminate variation in pressure due to changes in atmospheric pressure.

 

Mathematics

 

Capacity of Water Tank = 500 Lt

Water Tank Head = 30 Ft = 9.144 Mt = 914.4 Cm

Average Ambient Temperature = 25 deg Celcius

Specific Gravity of Tap water=0.997gm per cubic cm = 997Kg per cubic meter

Dia of Discharge Pipe=1/2 in = 1.27 Cm=0.0127 M

Pressure exerted at Base of Building by a column of Water at height

 

P = h * rho * g

where
P = Pressure
h = height of liquid column (2000 mm = 2m)
rho = Density of water (1000 kg/m^3)
g = Acceleration due to gravity (9.81 m/s^2)

 

P=9.14 x 997x 9.81 = 89394.4098 Pascals = 89.39kPa = 89.4 kPa (approx)

Similarly Pressure at Rooftop at the base of tank itself

P=1.5 x 997x 9.81 =14670.855 Pascals =14.67kPa = 14.7 kPa (approx)

 

Selected Sensor  :  Freescale Differential Pressure Sensor MPX2100DP  0-100 kPa

Selection  of MCU:-

Family : Microchip PIC

Why : Because I have necessary experience and required development tools.

Max No of ADC Channels Required = 3 (2xpressure Sensor + 1 spare)

No of Interrupts : 02 (Electrical Supply Available & Float Valve)

Digital I/O = 8

Operating Voltage= 5V

SPI Required = Yes (For Interfacing RTC)

MCU Selected  = 16F876A  or 16F877A Why? Because I already have!

Design of Pressure Sensor Amplifier:-

Design Guide/Reference : -

1. Freescale Application Note 
2. Op Amps for Everyone Design Guide.
3. 3 Op Amp Instrumentation Amplifier.
4. Five basic mistakes to avoid when using instrumentation amplifiers.
5. Bridge Measurements

Amplifier Design Objective : The output span of the transducer must be matched to the input span of the ADC to achieve optimum performance.

Mathematics for Amplifier Design

Span and Offset Characteristics at 10v,15v & 5V Excitation Voltage. As sensor is ratio-metric the the values are scaled for.

MPX2100DP

Excitation Voltage	Min Span  (mV)	Max Span  (mV)	Min. Offset  (mV)	Max Offset  (mV)
10V	38.5	40	-1	1
15V (scaled)	57.75	60	-1.5	1.5
5V (scaled)	19.25	20	-0.5	0.5
9V(scaled)	34.65	3.6	-0.9	0.9

 

 

MPX2010DP

 

Excitation Voltage	Min Span  (mV)	Max Span  (mV)	Min. Offset  (mV)	Max Offset  (mV)
10V	24	26	-1	1
15V (scaled)	36	39	-1.5	1.5
5V (scaled)	12	13	-0.5	0.5
9V(scaled)	21.6	23.4	-0.9	0.9

 

 

Desired Amplified Span and Offset


Desired Offset= 0.5V
Desired Span = 5V

Gain Range & Offset Calculation

Maximum Gain      =Desired Span (V)

                           Sensor’s Minimum Span

Desired Span = 5V

Excitation Voltage	Maximum Gain (MPX2100DP)	Maximum Gain (MPX2010DP)
10V	=5/38.5 = 129.8 = 130	208
15V	=5/57.75 = 86.58 = 87	139
5V	=5/19.25 = 259.75 = 260	417
9V	=5/34.65=144.300=144	231

 

Since Texas Instruments was kind to ship me samples of their instrumentation amplifiers, I would be basing my calculations on same. There is no rocket science involved since the datasheet is fairly self explanatory and the instrumentation amplifier itself requires only one external component for gain adjustment.

Texas Instruments was generous in mailing following samples to me

INA126, INA128,INA122,INA2126

 

I would be using INA126 amplifier for testing purpose

 

Brief description about INA126 ..”The INA126 and INA2126 are precision instrumentation amplifiers for accurate, low noise differential signal acquisition. Theirtwo-op-amp design provides excellent performance with very low quiescent current (175µA/channel). This, combined with awide operating voltage range of ±1.35V to ±18V, makes them ideal for portable instrumentation and data acquisition systems. Gain can be set from 5V/V to 10000V/V with a single external resistor. Laser trimmed input circuitry provides low offset voltage (250µV max), low offset voltage drift (3µV/°C max) and excellent common-mode rejection.”

 

Gain calculation formulae

 

Instr Amplifier	Gain Formula
INA 122	G=5+200kΩ           RG
INA 126	G=5+80kΩ           RG
INA 128	G=1+50kΩ           RG

 

RG (IN OHMS)values at Max Gain. Values in (brackets) indicate nearest standard resistor values.

 

MPX2100DP

 

Instr. Amplifier	10V	15V	5V	9V
INA 122	1602 Ω (1.6KΩ)	2452Ω(2.4KΩ)	785Ω (750Ω)	1439Ω(1.5K)
INA 126	641Ω (620Ω)	981Ω (1kΩ)	584Ω (560Ω)	576Ω(560Ω)
INA 128	388Ω (390Ω)	548Ω (560Ω)	193Ω (180Ω)	347Ω(360Ω)

 

MPX2010DP

 

Instr. Amplifier	10V	15V	5V	9V
INA 122	984 Ω (1KΩ)	1494Ω(1.5KΩ)	486Ω (470Ω)	885Ω(820Ω)
INA 126	393Ω (390Ω)	598Ω (560kΩ)	194Ω (200Ω)	356Ω(360Ω)
INA 128	241Ω (240Ω)	363Ω (360Ω)	120Ω (120Ω)	214Ω(200Ω)

 

Amplifier Interface Design

 

 

In the MPX sensor, each of the resistors in the bridge is an active strain gage, so this design illustration is applicable for the amplifier design.

 

 

Test Schematic

 

 

3D PCB Render (Using Diptrace)