microprocessor

EAT237 Microprocessors and PLCs

Referred/Deferred Coursework Assignment 2016/17 – Weight based potato packing system

Scenario

You have been contracted to develop a control system for a packing system which will load potatoes

into plastic punnets based on weight. Potatoes are to be loaded into punnets until the weight of the

punnet is 1kg +/- 100g. Each potato will weigh between 80g and 110 g.

The potatoes will be delivered one at a time on a conveyor belt with a minimum separation of 10cm

between each potato. When operational the conveyor will move at a speed of 5cm/s. When the

potatoes reach the end of the conveyor they will fall into a hopper which is equipped with a load cell.

The load cell will produce a voltage which is directly proportional to the weight contained in the

hopper. The output voltage will be 10mV per 5g.

The hopper will be controlled by two servos. If servo ‘A’ is activated the potato in the hopper is

emptied into the punnet. If servo ‘B’ is activated the potato is returned to the beginning of the system.

The punnets are carried on a second conveyor belt system at a spacing of 20cm. This belt moves at a

speed of 4cm/s. The punnet currently being loaded is weighed by a further load cell with the same

operational parameters as that for the hopper.

Project Objectives

1. To design a signal conditioning system to interface the two load cells to a PIC18F2455

microcontroller

2. To develop a microprocessor based system to weigh the current potato in the hopper, the

punnet and to decide whether to load or reject the current potato.

3. To design and develop a Micro820 PLC programme to control the two conveyors.

System Elements

Weight Control System

The two weight sensors will be interfaced to a PIC18F2455 microcontroller. The microcontroller will

perform the following functions.

1. Measure the weight of both the punnet and the current potato in the hopper.

2. Add the weight from the hopper and the punnet and determine whether to add or reject the

current potato.

3. Provide digital signals to the conveyor signals as per table 1 below.

Signal Value Meaning

Accept 0 No action

1 Hopper is emptied into the Punnet

Return 0 No action

1 Hopper is emptied into return silo

Full 0 No action

1 Punnet is full – move next punnet into position

Table 1 – Weight control system outputs

Conveyor Control System

The control system for the conveyor will be implemented by a Micro 820 PLC and will move the

potatoes through the system.

The PLC system should perform the following actions on the receipt of the signals from the

Microprocessor:

• When an ‘accept’ signal is received, the PLC should activate the ‘accept’ servo for 5 seconds,

then move the feed conveyor to bring the next potato into the hopper.

• When a ‘reject’ signal is received, the PLC should activate the ‘reject’ servo for 5 seconds

then move the feed conveyor to bring the next potato to the hopper.

• When the ‘full’ signal is received the PLC should move the next punnet into position.

Deliverables

Weight Sensor Signal Conditioning (20%)

You should provide a detailed description including circuit diagrams and relevant design calculations

explaining how the two weight sensors will be interfaced with the PIC18F2455 microcontroller. You

should explain the rationale for the design and how it is makes optimal use of the available feature

of the microcontroller.

Weight Control System Design Notes (10%)

You will carry out design work on the code and produce a set of design notes which may include:

• a flow chart of the operation of the program,

• a list of the I/O pin allocations

• A list of the subroutines you will use.

• Pseudocode outline for your program

Microcontroller Code (20%)

You should provide PIC18F2455 code to implement the required functionality. All code should be

appropriately commented and presented in a professional fashion using appropriate formatting and

indentation. You may write the program in Assembly Language OR C code suitable for the XC8

compiler.

PLC Design Notes (10%)

You will produce a set of design notes for the PLC program based on the required functionality

described above. This may include:

• Flowchart or Sequential Function Chart

• I/O list

• Variable list

• Comments on the purpose of each rung

PLC Program (20%)

You should provide a Micro820 Ladder Logic program to implement the required functionality for

the conveyor control system as described above.

Evaluation (20%)

You will complete the evaluation grid provided (see table 3 – a template will be made available via

SunSpace) where you will provide your critical reflections on how your work across all previous

deliverables addressed the original objectives and how the developed system could be improved.

This can include any changes you see fit to include different components or changes to the structure

of the system but the rationale for your changes must be made clear. There is no page or word limit

for the evaluation grid but it is expected that 2 pages will be adequate.

This is an individual submission so you work should be your own and should not be prepared with

your lab partner.

Guidance and Advice

Hardware Choices

The coursework will involve the use of the ECIO board (PIC18F2455) and the Rockwell Micro 820

PLC. Any submission for alternative devices e.g. Arduino will not be accepted.

Importance of Design

One of the most important themes in this module is the importance of design. It is important,

therefore, that you allow adequate time to design your program. If you fail to do so you will not only

jeopardise the design elements of the coursework but you will also make it extremely difficult to

score well for the coding activities.

Avoiding Accusations of Plagiarism

Any code you write must be entirely your own. You may bring listings into the coding sessions but if

these have been sourced from another source e.g. a text book or the internet, then this must be

fully acknowledged using comments which clearly and unambiguously identify the extent of the

code involved (e.g. by labelling the start and end of the section in question) and the source. You

should also ensure that all work in the written submissions is your own and correctly referenced.

Where you draw on references you should ensure that you clearly explain their relevance to your

own work. If you are unsure on the correct way to reference any material, please ask or refer to the

guidance available via the University Library Website:

http://library.sunderland.ac.uk/find-resources/referencing/

Presentation of Work

Presentation of work is of critical importance. The elements being submitted electronically should be

typed and clearly formatted making appropriate use of headings and subheadings.

SUBMISSION DEADLINE Please submit your work via SunSpace by Friday 25th August.

Marking Scheme

Weight Sensor Signal Conditioning (20%)

Explanation of optimal strategy to interface weight sensors with Microcontroller /20

Weight Measurement System Design Notes (10%)

Design notes which may include:

• a flow chart of the operation of the program,

• a list of the I/O pin allocations

• A list of the subroutines you will use

• Pseudocode outline for your program

/10

Microcontroller Code (20%)

Demonstration of required functionality:

• Read of each Load Cell

• Maintaining a record of current punnet weight

• Decision making algorithm

• Activation of appropriate outputs

/10

Code Structure

• Appropriate use of subroutines/functions

• Clear structure e.g through indentation etc

• Use of sensible/appropriate variable names

• Clear and appropriate use of comments

/10

PLC Design Notes (10%)

Design notes which may include:

• Flowchart or Sequential Function Chart

• I/O list

• Variable list

• Comments on the purpose of each rung

/10

PLC Program (20%)

Demonstration of required functionality:

• Read of output of weight measurement system

• Control of conveyor to move potatoes into hopper

• Control of solenoids

• Control of conveyor to move punnets

/10

Structure of PLC program:

• Appropriate use of instruction blocks

• Clear logical structure

• Use of sensible/appropriate variable and alias names

• Clear and appropriate use of comments

/10

Evaluation (20%)

Critical evaluation of degree of success achieved /10

Recommendations for further development and improvement /10

TOTAL: /100

Table 2 - Marking Scheme

N.B. Written elements will be assessed using the University of Sunderland Generic Assessment

Criteria https://docushare.sunderland.ac.uk/docushare/dsweb/Get/Document-7861/AQH-F6-

15%20Generic%20Assessment%20Criteria.pdf

Evaluation Table.

Objective Critical Evaluation of Degree of Success Achieved

Recommendations For Further Development And Improvement

1. To design a signal conditioning system to interface the two load cells to a PIC18F2455 microcontroller

2. To develop a microprocessor based system to weigh the current potato in the hopper, the punnet and to decide whether to load or reject the current potato.

3. To design and develop a Micro820 PLC programme to control the two conveyors.

Table 3 – Evaluation Grid