1. Introduction

At the end of my RMIT University engineering degree there were a number of 3-semester projects advertised to third year students. One of those projects was a post office automation system. I selected this project as my first preference. However, a few weeks later I received an email stating that I was selected for project number 1. It was Ericsson Australia optic fibre research three semester project that could lead to a career at Ericsson Australia. Thus I was forced to abandon my dream of creating the post office automation system that was not a paid project and someone else ended up working on this project. 

I completed my Ericsson Australia project with three other students that were also selected to work with me in November 2002. The following year our Ericsson project received an RS Component Award.

However, over the last two decades, occasionally I would think about the different electronic or mechanical solutions that I could have used to implement the post office automation system.

This system can be used not only for parcels but for letters as well.

2. Measuring the Box Size - Electronic

In the project description it was mentioned that the box size need to be measured with a light/infrared sensor array. The volume of the box covering the light will give an indication of how many sensors are receiving ambient/infra red light and thus determine the dimension of the box. The array can me implemented with light depended resistors (LDR)/photo diodes. Three arrays will me needed, an array for each of the three dimensions of the box. Each array must consist sensors placed in a straight line.

I considered the possibility of sensors being influenced by shadows. Thus I was considering the use of infrared sensors with infra-red emitters. Thus I began to think that using ultrasound to measure the distance to the box is a better option to determine the box dimensions. However, detecting small reflected ultrasound waves could be a more complicated challenge if this distance from sensor to box is very long. Then I realised that the parcel can me placed in light shielded box that will prevent any ambient light interference.

The best solution would be to use an array of laser tags, placed on the other side of the box that will shine on to the photo sensors. This method will give a clear indication if the box is covering the sensor or not.

3. Measuring the Box Size - Mechanical

A more reliable idea could be using an array of mechanical buttons. If you put the box on the edge of the post office automation encasement and apply as gentle force then you will be activating the buttons with the box volume. A reliable mechanical sensors might be needed that ensure a change in logic output because a possible low pressure from box might fail to turn a given button ON. If an parcel sachet/envelope is used instead of the box then long levers or shafts will be need to activate the buttons because we know that sachet/envelope parcels do not have flat sides like boxes.

4. Measuring the Box Weight

The box weight can be measured with pressure sensors. A pressure sensor can be created with a spring and photo sensor/ultrasonic sensor by measuring the length of the spring during spring deformation/compression. After I graduated I would study MEMS (micro-electromechanical systems) pressure sensors that are be used in digital scale devices.

5. Sensor Array Implementation

The photo sensors can be biased with a 1 kohm resistor. A box covering the photo diode array will ensure that no light can enter the photo diode/photo transistor. The laser tag is so powerful and directional that only one photo diode/LDR will sense the ambient light form the laser tag on the other side of the box. Each photo sensor corresponds to 1 cm or 1 inch dimension.

The photo sensor can be biased to provide a logic "1" or logic "0" output when light from the laser tag is generating sensor current. To switch from logic "1" to logic "0" you simply need to swap the position of photo sensor and the biasing resistor.

The 1 kohm resistor biasing can allow input from sensors to be connected directly to logic inputs. If this resistor value needs to be increased because of poor photo sensor sensitivity then op-amp voltage followers, logic buffers or inverters can solve this problem. The use of transistor inverters will require additional resistors and they will be affected by ambient temperatures. However, transistors are still useful if you are implementing this solution with recycled components from old appliances. You can use BJTs or MOSFETS.

6. Analysing Data From Sensor Array

The output from sensors can be connected to decimal to binary converters or to 7-segment/numeric display converters. There are digital ICs or complete circuits that can convert multiple digital channels to numeric display outputs and thus show the number in centimetres/inches. You might also need to use a binary summation IC if you are outputting a number that is greater than 9 cm.

A very bad idea is to use an analogue summation circuit. The main aim of this post office automation system is to provide a precise dimension of the box, measured to a nearest centimetre precision and an analogue circuit cannot do this properly unless you apply op-amp offset trimming and user low tolerance resistors for your op-amp summation circuit. However, similarly to the transistor inverters you can try this idea if you have spare old components. A voltage summer can be implemented not only with an op-amp but with a resistors alone, BJT transistors or BJT transistor current mirrors source. Transistor current mirror sources is not affected by temperature drifts unlike a single transistor summer. You can connect summer output to analogue voltmeter because using a digital voltmeter could be a waste for this circuit because precise output cannot be achieved easily.

Another bad idea is the use of microprocessor or micro-controller. In my personal opinion those device are not needed to this particular circuit. You need to sample data from many channels and all you are doing is converting decimal output to 7-segment display output. Most microprocessors do not have to many digital inputs anyway and are needed for more complicated control applications. However, as I mentioned before, you can try looking for a complete circuit that converts decimal inputs to numeric display outputs on the internet (eBay, Amazon, Aliexpress).

7. Conclusion

I never found out what happened to the group of students that were working on the post office automation project. Even today when I go to the local post office, the staff always measure the box with a measuring tape. However, the scale has been digital for many decades now. I guess most postal/courier companies decided that they do not want to waste extra money if the staff are still required to accept the payment for the parcel.

However, the parcel sorting has been automated by robots for many years now. Similar robotic technology was also used for newspaper printing and distribution in 1990s.