What is a system? Boundary and environment
A system is a set of parts joined together to do one job. A bicycle, a phone, a water pump and a metro line are all systems.
The system boundary is an imaginary line around the parts we study. What is inside is the system. What is outside is the environment. Things that cross the line are inputs (they enter) and outputs (they leave).
- Product systems: things we use (a fan, a car).
- Process systems: steps that make something (a bottling plant).
- Service systems: help given to people (a bus network).
Draw the boundary first. If you draw it in a different place, you study a different system.
From need to requirement
Good engineering starts with a need: who is the user, and what do they want? A use case is one short story of the user using the product ("A child walks up to the door").
A need is vague. A requirement is clear and measurable: "The door opens in less than 2 seconds." Functional analysis then lists the main function (open the door) and the constraints (safe, cheap, low power).
Engineers draw the system as boxes and arrows. In a block diagram (SysML is a standard way to draw it) each box is a part and each arrow shows what passes between parts.
Flows of matter, energy and information
Three things move through a product:
- Matter (water, air, material), measured for example in kg/s.
- Energy (electric, heat, motion), measured in watts (W) when it is a flow.
- Information (signals, data), measured for example in bits per second.
A stock is an amount stored (a full battery, a tank). A flow is an amount moving per second. The battery is the stock, the current it gives is the flow.
The energy chain has four jobs: supply the energy, convert it (battery to motor), transmit it (gears, belt) and act (the door moves). The information chain has three: acquire (sensor), process (controller), communicate (send a command or show a result).
Every step wastes some energy, usually as heat. Efficiency = useful power ÷ input power. A Sankey diagram draws each flow as a band: the wider the band, the bigger the flow, and the loss shows as a band leaving the side. If the parts are in a chain, multiply their efficiencies.
The V-cycle: design, build, verify, validate
The V-cycle is a picture of a project. The left arm goes down: needs, then the whole design, then the design of each part. The bottom is building the parts. The right arm goes up: test each part, put (integrate) them together, test the whole product, and finally try it with the user.
- Verification: "Did we build it right?" Compare measured results with the requirements (expected vs measured).
- Validation: "Did we build the right thing?" Check it really solves the user's need.
- Qualification: the formal approval that the product may be sold.
A product can pass verification and still fail validation: it opens in 1.6 seconds, as asked, but is so fast it scares small children. Then the requirement itself was wrong.
To plan behaviour, engineers use states (door closed, opening, open, closing) and the sequence of events that moves the system from one state to the next. A computer simulation lets them test a design before building it.
Try it
Pick a product at home: a table fan. Draw its boundary. List one input and one output. Write the energy chain (socket, motor, blades, moving air) and the information chain (switch or remote, speed knob). Then write one measurable requirement, such as "air speed of at least 2 m/s at 1 m".
Key formulas and definitions
- Efficiency η = useful output ÷ input × 100%
- Lost power = input power − useful power
- Energy E = power P × time t (J = W × s; 1 kWh = 1000 W × 1 h)
- Chain of parts: η total = η1 × η2 × η3
- Information rate = items per second × bits per item
Worked examples
1. A motor takes 200 W of electric power and gives 150 W of motion. Find the efficiency and the power lost.
η = 150 ÷ 200 × 100 = 75%. Lost power = 200 − 150 = 50 W (mostly heat).
2. A drive has a battery (90% efficient), a motor (80%) and a gear box (95%). What is the efficiency of the whole chain?
η total = 0.90 × 0.80 × 0.95 = 0.684, so 68.4%. Each part loses some, so the whole chain is lower than any single part.
3. A fan uses 50 W for 8 hours. How much energy is that in kWh?
E = 50 W × 8 h = 400 Wh = 0.4 kWh.
4. A door sensor sends 10 readings every second. Each reading has 16 bits. What is the information flow?
Flow = 10 × 16 = 160 bits per second.
5. The requirement says "opens in less than 2 s". The test gives 1.6 s. Children say it is too fast and scary. Which check passed and which failed?
Verification passed: 1.6 s is less than 2 s. Validation failed: it does not give a safe, comfortable result for the real user. The requirement must change (for example, 2 s to 3 s).
6. A lamp takes 10 J every second and gives 2 J of light. Find its efficiency and say where the other energy goes.
η = 2 ÷ 10 × 100 = 20%. The other 8 J every second is lost as heat. This is why the bulb feels warm.
Common mistakes
- Drawing no boundary. Without a boundary you cannot tell what is inside the system and what is the environment.
- Confusing verification and validation. Verify = matches the requirements. Validate = satisfies the real user.
- Writing a requirement that cannot be measured ("fast", "strong"). Give a number and a unit: "opens in under 2 s".
- Adding efficiencies in a chain. You multiply them: 0.9 × 0.8 = 0.72, not 1.7.