>>6 minute read
Have you ever had a problem with your computer or phone and had to call support? Next thing you know, they’re naming off components and programs you thought only existed in Back to the Future II.
Fortunately, we’re here for you to help remedy that situation. Not only will knowing the parts of scale help with understanding really how one works, but it can also give you an idea of what to expect in terms of cost of parts and labor services.
Think of it like your car; because you most likely have a general understanding of how a car operates and the parts and mechanisms associated with it, you can infer different causes to your issue. For instance, your car is consistently dead when you turn it on every morning. Well, there are a couple things that can be going on.
Now you have three ideas in your head in terms of cost: Free (no service), less than $100 (+ optional service), and $500-$1,000 (+ service) (unfortunately, I’ve experienced all of these).
Scales work the same way. Ideally, you can figure out roughly what is wrong with one if you know how it operates, but ultimately it should be serviced by a certified technician. But this can’t happen if the terms are a stranger to you. So let’s begin!
We’ll start with the most basic part of the scale, the base and what makes it up, and work our way towards the terminal.
Load Cell – This is a typically metal block (steel or aluminum), rectangular in shape. Load cells come in all shapes and sizes including S-shaped and cylindrical. This is where the load/weight rests. This block combined with strain gauges, is what measures the actual force of the weight being applied. It is the heart of a scale.
Strain Gauge – The strain gauge is a wire connection on either side of the load cell that measures the bending or shearing of the cell when weight is applied. This fixture almost resembles a sticker on the walls of the cell. When the cell bends, one side of the steel block expands while the underside contracts, causing a varying readout in millivolts. These millivolt signals are what is communicated to the terminal to give a translation of weight.
Weigh Module – A weigh module refers to all the necessary components used to make up the load cell, its fittings, the strain gauges, and any other necessary requirements to provide a reading of weight. So think of it like this, if you can put a weight on a load cell, and it was connected to a terminal, would it read anything as it is supposed to? If so, that is a weigh module.
Spider/Weighbridge – A spider is only applicable to smaller bench scales with single load cell applications. The spider is a design that distributes the weight that is applied to the platter, or deck, onto the load cell evenly. The idea is that even if you have a weight in the corner, because of the distributive design, the weight will remain the same as if it were placed in the center. On truck scales, this is simply called the weighbridge. This would be the components that sat directly on top of the load cells, the I-beams.
Platter/Deck – We’re moving slowly up. The platter, or deck, is the pan that sits on top of the weigh modules and weighbridge/spider. Without this, you would not have a full or level surface to set a weight on. These are designed to be smooth and level with minimal, yet necessary, movement. This is going to be the concrete or steel deck on a vehicle scale.
Platform/Base – The platform or base is the entire ‘scale’ portion of a weighing system. This is what the average person would simply refer to as a ‘scale’. Without the terminal.
Junction Box (J-Box) – The junction box is only applicable in scales that contain more than one load cell or weigh module. It generally sits inside of the scale and faces closest to the terminal. The duty of the j-box is to sum all voltage readings from each load cell and send the data to the terminal to be read. Since j-boxes cannot be hermetically sealed, they are the most prone piece of equipment to moisture damage.
Board – The electrical component within the j-box that contains the software and algorithm to sum the counts from the load cells and send the data to the terminal.
Potentiometers (Pots) – Pots are circular adjustment bolts on the board of a junction box. They are designed to be used to calibrate the analog reading a terminal receives from the j-box. Turning these increases or decreases the resistance received by specific load cells or entire sections of a scale.
Homerun Cable – This is the cable that runs from the junction box to the terminal. It is typically more rugged than a load cell cable within the scale due to the journey it has to travel. Homerun cables can be as short as 1 foot or as long as hundreds of feet.
Terminal/Indicator/Instrument – The terminal is much more than simply a display for a weight. This is the brain of the scale system. This is the primary piece of equipment that reads the weight of the force applied to the base. The primary functions of the terminal is to send a voltage through the scale (excitation voltage), based on the readings it takes from the signal inputs and outputs of the Wheatstone bridge built using the strain gauges, to determine the force. And to display the weight on the screen.
Housing – The Housing is the casing around the board, connections, and screen. These are typically plastic, aluminum, or stainless steel depending on the environment.
Board – Guess what. These have a board too. And are much more complicated (see junction box section).
Power Supply – What gives power to the terminal is the power supply. This could be an internal or external battery. Additionally it can be connected to the wall or PLC as a panel mount.
EtherNet/IP – Ethernet TCP/IP – Modbus – All examples of a terminal’s connectivity based on the needs of what the scale needs to do. Does it need to send data to a database? Control a valve in a PLC? Many manufacturers and businesses use different forms of communication within their own facility. Most indicators nowadays have network connectivity and advance controlling functions that integrate into PLCs and can often outright replace the need for PLCs. METTLER TOLEDO offers the most advanced solutions with these such as the IND780 terminal.
RS232/RS482/RS485 Serial Communication – A serial, binary connection from one device to another. Typically used from the indicator to a printer or a remote display. These tripped me up for the longest time. All of these look very similar and they all do the exact same thing. The only difference is how they are wired. Think of them like spark plugs. One kind will work for one car, but not for another, however, they do the same things. If you’re replacing a peripheral, it is important to know what connection it receives so it can connect to your indicator correctly.
Peripheral – A device that connects to the instrument that is not necessary to the sole application of weighing. As mentioned before this could be a printer or a remote display or even a scanner.
Remote Display (Display) (Remote) – All these terms refer to a device that only displays a weight. The data is sent to this device from the terminal. These are common on truck scales so drivers can see how much they are weighing while on the deck. Display often gets confused with the indicator/terminal/instrument.
There you have it! Just about everything you need to know about how your scale operates and then some. Knowing these terms can definitely help with keeping a clear expectation of how things work and what to expect if one of these things isn’t working. It will also make talking about the issues with a service technician much easier and smoother when both parties are using the same terminology to help each other identify a cause to the issue.