1. 🎯 What is an Encoder?
Simply put, an encoder is a sensor that converts mechanical motion (rotary or linear) into an electrical signal.
Its primary function is to provide feedback to a control system (like a PLC or servo drive). It acts as the “eyes” of the machine, telling the “brain” (the PLC) exactly what is happening.
It can measure:
- Position: Where is the moving part right now?
- Speed: How fast is it rotating?
- Direction: Is it turning clockwise or counter-clockwise?
- Distance: How far has it moved?
2. ⚙️ How Does it Work? (The Optical Principle)
The most common type of encoder (optical) works on this principle:
- Light Source (LED): An LED emits a beam of light on one side.
- Code Disk: In the middle, a very thin glass or metal disk is attached to the encoder’s shaft. This disk is printed with precise, fine lines (transparent and opaque areas).
- Photodetector (Sensor): On the other side, a light sensor is positioned to receive the light.
As the encoder rotates:
The disk spins with it. When a transparent area passes, the light goes through to the sensor, which generates an (ON) signal. When an opaque area passes, the light is blocked, generating an (OFF) signal.
This ON/OFF process creates a series of electrical pulses. The control system (PLC or drive) then counts these pulses to precisely determine the amount of movement and its speed.
3. ⚖️ The Most Important Part: Types of Encoders
This is the key distinction. Encoders are divided into two main types based on how they send their signal:
A) The Incremental Encoder
This is the most common type, used for measuring speed or relative motion.
- How it works:
- It outputs a continuous stream of pulses as long as it is rotating. It doesn’t know its specific position, but it knows that it “has moved” and by how much since it was turned on.
- Channels:
- Channel A & Channel B: It always has two pulse channels (A and B). They are “out of phase,” meaning they don’t pulse at the same time. This phase shift is what allows the PLC to determine the direction of rotation (clockwise or counter-clockwise).
- Channel Z (or 0): Most models also have a third channel (Z-Phase or Index). This channel gives only one pulse per full 360° revolution. It is used as a reference point or for “homing” (zeroing) the counter.
- The Critical Disadvantage:
- It loses its position when power is lost. When the machine is restarted, the encoder has no memory of where it was (its count starts from zero). It must perform a “homing” sequence (e.g., return to a home-position sensor) to re-learn its position.
B) The Absolute Encoder
This is the smarter, more advanced type, used for knowing the exact position.
- How it works:
- Its internal disk does not have repeating lines. Instead, it has a unique code (like a “Binary” or “Gray Code”) for every single position on the disk.
- Instead of outputting “pulses,” it outputs a unique “digital word” that represents the current, exact position (e.g., “I am currently at angle 214.5 degrees”).
- The Critical Advantage:
- It knows its exact position even after a power failure. As soon as the machine is powered on, the encoder reads its current position and sends the absolute value. It never needs a homing sequence.
- Its Types:
- Single-Turn: Knows its precise position within one 360° revolution. If it completes a turn, the count starts over.
- Multi-Turn: The most advanced. It uses internal gears. It knows its precise position within the revolution AND it also counts the number of full revolutions it has turned (e.g., “I am at angle 214.5 degrees, on revolution #52”).
4. 📋 Technical Specifications (When Buying or Selling)
When you describe or look for an encoder, these are the specs you need:
1. Resolution
- For Incremental Encoders: Measured in PPR (Pulses Per Revolution). (e.g., 1024, 2048, 10000 PPR). The higher the number, the higher the precision.
- For Absolute Encoders: Measured in Bits. (e.g., 12-bit, 13-bit).
- 12-bit = $2^{12}$ = 4096 steps per revolution.
- 13-bit = $2^{13}$ = 8192 steps per revolution.
2. Output Type
This defines the electrical signal and how to wire it:
- For Incremental Encoders:
- HTL (Push-Pull): The most common. Runs on 10-30 VDC and is directly compatible with standard 24VDC PLC inputs.
- TTL (Line Driver / RS-422): A 5VDC only output. Used for very high speeds, long-distance signal transmission, and high noise immunity.
- $1 V_{PP}$ (Sin/Cos): An analog (sine wave) output. Not used with PLCs. It is used exclusively for high-precision Servo Drives.
- For Absolute Encoders:
- SSI (Serial Synchronous Interface): The most common digital standard. It sends the “data word” serially (using Clock/Data signals) over just a few wires.
- Fieldbus (PROFINET, EtherNet/IP, CANopen): Smart encoders that connect directly to an industrial network, just like a PLC.
3. Mechanical Connection
This defines how the encoder is physically mounted:
- Solid Shaft:
- The encoder has a shaft protruding from its body (e.g., 6mm or 10mm diameter).
- It is connected to the motor’s shaft using a Coupling.
- Hollow Shaft:
- The encoder has a hole (bore) through its center.
- It is mounted directly onto the motor’s shaft (slips over the shaft), which saves space and eliminates the need for a coupling.
- Blind Hollow: The bore is “blind” (closed on one side).
- Through Hollow: The bore is “through” (open on both sides).
5. 🏭 Where are Encoders Used? (Examples)
Encoders are used in any application that requires precise motion:
- Servo Motors: An encoder is an essential part of all servo systems to provide feedback to the drive.
- CNC Machines & Routers: To determine the exact position of the cutting tool.
- Industrial Robots: To know the precise angle of every joint in the robot’s arm.
- Conveyors: To measure the speed of the belt or measure the length of products moving on it.
- Elevators: To determine the exact position of the elevator car at each floor.
- Cutting Machinery: To ensure products (paper, plastic, etc.) are cut to the exact same length every time.
No comment