Shielding - Keeping the Sun at Bay: How Barcode Scanners Protect Themselves from the Harsh Outside World |
Subtitle: A Deep Dive into Optical Filters, EMI Shielding, ESD Protection, and Environmental Sealing - with Real-World Examples from Symbol, Zebra, Honeywell, Datalogic, Cognex, and Keyence |
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Opening Summary |
The barcode scanner is a delicate instrument. Its heart is a photodetector that measures currents in the nanoampere range. Its brain is a microcontroller that operates at high frequencies. Both are vulnerable to interference from the outside world - the blinding glare of the sun, the crackle of a nearby motor, the shock of a static discharge, the corrosion of moisture and dust. Shielding is the scanner's armor. It protects the sensitive electronics from these environmental threats, ensuring reliable operation in the harshest conditions. |
This article is dedicated to shielding - the various techniques that engineers use to protect the scanner from the outside world. We will explore the different types of shielding: optical filters that block unwanted light, electromagnetic interference (EMI) shields that block radio frequency noise, electrostatic discharge (ESD) protection that absorbs static shocks, and environmental seals that keep out moisture and dust. We will look at how major companies have implemented shielding in their products. We will examine the optical filters used in Symbol's (now Zebra's) laser scanners. We will explore Honeywell's use of EMI shielding and ESD protection. We will examine Datalogic's ruggedized designs for industrial applications. We will also look at the advanced shielding techniques used in Cognex's and Keyence's industrial scanners. |
By the end of this journey, you will understand that shielding is not an afterthought but an integral part of the scanner's design. It is the armor that allows the scanner to operate reliably in the real world. |
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Full Article |
Section 1: The Threats - What the Scanner Must Withstand |
A barcode scanner must operate in a wide variety of environments. It may be used in a bright, sunny retail store, a dusty warehouse, a humid factory, or a cold outdoor loading dock. It must withstand: |
Ambient Light: Sunlight, fluorescent lights, and incandescent lights can all overwhelm the photodetector. |
Electromagnetic Interference (EMI): Radios, motors, and other electronic devices can radiate noise that interferes with the scanner's circuits. |
Electrostatic Discharge (ESD): A static shock from a user's finger can destroy the sensitive electronics. |
Moisture and Dust: Water and dust can corrode the contacts and short out the circuits. |
Mechanical Shock: Drops and impacts can damage the components. |
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Section 2: Optical Filters - Blocking Unwanted Light |
The first line of defense against ambient light is the optical filter. An optical filter is a device that transmits only certain wavelengths of light and blocks others. In a barcode scanner, the optical filter is typically placed in front of the photodetector. |
The most common type of optical filter is a band-pass filter. A band-pass filter transmits a specific band of wavelengths and blocks all others. For a red laser scanner, the filter is a red-pass filter that transmits red light (around 650 nm) and blocks all other wavelengths. This blocks sunlight and fluorescent light. |
Section 3: Symbol's LS2208 - The Red-Pass Filter |
Symbol's LS2208 uses a red-pass filter that is integrated into the scanner's window. The window is made of a red-tinted polycarbonate that transmits red light and blocks green and blue light. The red-pass filter effectively blocks sunlight, which contains a lot of green and blue light. |
The red-pass filter is a simple, inexpensive solution. It is effective for a red laser scanner. |
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Section 4: Honeywell's 1900 - The IR-Cut Filter |
Honeywell's 1900 imager uses an IR-cut filter. The IR-cut filter blocks infrared light. Infrared light is emitted by sunlight and by incandescent lights. If the infrared light is not blocked, it will create a DC offset on the photodetector. |
The IR-cut filter is a dielectric coating that is deposited on the glass window of the imager. The dielectric coating reflects infrared light. |
Section 5: Datalogic's PowerScan - The Multi-Layer Coating |
Datalogic's PowerScan series uses a multi-layer optical coating on the scanner window. The multi-layer coating provides both IR rejection and visible light transmission. The coating is designed to be durable and scratch-resistant. |
The multi-layer coating is more expensive than a simple dye or coating, but it provides better performance. |
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Section 6: Electromagnetic Interference (EMI) Shielding - Blocking Radio Frequency Noise |
Electromagnetic interference (EMI) is a disturbance that affects an electrical circuit due to electromagnetic radiation emitted from an external source. EMI can be caused by radios, motors, and other electronic devices. The EMI can couple into the scanner's circuits, causing errors. |
The solution to EMI is shielding. A shield is a conductive enclosure that blocks the electromagnetic radiation. The shield is typically made of metal. The shield is connected to the ground plane. |
Section 7: The Shield Can - The Metal Enclosure |
A shield can is a metal enclosure that covers the sensitive circuits. The shield can is typically made of tin-plated steel or copper. The shield can is soldered to the ground plane. |
The shield can prevents EMI from reaching the sensitive circuits. It also prevents the scanner from radiating EMI. |
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Section 8: Honeywell's EMI Shielding - The Shield Can |
Honeywell's 1900 imager uses a shield can over the analogue front-end. The shield can is made of tin-plated steel. The shield can is soldered to the ground plane. |
The shield can effectively blocks EMI from the microcontroller and the communication interface. |
Section 9: Datalogic's EMI Shielding - The Conductive Coating |
Datalogic's PowerScan series uses a conductive coating on the inside of the scanner's housing. The conductive coating is a paint that contains metal particles. The conductive coating provides EMI shielding. |
The conductive coating is less expensive than a shield can, but it is also less effective. |
Section 10: Electrostatic Discharge (ESD) Protection - Absorbing the Shock |
Electrostatic discharge (ESD) is a sudden flow of electricity between two objects. ESD can be caused by a user touching the scanner. The static discharge can destroy the sensitive electronics. |
The solution to ESD is protection. ESD protection components are placed on the input and output pins. The ESD protection components shunt the ESD current to the ground. |
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Section 11: The ESD Protection Diode - The Clamp |
The most common ESD protection component is a clamping diode. The clamping diode is connected between the protected pin and the ground. When an ESD event occurs, the diode conducts, clamping the voltage to a safe level. |
The clamping diode must be placed very close to the protected pin. |
Section 12: Honeywell's ESD Protection - The Network |
Honeywell's 1900 imager uses a network of ESD protection devices. The network includes clamping diodes on the USB and I2C interfaces. The ESD protection devices are placed close to the connectors. |
Section 13: Datalogic's ESD Protection - The Ruggedized Design |
Datalogic's PowerScan series is designed for rugged industrial applications. The PowerScan uses a combination of ESD protection diodes and spark gaps. The spark gaps are small gaps in the PCB that allow the ESD current to arc to the ground. |
The spark gaps are a more robust solution for industrial applications. |
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Section 14: Environmental Sealing - Keeping Out Moisture and Dust |
Moisture and dust are the enemies of electronics. Water can cause corrosion and short circuits. Dust can clog the moving parts and reduce the light transmission. |
The solution to moisture and dust is sealing. The scanner's housing is sealed with gaskets and O-rings. The scanner's window is sealed with adhesive. |
Section 15: The IP Rating - The Standard |
The IP (Ingress Protection) rating is a standard that defines the level of protection against solids and liquids. The IP rating is a two-digit number. The first digit indicates the level of protection against solids (dust). The second digit indicates the level of protection against liquids (water). |
For example, an IP67 rating means that the device is dust-tight (6) and can withstand immersion in water up to 1 meter (7). |
Section 16: Honeywell's Sealing - The IP Rating |
Honeywell's 1900 imager has an IP41 rating. It is protected against solid objects larger than 1 mm (4) and against vertically falling water drops (1). The IP41 rating is adequate for retail and office environments. |
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Section 17: Datalogic's Sealing - The IP67 Rating |
Datalogic's PowerScan series has an IP67 rating. It is dust-tight and can withstand immersion in water up to 1 meter. The IP67 rating makes the PowerScan suitable for industrial environments. |
Section 18: Cognex's Sealing - The IP65 Rating |
Cognex's DataMan series has an IP65 rating. It is dust-tight and protected against water jets. The IP65 rating makes the DataMan suitable for machine vision applications. |
Section 19: Keyence's Sealing - The IP67 Rating |
Keyence's SR-1000 series has an IP67 rating. It is dust-tight and can withstand immersion in water. |
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Section 20: The Optical Filter and the Window |
The optical filter is often integrated into the scanner's window. The window is the transparent cover that protects the photodetector. The window can be made of glass or plastic. The window may have a coating that provides optical filtering. |
Section 21: The Window and the Sealing |
The window must be sealed to prevent moisture and dust from entering the scanner. The window is typically sealed with an adhesive or a gasket. |
Section 22: The Window and the Scratch Resistance |
The window is exposed to the environment. It must be resistant to scratches. The window is typically made of scratch-resistant glass or coated with a hard coating. |
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Section 23: The Window and the Anti-Reflection Coating |
The window can be coated with an anti-reflection coating. The anti-reflection coating reduces the reflection of light at the air-glass interface, improving the light transmission. |
Section 24: The Window and the Moisture Repellent Coating |
The window can be coated with a moisture repellent coating. The moisture repellent coating prevents water droplets from forming on the window. |
Section 25: The Shielding and the PCB |
The shielding is connected to the PCB's ground plane. The shielding must have a low-impedance connection to the ground plane. The connection is typically made by soldering the shield can to the ground plane. |
Section 26: The Shielding and the Housing |
The scanner's housing can also provide shielding. The housing is typically made of a conductive material or coated with a conductive paint. The housing is connected to the ground plane. |
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Section 27: The Shielding and the Cable |
The cable that connects the scanner to the host can also be a source of EMI. The cable should be shielded. The shield of the cable is connected to the scanner's ground. |
Section 28: The Shielding and the Connectors |
The connectors on the scanner should have EMI shielding. The connectors should have a metal shell that is connected to the ground. |
Section 29: The ESD Protection and the Human Body Model |
The ESD protection is designed to protect against the human body model (HBM). The HBM is a model of the static discharge from a human body. The HBM specifies a discharge of up to 8 kilovolts. |
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Section 30: The ESD Protection and the Machine Model |
Some scanners are designed to protect against the machine model (MM). The MM is a model of the static discharge from a machine. The MM specifies a discharge of up to 400 volts. |
Section 31: The ESD Protection and the Charged Device Model |
Some scanners are designed to protect against the charged device model (CDM). The CDM is a model of the static discharge from a charged device. |
Section 32: The Environmental Sealing and the Temperature |
The environmental sealing must be able to withstand the operating temperature range. The gaskets and O-rings must be made of materials that are stable over the temperature range. |
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Section 33: The Environmental Sealing and the Chemical Resistance |
The environmental sealing must be resistant to chemicals. The gaskets and O-rings must be made of materials that are resistant to the chemicals that the scanner may encounter. |
Section 34: The Environmental Sealing and the Abrasion Resistance |
The environmental sealing must be resistant to abrasion. The gaskets and O-rings must be made of materials that are durable. |
Section 35: The Shielding in Texas Instruments' Reference Design |
Texas Instruments' TIDA-00857 reference design uses a shield can on the analogue front-end. The shield can is soldered to the ground plane. The reference design also includes ESD protection on the photodiode input. |
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Section 36: The Shielding - A Summary of Best Practices |
Based on our exploration, let us summarize the best practices for shielding in a barcode scanner: |
1. Use an Optical Filter: Block unwanted light with an optical filter. |
2. Use EMI Shielding: Shield the analogue circuits with a shield can. |
3. Use ESD Protection: Protect the input and output pins with clamping diodes. |
4. Use Environmental Sealing: Seal the housing with gaskets and O-rings. |
5. Consider the Environment: Choose the appropriate IP rating for the application. |
6. Design the Window Carefully: Choose the right window material and coatings. |
7. Test the Shielding: The shielding must be tested for effectiveness. |
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Final Summary |
Shielding is the scanner's armor. It protects the sensitive electronics from the harsh outside world. The scanner must be protected from ambient light, EMI, ESD, moisture, and dust. The shielding includes optical filters, EMI shields, ESD protection, and environmental seals. |
We have seen how major companies have implemented shielding in their products. Symbol's LS2208 uses a simple red-pass filter. Honeywell's 1900 uses an IR-cut filter, an EMI shield can, and ESD protection. Datalogic's PowerScan uses a multi-layer optical coating, conductive coating, and IP67 sealing. Cognex and Keyence also use advanced shielding techniques for their industrial scanners. |
Shielding is not an afterthought. It is an integral part of the scanner's design. By following the best practices, you can create a scanner that operates reliably in the real world. |
cs@easiersoft.com