Machinedesign 1589 Adaptive Control 0 0

Selecting sensors for packaging motion control

Nov. 1, 2000
With computers and controls, device-level networks, and microprocessor-based motion-control devices, equipment designers can make packaging lines more versatile than ever before. All this flexibility though, means data input from sensors must be reliable. Engineers can help ensure the needed reliability with the right sensor

In today’s packaging equipment, the integrity of sensor input data to the control system is important to proper operation. Thus, the choice of sensor is critical.

Packaging equipment is demanding because the product or its packaging is constantly changing. For example in the beverage industry, more new beverages and new types of beverage containers were introduced in the last two years than in the previous five years. Beverage machines must maintain reliability while handling many small, high-speed runs with fast change-overs.

Advantages of photoelectric sensors

Packaging equipment can use many types of sensing technologies in linear and non-linear motion-control applications. Photoelectric sensors, though, are the most popular choice because they offer several advantages:

• Fast data collection — response times are typically less then 1 ms.
• Few limitations on the shape or type of sensed object — can detect objects of all shapes, sizes, and colors, including clear objects such as plastic beverage bottles.
• Repeatable outputs at high speeds for motion events — accurately and repeatedly sensing every object, usually by scanning for the object’s leading or trailing edge.
• Non-contact sensing.

Selecting the best photoelectric sensor

Sensor manufacturers can help determine the best technology for specific applications and will work with engineers to identify solutions for challenging circumstances.

Equipment design, interfacing, and mounting considerations affect the choice of sensor. Before making a phone call, you will need to answer the following equipment design questions, determine your electrical requirements, and choose among various mounting configurations.

Equipment design.

• What is really required of the machine? How frequently will change-overs be needed? What are the versatility requirements for different travel paths as well as for filling, bagging, or labeling functions?
• What will be sensed — whether a part is missing or not, or color? This information determines the sensing tasks required of a sensor.
• What are the characteristics of the target objects? What is the size of the target object? It may be large or small or just a mark on a product.
• How fast will the object move? Sensor response time is the amount of time it takes a sensor to respond to object detection. Today’s electronic sensors should not be a limiting factor in line speed. Other considerations that affect object detection include: the amount of time that the objects will be in the beam path and the maximum number of objects to be packaged or traveling per second.
• How important are quick installation and maintenance?
• How will the target be illuminated and how will it interact with light (reflect, absorb, or scatter the light for instance)?
• Where will the sensor be placed on the equipment? The location of the sensing application determines the sensing zone or distance between the sensor and the object being sensed. In tight sensing zones, diffuse-proximity sensing works well. When the sensing distance is more than 6 in. but less than 30 feet, retrore flective sensing is usually recommended.

Sensor interfacing considerations.

Once engineers determine speed and response time requirements, the next step is to determine the sensor’s control voltage and its electrical compatibility with the interfacing equipment. Sensor manufacturers are responding to the increasing preference for dc supply and high control voltages (to 250 V in some products), and most photoelectric sensors are now available with solid state outputs.

Mounting considerations.

Sensors are available in several mounting styles. The most widely used are panel or frame-mountable sensors with remote optics. These sensors, especially fiber-optic sensors, are often chosen for their small size and ability to fit in tight spaces on machines with limited access. Direct mounting is an option when more space is available.

Sensor performance

The environment, cleaning processes, and power supply voltages can affect sensor performance. One of the most repeated mistakes in specifying sensors is failure to check environmental conditions for sensor compatibility. For example, consider if the sensor will be exposed to extreme temperatures or rapid changes in temperature. Environment incompatibility is one of the leading causes of application failure, often with consequences more costly than the initial sensor investment.

Cleaning processes can affect sensor performance if the sensor is not packaged and sealed to withstand harsh chemicals or high-pressure washdowns, Figure 1. Sensor manufacturers have greatly improved sealing, but not all claims for harsh-duty performance stand up to demanding clean-in-place processes. Make sure the manufacturer performs tests similar to the conditions the product will face and request test results.

Unwanted EMI and RFI energy, from many sources, can pollute power supplies. These conditions can cause false data or false triggering from the photoelectric sensor. Not all manufacturers provide the same degree of electrical immunity. Look for products specifically tested for EMI and RFI immunity.

Adaptive control

Sensor manufacturers continually develop features to improve scanning functions and enhance sensor performance. One of the latest is adaptive control.

Adaptive control removes the necessity to set a sensor to a specific illumination- detection setpoint. Traditional sensors are set to respond to a specific level of returned illumination, and to read this fixed level through a gain circuit. These sensors use excess gain or high gain approaches. However, this method of sensing could miss objects moving along a conveyor. This was especially true with glass or clear-plastic bottles, which alter the returned illumination in too many ways to account for in the sensor.

In adaptive control, the sensor has a microprocessor that constantly takes in data on the returned light, analyzes the signal conditions in a constant mode, and adapts the sensor’s decision point — when it decides it did or did not detect an object — based on the analyzed data. Sensing decisions are made closer to real time, and this feature improves the repeatability and reliability of the sensor signal to the control system.

On high-speed packaging equipment, sensors with this feature provide reliable data that a controller uses to monitor and control the motion of infeed and discharge feed lines. The sensors report on the presence or absence of clear and opaque containers and on their speed and motion. Such information is critical at the discharge feed line, because a false detection can cause machine damage, resulting in down time and costly repairs.

Another application for adaptive photoelectric sensors is automatic bagging. The task of a sensor in this application is to sense the bag’s position, (indicate if it’s skewed, for example), or assist with motion and flow sequencing. Non-adaptive control sensors, particularly opticalbased sensors, may experience scanning problems because bag dust from autobaggers can accumulate on the lenses. Adaptive sensors are becoming a preferred solution in this application because they automatically adjust the decision point, and are unaffected by new bag types and dust built-up.

See Associated Table

Jeff Otterstein is manager of business development, Micro Switch Div., Honeywell Inc., Freeport, Ill.

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