7 Important Considerations When Purchasing Custom Sensors for Your Next Appliance Design
A variety of factors need to be weighed in the purchasing decision.
For many purchasing professionals, sensors are just another type of component. Among these buyers, the choice of vendors is often based heavily on price and delivery. For the more discriminating buyer, however, a variety of other factors need to be weighed in the purchasing decision, many of which don’t necessarily appear on the invoice.
Here are some questions that purchasing managers need to ask themselves and the engineers who specify components before placing an order.
Are the vendor and manufacturer the same organization?
Not surprisingly, many organizations that sell a specific type of sensor don’t actually manufacture the discrete component itself. A manufacturer will always have more control over the quality of the sensor sold than a buyer/reseller simply because the manufacturer has the ability to investigate the root cause of a quality problem. A buyer/reseller can only report a problem, not fix it. The manufacturer also has greater control than a buyer/reseller in terms of increasing production capacity to meet increased demand for a specific sensor.
Does the vendor offer a broad range of sensors that address the majority of your sensing requirements, both now and in the future?
Some manufacturers specialize in only a single type of sensor (Hall effect, magnetic, reed switches/sensors, temperature, etc.). However, sensor manufacturers that produce multiple sensor types are far more likely to have a broader range of up-to-date sensor R&D capabilities and the design personnel necessary to develop new or custom products.
What other kinds of components does a specific sensor supplier offer?
It’s important to look beyond the specific sensor you are purchasing today to the rest of the supplier’s component portfolio. For example, the appliances your company is manufacturing probably also require other electronic components like TVS diodes and arrays, GDTs, thyristors, varistors, fuses, switches, semiconductors, etc. Choosing a vendor capable of supplying a significant percentage of your company’s purchased parts requirements would allow paring down the number of vendors you work with, which is always advantageous. Similarly, suppliers that can offer a broad portfolio are also likely to be able to offer you higher volume purchasing discounts than a vendor that only supplies a handful of parts.
Figure 2. Littelfuse custom sensor designers employ 3D CAD design tools to create a new design or modify existing standard product packages, such as adding connectors or changing wire size or length, as well as offering special reed sensitivities.
In addition, a vendor with extensive portfolio range and a leading position in diverse markets gives customers an opportunity to offer new technologies, products and solutions to problems. The knowledge and experience the vendor develops gets passed along to its customers so they can tackle complicated new projects.
Does the sensor supplier offer applications assistance to the engineers who specify sensors at your company?
Expert applications assistance provided by a sensor supplier can help your engineers make better specifying decisions faster and determine the optimal sensor for the job the first time. When engineers encounter a problem with a new sensor design, vendor applications personnel can help engineers determine if they are installing/using the sensor properly or if the part is simply unsuited for the application.
Does the supplier offer custom engineering and development capabilities to its customers?
If your company is a leader in innovative design, it might not be possible to find a standard sensor that suits a new application’s requirements precisely. However, manufacturers with in-house customization capabilities can often modify one of their standard products relatively quickly and cost-effectively, such as by adding connectors or changing the wire gauge or length, offering various levels of sensitivity, custom lead forming and bending or modifications to bare reed switches.
Figure 3. Magnetic sensor developers use magnetic sensor simulation tools to gain a thorough understanding of a sensor’s magnetic operating environment and project sensor performance under the full range of operating conditions. Simulation work like this makes it possible to consider design, manufacturing, performance, and environmental parameters and tolerances during the initial phases of a design. Using magnetic simulation often allows a variety of magnet and sensor configurations to be studied and analyzed in a much shorter time period than trying to physically assemble and test sensors.
If a minor modification isn’t sufficient and the job would require a more significant design development project, look for sensor manufacturers with engineering capabilities like these:
- 3D CAD mechanical design services
- Electronic circuitry design
- Magnetic simulation support analyzing feasibility of the design options
- 3D mapping of magnetic actuation of the sensor
- Rapid prototyping and quick-turn concept parts including 3D printed parts
- Reliability/validation testing options
Once engineering is complete, producing these custom designs requires a broad range of manufacturing capabilities, including plastic assembly (injection molding, ultrasonic welding, heat staking), encapsulation (over-molding, potting), wire hardness and termination, assembly, tape & reel packaging, circuit board assembly and testing, and final testing.
Often, if the manufacturer can count on producing the customized sensor in sufficient quantities over an extended period, these types of design assistance and modifications can be factored into the product pricing.
Figure 4. Magnetic sensor developers also use multiphysics simulation packages to predict the mechanical deflection of reed blades from the magnetic field source, such as coils or permanent magnets. This allows for quick design of reed sensors for various customer projects with minimum material cost.
Can the sensor supplier guarantee sufficient capacity to handle the largest orders reliably?
Unlike sensor manufacturers, who have both the resources and the motivation to invest in capacity expansion to meet demand and keep delivery lead times reasonable, buyer/resellers are at the mercy of their own suppliers. That can make a critical difference when demand for a specific sensor surges quickly.
Figure 5. Littelfuse manufactured more than 300 million reed switches in this facility in 2017 and is expanding its capacity to produce more than 400 million annually.
For magnetic sensors, does the sensor supplier also provide the magnetic actuator?
It is always recommended that the sensor supplier review the magnetic actuator that the customer selects. Together, the sensor and actuator form a “magnetic circuit” that must work together within the application. Don’t attempt to buy a sensor from one supplier and an actuator from another one, without at least reviewing your choice of actuator with the sensor supplier.
Is the sensor manufacturer involved in industrial standards development efforts?
Unlike buyer/resellers, sensor manufacturers often encourage their personnel to contribute to the development of new industry standards for safety and performance through organizations like UL and ATEX. Not only does this help suppliers stay on the leading edge of new technical developments, but they can help direct the next generation of sensor designs.
Pricing and delivery will always be important factors in purchasing decisions, but a more comprehensive perspective on supplier selection can often save more money and reduce buying headaches in the longer term.
Making Sense of Sensors
A Hall effect device is a semiconductor-based integrated circuit with Hall plates that respond to magnetic fields. Additional circuitry is required for power supply and signal conditioning, temperature compensation and EMC/ESD protection. Hall effect devices provide digital or analog output signals that are used for proximity and continuous rotary or linear positioning. Unlike a reed switch, a Hall effect device contains active circuitry, so it draws a small amount of current at all times.
A TMR (Tunnel Magnetoresistance) sensor is a semiconductor- based integrated circuit somewhat similar to a Hall effect device but with a quantum mechanical effect. The advantage of a TMR sensor is that it can be much more sensitive than a Hall effect device, so a smaller magnet can be used. A TMR sensor will operate at a lower voltage than a Hall effect device and requires less current, making the TMR sensor ideally suited to DC battery applications. TMR sensors are also available with omnipolar, unipolar, or bipolar magnetic field response and have options for three different sensing axes. Custom sensor designs based on TMR technology are available.
Reed switches have two ferromagnetic blades (reeds) hermetically sealed within a tubular glass envelope. The contacts on each reed blade have a thin layer of precious metal deposited on them. The glass envelope is filled with nitrogen gas to eliminate oxygen and prevent contact oxidation. Reed switches can be activated by either a permanent magnet or an electromagnet. The reed switch and magnetic field combination is referred to as the “magnetic circuit.” The relative stiffness of the reed blades and the small gap and overlap between the two contacts controls the switch’s sensitivity, that is, the amount of magnetic field required to actuate the contact into an open or closed mode. Unlike an integrated circuit, a reed switch requires no power to operate, which makes it a very popular choice for battery-powered applications.
Although a reed switch can easily be mounted on circuit boards, for mechanical system applications like a door security sensor, greater protection is essential. Reed sensors are reed switches packaged inside a housing to simplify mounting/connection and enhance environmental protection. Reed sensors offer protection from mechanical stress by shielding the reed switch’s glass envelope.
A reed relay combines a reed switch with a copper coil. Like other relays, this provides galvanic isolation between the coil input and the controlled contact(s). However, because of the reed switch’s small size and magnetic efficiency, the power required to drive the coil is lower than for most other relay types. Other advantages include high insulation resistance, low contact resistance and long contact life. Reed relays are used in many applications, including automotive, test equipment, security, medical and process control equipment.
Thermistors are thermally sensitive resistors that exhibit a large, predictable and precise change in electrical resistance when subjected to a corresponding change in body temperature. Negative temperature coefficient (NTC) thermistors exhibit a decrease in electrical resistance when subjected to an increase in body temperature and positive temperature coefficient (PTC) thermistors exhibit an increase in electrical resistance when subjected to an increase in body temperature. Their highly predictable characteristics and excellent long-term stability make thermistors ideal for temperature measurement and control applications.
Platinum Resistance Temperature Detectors (Pt-RTDs) are temperature sensors with a positive, predictable and nearly linear change in resistance when subjected to a corresponding change in their body temperature. Platinum RTDs will maintain a significant and uniform rate of range of resistance vs. temperature over an extended operating temperature range, which is ideal for digital measurement and control applications.