All in the Ingredients

The working environment of industrial and utility vehicles can be hard on the sensors inside. These vehicles can experience extreme environmental conditions, including temperatures from –40 to 150 C, shock and vibration to 50gs, exposure to particulate matter and liquid spills.

In order to offer an acceptable level of field reliability, the sensors used within such vehicles to measure speed, direction and angular displacement in the rotating parts must meet two main criteria: accurate sensing and robust packaging that can stand up to physical degradation.

Cherry Electrical Products, Pleasant Prairie, WI, an ISO 9001-certified manufacturer of proprietary and custom electrical switches, sensors and keyboards, has combined the strengths of several proven technologies: magnetic- and inductive-based sensing and polyamide thermoplastic packaging to encase the sensors, using low pressure injection molding (LPIM) to form the parts. This molding process delivers measurable improvements in manufacturing efficiency, quality and reduced costs.

Hall effect sensors

Hall effect sensing provides a means of sensing a measurement in a non-contact manner. When packaged appropriately via LPIM, this technology is excellent for rugged environment applications.

Cherry GS and SD sensors contain integrated circuit Hall effect sensors biased by an internal magnetic field. As a ferrous gear-toothed wheel spins, the Hall effect sensor detects a changing magnetic field when each gear crest transitions to a root. A voltage pulse output is fed into an ECU, which provides a digital reading of speed, speed and direction, or angular position at speeds to 100,000 Hz. The sensors can be used in a wide range of applications in engines and powertrain components.

Cherry's AN angular displacement sensor, on the other hand, uses a programmable Hall IC. Magnets or a flux concentrator are swept in a circular manner past the IC so that magnetic field strength changes from a positive north field to a negative south field. A linear voltage response is obtained from the magnetic-magnitude change. The angular movement sensor is available in either a digital or analog output format.

Thermoplastic polyamide hot melt

Henkel KGaA of Dusseldorf, Germany, developed the idea of supplying customers with its own proprietary formulations of a polyamide resin that would protect electrical components for the typical stresses seen in harsh applications. Henkel introduced the Macromelt OM trade name for the polyamide formulations.

The polyamide thermoplastic material used to fabricate Cherry's family of Hall effect sensors for harsh environments is a version of nylon, developed specifically for insert-molding. However, the two plastics have very marked differences. The polyamide is produced in a somewhat similar way to nylon but its stoichiometric balance is not 1:1. The imbalance leads to a lower viscosity, lower molecular weight and a much more flexible plastic than nylon. Nylon is essentially a harder plastic, used wherever its good abrasion resistance and self-lubricating features are useful, e.g. in gears, bearings and door latches. The lower viscosity and molecular weight in Macromelt OM allows it to be injection molded at a lower pressure than nylon.

The low viscosity of the Macromelt OM resin is perhaps the principal factor in its success in the low pressure injection molding process. The material will flow around the tightest dimensions and smallest spaces without need for the high injection pressure necessary when using traditional resins. This low pressure, typically 50 psi to 150 psi, means the electronic components being encased do not experience the mechanical stresses of flow seen during conventional molding.

A major benefit of Henkel's polyamide material is its exceptional TCE-matching (thermal coefficient of expansion) ability throughout the temperature ranges expected for industrial vehicles. From -40 C to 150 C, Macromelt OM will allow expansion and contraction of the electrical components with no deleterious effect.