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High-Performance Power ICs and Hall-Effect Sensors
Vehicle Safety Systems 
by Christine Graham, Systems Engineer
Seat Position Sensor
Occupant safety is one of the most critical elements of the automobile design. As a result, safety systems continue to become more sophisticated in order to limit, and ultimately prevent, personal injury in the case of an accident.
Seat position sensing is used in safety systems to determine the position of an occupant in relation to the steering wheel, preventing the air bags from deploying with excessive force.
The most common solution today incorporates two-wire, unipolar, Hall-effect switches to sense discrete seat position zones. The sensor must relay this information in the form of a digital output to the controller unit indicating a particular zone. This information must be correct at start up of the vehicle, so the sensor output must decode without any user action.
The seat track is typically a ferrous metal material capable of interrupting the magnetic field between the Hall-effect sensor and a magnet. The ferrous metal of the seat track passes between the switch and the magnet causing the switch to turn on or off, relaying seat position information to the controller unit. A change in the output state of the sensor indicates to the controller unit that the seat has passed into a particular zone.
There can be any number of zones depending on how many Hall-effect sensors are used, assuming two sensors per seat track, four zones would be possible. The information, provided by the Hall sensor, is processed by the controller to determine the seat position relative to the steering wheel. A seat that is in one of the closer zones to the steering wheel would indicate to the controller unit that a lower force deployment is necessary. Seat positions that are in one of the rear zones, furthest from the steering wheel, require a higher force deployment. The controller unit decodes the output states of the Hall-effect sensors to determine in which zone the seat is positioned. Two sensors will provide a convenient Grey Code output as shown in figure 1 and the table below.
Figure 1. Position sensors relay proper seat location to the controller unit the entire time the vehicle is on. Occupants are unaware of the fact that the vehicle is making life or death decisions automatically with no user interface required.
| Zone | Sensor 2 Output | Sensor 1 Output |
| 1 | 0 | 0 |
| 2 | 0 | 1 |
| 3 | 1 | 1 |
| 4 | 1 | 0 |
The vast selection of Hall-effect sensors allows different solutions for the same application. A higher resolution may be required to determine exactly where the seat is at all times. The highest resolution solution is to use a linear, analog Hall sensor, which produces a voltage output proportional to the strength of the magnetic field. A dual pole magnet in a slide-by configuration with the linear will produce an output ranging from 0 volts to 5 volts with the proper design.
Hall-effect technology is highly reliable and relatively inexpensive. If automatic sensing is required the solution must be dependable.
If higher precision is required, programmable switches and linear devices are available, and can minimize stack up tolerances by allowing end-of-line programming.
Ferrous targets can be detected using a back-biased Hall-effect sensor. These sensors incorporate a Hall IC and magnet in one over molded assembly. Back-biased solutions are offered for switch and linear designs. These assemblies simplify manufacturing and offer an optimized electrical and magnetic design in a single over molded package.
Seat Belt Buckle Sensor
The seat belt buckle, SBB, is another area where Hall-effect technology has been used as a part of the safety system. The two-wire, unipolar switch is again a simple, yet reliable, solution common to many automobiles on the road today. The purpose of the Hall-effect device (HED) is to guarantee proper latching of the buckle whereby ensuring the occupant is properly restrained in the event of an accident or sudden stop.
Similar to the seat position sensing application, seat belt buckle switches operate using a vane interrupt concept. In this case the buckle, made of a ferrous material, is responsible for interrupting the magnetic field between a magnet and the Hall effect device. Typically when the field is interrupted, the device output switches on and when the buckle is removed the device switches off. This information is sent to the controller, which then processes the data in conjunction with data from the seat position sensor and other outputs in order to reliably deploy air bags in the event of an accident.
Application Hurdles
- The SBB sensor has tight spatial constraints making the use of a printed wafer board difficult. Therefore, welding the wires to the leads is the more common approach as part of the packaging process to minimize size. However, welding to the leads takes expertise in the area of welding and is typically contracted out to a welding facility. One of the most common errors seen in welding Hall-effect devices is the amount of heat/power allowed to reach the IC, causing wire bonds to be catastrophically damaged. Another common error seen in new welding processes is insufficient clamping of the leads allowing the leads to twist or pull during the contact with the weld tip. This will also cause catastrophic damage to the wire bonds.
- In addition to the spatial constraints the sensor is subjected to high ESD levels (15kV) due to customer accessible points within the vehicle such as the tongue of the buckle assembly, shunting effects on the magnetic field to the sensor due to the ferrous properties of the buckle assembly, and wide tolerances of the mechanical buckle assembly causing large variations in the magnetic field seen by the Hall sensor. Choosing the right sensor is critical to meeting all the requirements.
Application Solutions
- Transient/ESD protection has been accomplished with the use of a 0.1 µF bypass capacitor welded between sensor supply and sensor ground. In the case of a PCB, an MOV has been used in addition to the bypass capacitor to protect the sensor against harsh EMC/ESD conditions due to the use of a chassis ground. Just a bypass capacitor may be sufficient when the sensor is robust against EMC/ESD.
- A sufficiently large magnet is required to over come the shunting effect caused by the buckle assembly itself. SmCo or Neodymium is common magnet materials used in Seat Belt Buckle applications. They provide large field levels to compensate for the mechanical tolerances and possibly large air gaps > 3mm) seen in SBB applications.
-
Tolerances
of the mechanical assembly can cause a large gauss variation (hundreds
of gauss) in the field level seen by the sensor; so all conditions
must be characterized to ensure the sensor never switches in to the
incorrect state. The conditions that must not cause false switching
of the Hall sensor are as follows.
- Normal buckled position with the tongue in place.
- Normal unbuckled position with the tongue removed.
- Over-travel of the tongue when pushed in and held by a person sitting on it or a child seat resting on the buckle assembly.
- False latch condition when something other than the actual tongue is pushed in, holding the buckle in a falsely latched condition (Popsicle stick, toy, etc).
Suggested Devices
| Allegro Part Number | Temperature Range(s) |
Package Type(s) | Tape & Reel Available |
Comments |
|---|---|---|---|---|
| A1142 | E, L | LH, UA | Yes | Externally connected for 2-wire operation |
| A1143 | E, L | LH, UA | Yes | Externally connected for 2-wire operation |