August 03, 2006
Improve safety and function from auto-seat-heating flexible conductors
Robust wiring and thermal monitoring and control avoid faults such as breaks, burns, and fires. But the design engineering approaches necessary are often counter intuitive.
Automotive electronics wiring has aspects other than running electrical power or control and sensor signals from one location in a car to another. One of these applications is seat heater wiring. Faults in this area not only can result in a cold seat but could encompass burns and fires.
Designing a heating conductor for seat heaters has many challenges, the principal ones being mechanical flexing and temperature control. The two materials most widely used today for the conductors are:
Carbonated fibers
Stranded pure copper wires
The insulation on the wires varies from fluropolymer to enameling or just bare copper. Typically, the conductors are then encapsulated within a fabric/foam substrate creating a shell either side of the conductor, usually secured by adhesives.
Conductor dynamics
An occupant's entry onto and exit from the seat necessitates a sliding action over the side bolsters. This action produces a gradual rubbing and scuffing of the bolster's internal heating conductors, inducing considerable stress forces. Conductor stress is also produced by a person twisting and moving around in the seat.
For metallic conductors, logic may suggest that the greater the cross sectional area of the wire, the greater the tensile strength—resulting in improved flex performance. Nothing could be further from reality!
Tensile strength is important but equally so are the ductile properties of the wire. This can be seen in existing seat heaters that use stranded copper wires. Such heaters are constructed of many narrow gauge wires bunched together. These smaller wires are more ductile than one large wire of the same electrical resistance. Unfortunately, the stranding is longitudinal in direction, resulting in less than optimum flex performance.
A design refinement is to spirally (helically) wrap the conductors around a flexible core. This winding enhances the ductile properties of the conductor producing considerable improvements in its dynamic performance. Spirally wrapped conductors consistently outperform stranded conductors by upward of 10 times.
Thermocable has developed its own flex test (see below). This test's objective is to apply a more robust and severe test in an effort to design spirally wrapped conductors that are fit-for-purpose regarding the practical dynamics being experienced in the field. This test is in addition to International Electrotechnical Commission (IEC) tests.
The image below is an example of a seat heater where the conductor started a fire. The first priority in preventing these seat fires is to improve the simple mechanical ability of the conductor to flex repeatedly without premature breakage. The second priority is accurate temperature monitoring of the conductor.
Overheat protection
Heating conductors within existing seat heaters are controlled by a single thermostat. There is no doubt, that the "area of thermal detection" of a single thermostat is limited, particularly in the case of a localized overheating hot spot occurring at the greatest distance from the location of the thermostat.
For simple overheat protection, a temperature sensing polymer layer may be used which melts at a predetermined temperature—lower than the combustion temperature of the seat cloth. Conventionally a meltdown layer is used to create a dramatic decrease in resistance which will correspondingly draw a large current from the power supply and blow a fuse. However, to achieve the required resistances in a seat heating pad, two helically wound element wires are attached in parallel. In this situation, a fusible layer no longer causes an increase in current because the voltage between the two conductors is the same at each point on each conductor.
For a meltdown layer to be used effectively, the connection between the two conductors should be broken briefly to leave only one leg of the parallel resistor combination attached to the supply. If the current flowing through the combination is monitored during this period the current draw should be much lower than normal. In the event that the fusible layer has melted the current draw will still remain relatively high and therefore indicative of an overheat condition (see below).
Temperature Monitoring
Accurate temperature control can be achieved by the conductor also being a thermistor. Thus temperature sensing can be measured on the conductor itself. The image below shows a helically wound conductor with an NTC (negative temperature coefficient) thermistor layer.
This conductor has two heated circuits separated by a NTC thermistor plastic. The NTC layer has two functions:
Overall temperature monitoring
Hot spot detection
An important consideration in the design of feedback sensor conductors is the long term stability of the thermistor polymer. It is essential that the control module's electronics receive repeatedly consistent signals from the conductor within a narrow band of tolerance. This consistency is achieved by controlled fabrication processing techniques of the thermistor polymer.
NTC and PTC (positive temperature coefficient) technology allow the controller to determine localized areas of over-heating and average element temperature, respectively. Both technologies rely on a change in temperature creating a change in resistance which is both repeatable and predictable. At low frequencies, the NTC separation layer can be modeled as many resistors in parallel, as shown below.
The value of each resistor varies with temperature as a logarithmic function. Hence, if two conductors are separated by an NTC material the resistance between the two conductors can be calculated using the equation:
where a and b are constants and depend upon the volume of NTC material, surface area contact between the conductors, NTC material and unit length, and T is the temperature of the NTC material. The overall NTC resistance is the reciprocal of the sum of all unit lengths.
When resistors are connected parallel, if a resistor has a much lower resistance than other resistors the overall resistance can be approximated to the lowest resistance.
It can be seen, therefore, that if a portion of NTC material significantly increases in temperature the overall resistance between the two conductors is approximately equal to the resistance of the hottest part of the NTC material. For short lengths of NTC material (i.e. less than 20m) an increase of 30C in a small amount of element wire (say, less than 0.5m) has a pronounced effect on the overall resistance of the NTC material. Therefore, by monitoring the resistance of NTC separation layer the controller may determine a localized area of over-heating is arising if a dramatic decrease in resistance is seen.
Measuring the NTC resistance
Several methods of measuring the NTC resistance are possible and have different benefits and considerations which need to be taken into account when designing such circuits. Primarily, it should be noted that inherently the resistance of NTC material is very high at low temperatures, in the order of tens of Mohms. Accordingly to Ohm's law at low voltages (such as 12V for automotive applications) this equates to very low current through the separation layer. For example, if the resistance of the NTC separation layer was say 30 Mohms at 20C, at 12V the current through the layer is 0.4 microA. One method of measuring the NTC resistance may be achieved using a differential amplifier and a small value resistor in series with the NTC layer (see below).
View a full-size image
A further method for measuring the resistance of the NTC layer is to create an RC filter where a fixed capacitor is placed in series with the NTC resistance. As the resistance of the NTC layer changes the transfer function of the filter will change accordingly.
View a full-size image
Therefore for an AC signal of the appropriate frequency (i.e. close to the cut-off frequency of the filter when the NTC material is at 20C), as the temperature of the NTC material increases, the cut-off frequency will either become lower or higher depending upon the configuration of the filter. The AC signal will also be subject to a phase shift, again dependant on temperature. This introduces two possible ways of measuring the NTC resistance.
Measuring the PTC resistance
PTC technology uses the known temperature coefficient of resistance of specific metals to allow the average element temperature to be calculated. As the temperature of such a metal increases the resistance increases also in a linear fashion. Conversely to an NTC separation layer, a PTC wire may be modeled as many resistors in series. This has the effect that any localized area of over-heating will have a negligible impact on the overall measured PTC resistance.
The simplest method of measuring the PTC resistance is to apply a constant current through PTC wire. As the temperature of the wire increases and hence the resistance increases, the voltage across the PTC wire will decreases according to Ohm's law. The voltage across the PTC wire may be monitored by an analogue-to-digital converter on a micro-processor which can then calculate the element temperature.
Economics
The total technology of thermistor conductor and control module described above is economically competitive against existing seat heating technology while providing robust and safe use.
Mike Daniels is development director and Tom Robst is an electronic design engineer at Thermocable UK. They can be reached at: MikeDaniels@Thermocable.com and tomrobst@thermocable.com.
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