1. Introduction
Rapid developments in the fields of control engineering have brought the widely use of electronically controlled systems in all industries today. This has created a large demand for sensors with perfect performances, low cost and capabilities of working under hostile environment such as -40℃ to 150℃. This paper is concerned with quality criterias of conductive plastic potentiometers for being used as sensors for angular and linear motion. Such potentiometers primarily comprise the following components：
a. The resistance element (support material + a resistance track of conductive plastic) .The resistance track is the most important element of the position sensor.It has a conductive plastic paste laid on the substrate and can run smoothly with almost no abrasion. Minor uses Trimming track technology for trimming the resistance track to the required linearity. (Fig1)
Fig1
b. A wiper (precious metal alloy). Wiper is the critical part of the transducer. It is supported by precious metals and of multi-fingers structure to provide long serving life (>100million cycles), low torque and high speed operation (10m/s) c. A driving shaft or actuating rod .
d. Bearings (ball bearing or plain bearing) .
e. Housing. It is normally made of aluminium with surface anodized. It has a strong structure and outstanding corrosion resistance.
2. Travel
L1 indicates the defined electrical travel. L2 indicates the continuous travel which also includes non-linear connection fields. L3 indicates the total electrical contact travel of the potentiometer. L4 indicates the mechanical travel. There are other parts which can not be brought into the whole travel. （Fig2）
Fig2
3. Independent linearity
If the wiper moves in a fixed direction, the relationship between the output voltage and the mechanically input value will be that as shown in Fig. 3. The maximum deviation of the potentiometer curve from an ideal straight line is referred to as independent linearity error. The error ±f is indicated as a deviation in percentage terms of the output voltage from the theoretical in relation to the input voltage. Typical values for independent linearity nowadays are between 0.2 % and 0.02 %. There is a test result of position sensor product by Minor as shown in fig.4
Fig3
Fig4
Testing results of the independent linearity of a potentiometer
Testing conditions：temperature 26℃，humidity 52%
Length of resistance track：150mm
Resistance: 5184Ω
Testing result：linearity 0.058%
4. Absolute linearity
Unlike independent linearity ，the reference slope of absolute linearity is fully defined so that there is no need for subsequent system trimming and the value of it becomes more and more essential. The definition of an index point establishes a relationship between the mechanical input value (travel or angle) and the output voltage. Because this index point is set according to work conditions, absolute linearity is more suitable for industrial process control. It is frequently necessary for the tolerance fields to be stepped Fig5 shows a practical example.
Fig5
5. Contact resistance
Contact resistance is the resistance between the wiper terminal and the wiper's immediate point of contact on the potentiometer's resistive track. As will be subsequently explained, this contact resistance affects all the important quality features of a potentiometer. Contact or transition resistance can be broken down into three components:
The first component, describes the integral voltage drop between the current-carrying track and the contact surface. This component is largely dependent on technology factors and amounts to several hundred Ohm.
The second component, the external component, is far more difficult to master than the first. This external transition resistance has much in common with the contact resistances occurring in switches and plug- and socket-connectors. It is caused by the transition between the wiper and the potentiometer track not being ideal from an electrical viewpoint. Metal oxides, chlorides and sulphides, mixed with various organic substances, can result in the formation of thin nonconductive facings at the interface. If not kept within bounds, this external transition resistance can, under unfavourable conditions, lead to complete failure to keep within a tolerance range. It is absolutely essential that the materials used in potentiometer manufacutre be subjected to stringent quality control and be matched one with another.
The third component, the dynamic component, is related to high-speed movement of wiper driven by driving force. With the help of damped wipers, actuation speeds up to 10 m/sec will not create obvious resistance increase.
6. Causes of Linearity errors
There are two rensons for linearity errors:
Firstly, linearity errors could be caused due to electrical circuitry. Because of the exit of contact resistance, the wiper will be an ohm load when weak working current goes through it .For example ,the precision will be reduced by 1% when the current of the wiper is increased from 1μA to 10μA( Fig6).The smaller the contact resistance is, the less influence to linearity will be; the smaller the work current is, the less influence to linearity will be.
Fig6
Secondly, linearity errors could be caused due to mechanical coupling. If there is an axial misalignment (eccentricity) between the driving shaft and the shaft of a potentiometer for sensing angular, it will cause a linearity error.
7 .Output Smoothness
Output smoothness is an index for measuring any variation in the electrical output. It is expressed as a rate of the extra voltage exceeding theoretical travel increase to applied voltage. Output smoothness is influenced by contact resistance, resolution and other nonlinear factors.Fig7
Fig7
8 .Resolution
Resolution is used to measure the sensitivity of the output of the potentiometer. Because of conductive plastic’ characteristics, the resolution is infinitesimal and any tiny movement will not be ignored.
9 .Repeatability
During the observation of sensor’s repeated movements, we will find that even the value of mechanical input is the same every time, the outputs may slightly different from each other. Repeatability is expressed as the maximum difference of these outputs. Repeatability is mainly influenced by temperature, humidity, mechanical connections and other factors. Position sensor product of Minor in fig.8
Fig8
Records of the Repeatability of a potentiometer
Test condition：temperature 26℃，humidity 52%
Length of resistance track：150mm
Test result：Repeatability 0.01mm
10. Temperature and humidity coefficients
In many data sheets issued by potentiometer manufacturers, reference is frequently made to the temperature coefficient (Tk) and humidity coefficient (Fk) of the nominal resistance. Where potentiometers are used as voltage dividers (Fig. 2), these values are irrelevant. For this application, it is the Tk and Fk applicable to the voltage divider behaviour which are of significance. It often also happens that the humidity is not kept constant while making Tk measurements, with a result that a mixture of Tk and Fk is often given as a temperature coefficient. Detailed measurements made by Novotechnik have show that the Tk and Fk of the nominal resistance in conductive-plastic potentiometers (without housing) are of an order of magnitude somewhat less than 200 ppm/°C and 500 ppm/% RH respectively. The Tk and Fk of the voltage-dividing behaviour are some two orders of magnitdue lower which means that here changes within a range of less than 5 ppm/°C and 5 ppm/% RH can be expected, ensuring constancy over a wide temperature and humidity range. However, this advantage can only be utilized given a suitably designed housing and if, for example, no trimming resistances are used in the potentiometer circuit.
11. Hysteresis
Hysteresis is expressed as the diffenence between forward characteristic(input increases) and backward characteristic(input decreases) of the displacement transducer.It is usually determined by experiment ,the reasons of hysteresis include the gap of the drive mechanism, friction and elastic lag of elastic components and so on.
12. Service life
The magnitude of the contact resistance and the wear to which the resistance track is subjected and the resulting change in electrical characteristics dertermine the number of operations to which a potentiometer can be subjected and thus also its service life.
Although of considerable importance for industrial applications, no standard has so far been issued that specifies a service-life definition or a particular method of testing. It is, of course, most difficult to specify a value for wear or for an increase in contact resistance for a given number of actuation cycles since such values are markedly influenced by such external factors as temperature and humidity, and by mechanical and chemical influences.
Such values need to be established for each particular application. This applies to a lesser extent for the method of testing, and here the establishment of standard method would facilitate comparing the service life of various potentiometers.
Rapid developments in the fields of control engineering have brought the widely use of electronically controlled systems in all industries today. This has created a large demand for sensors with perfect performances, low cost and capabilities of working under hostile environment such as -40℃ to 150℃. This paper is concerned with quality criterias of conductive plastic potentiometers for being used as sensors for angular and linear motion. Such potentiometers primarily comprise the following components：
a. The resistance element (support material + a resistance track of conductive plastic) .The resistance track is the most important element of the position sensor.It has a conductive plastic paste laid on the substrate and can run smoothly with almost no abrasion. Minor uses Trimming track technology for trimming the resistance track to the required linearity. (Fig1)
Fig1b. A wiper (precious metal alloy). Wiper is the critical part of the transducer. It is supported by precious metals and of multi-fingers structure to provide long serving life (>100million cycles), low torque and high speed operation (10m/s) c. A driving shaft or actuating rod .
d. Bearings (ball bearing or plain bearing) .
e. Housing. It is normally made of aluminium with surface anodized. It has a strong structure and outstanding corrosion resistance.
2. Travel
L1 indicates the defined electrical travel. L2 indicates the continuous travel which also includes non-linear connection fields. L3 indicates the total electrical contact travel of the potentiometer. L4 indicates the mechanical travel. There are other parts which can not be brought into the whole travel. （Fig2）
Fig23. Independent linearity
If the wiper moves in a fixed direction, the relationship between the output voltage and the mechanically input value will be that as shown in Fig. 3. The maximum deviation of the potentiometer curve from an ideal straight line is referred to as independent linearity error. The error ±f is indicated as a deviation in percentage terms of the output voltage from the theoretical in relation to the input voltage. Typical values for independent linearity nowadays are between 0.2 % and 0.02 %. There is a test result of position sensor product by Minor as shown in fig.4
Fig3Fig4Testing results of the independent linearity of a potentiometer
Testing conditions：temperature 26℃，humidity 52%
Length of resistance track：150mm
Resistance: 5184Ω
Testing result：linearity 0.058%
4. Absolute linearity
Unlike independent linearity ，the reference slope of absolute linearity is fully defined so that there is no need for subsequent system trimming and the value of it becomes more and more essential. The definition of an index point establishes a relationship between the mechanical input value (travel or angle) and the output voltage. Because this index point is set according to work conditions, absolute linearity is more suitable for industrial process control. It is frequently necessary for the tolerance fields to be stepped Fig5 shows a practical example.
Fig55. Contact resistance
Contact resistance is the resistance between the wiper terminal and the wiper's immediate point of contact on the potentiometer's resistive track. As will be subsequently explained, this contact resistance affects all the important quality features of a potentiometer. Contact or transition resistance can be broken down into three components:
The first component, describes the integral voltage drop between the current-carrying track and the contact surface. This component is largely dependent on technology factors and amounts to several hundred Ohm.
The second component, the external component, is far more difficult to master than the first. This external transition resistance has much in common with the contact resistances occurring in switches and plug- and socket-connectors. It is caused by the transition between the wiper and the potentiometer track not being ideal from an electrical viewpoint. Metal oxides, chlorides and sulphides, mixed with various organic substances, can result in the formation of thin nonconductive facings at the interface. If not kept within bounds, this external transition resistance can, under unfavourable conditions, lead to complete failure to keep within a tolerance range. It is absolutely essential that the materials used in potentiometer manufacutre be subjected to stringent quality control and be matched one with another.
The third component, the dynamic component, is related to high-speed movement of wiper driven by driving force. With the help of damped wipers, actuation speeds up to 10 m/sec will not create obvious resistance increase.
6. Causes of Linearity errors
There are two rensons for linearity errors:
Firstly, linearity errors could be caused due to electrical circuitry. Because of the exit of contact resistance, the wiper will be an ohm load when weak working current goes through it .For example ,the precision will be reduced by 1% when the current of the wiper is increased from 1μA to 10μA( Fig6).The smaller the contact resistance is, the less influence to linearity will be; the smaller the work current is, the less influence to linearity will be.
Fig6Secondly, linearity errors could be caused due to mechanical coupling. If there is an axial misalignment (eccentricity) between the driving shaft and the shaft of a potentiometer for sensing angular, it will cause a linearity error.
7 .Output Smoothness
Output smoothness is an index for measuring any variation in the electrical output. It is expressed as a rate of the extra voltage exceeding theoretical travel increase to applied voltage. Output smoothness is influenced by contact resistance, resolution and other nonlinear factors.Fig7
Fig78 .Resolution
Resolution is used to measure the sensitivity of the output of the potentiometer. Because of conductive plastic’ characteristics, the resolution is infinitesimal and any tiny movement will not be ignored.
9 .Repeatability
During the observation of sensor’s repeated movements, we will find that even the value of mechanical input is the same every time, the outputs may slightly different from each other. Repeatability is expressed as the maximum difference of these outputs. Repeatability is mainly influenced by temperature, humidity, mechanical connections and other factors. Position sensor product of Minor in fig.8
Fig8Records of the Repeatability of a potentiometer
Test condition：temperature 26℃，humidity 52%
Length of resistance track：150mm
Test result：Repeatability 0.01mm
10. Temperature and humidity coefficients
In many data sheets issued by potentiometer manufacturers, reference is frequently made to the temperature coefficient (Tk) and humidity coefficient (Fk) of the nominal resistance. Where potentiometers are used as voltage dividers (Fig. 2), these values are irrelevant. For this application, it is the Tk and Fk applicable to the voltage divider behaviour which are of significance. It often also happens that the humidity is not kept constant while making Tk measurements, with a result that a mixture of Tk and Fk is often given as a temperature coefficient. Detailed measurements made by Novotechnik have show that the Tk and Fk of the nominal resistance in conductive-plastic potentiometers (without housing) are of an order of magnitude somewhat less than 200 ppm/°C and 500 ppm/% RH respectively. The Tk and Fk of the voltage-dividing behaviour are some two orders of magnitdue lower which means that here changes within a range of less than 5 ppm/°C and 5 ppm/% RH can be expected, ensuring constancy over a wide temperature and humidity range. However, this advantage can only be utilized given a suitably designed housing and if, for example, no trimming resistances are used in the potentiometer circuit.
11. Hysteresis
Hysteresis is expressed as the diffenence between forward characteristic(input increases) and backward characteristic(input decreases) of the displacement transducer.It is usually determined by experiment ,the reasons of hysteresis include the gap of the drive mechanism, friction and elastic lag of elastic components and so on.
12. Service life
The magnitude of the contact resistance and the wear to which the resistance track is subjected and the resulting change in electrical characteristics dertermine the number of operations to which a potentiometer can be subjected and thus also its service life.
Although of considerable importance for industrial applications, no standard has so far been issued that specifies a service-life definition or a particular method of testing. It is, of course, most difficult to specify a value for wear or for an increase in contact resistance for a given number of actuation cycles since such values are markedly influenced by such external factors as temperature and humidity, and by mechanical and chemical influences.
Such values need to be established for each particular application. This applies to a lesser extent for the method of testing, and here the establishment of standard method would facilitate comparing the service life of various potentiometers.