Interlink Electronics
®
FSR
Ring Sensor
Force Sensing Resistors®
Ring Sensor
Integration Guide
Integration Guide
Document P/N: EIG-10006 Rev. A
Interlink Electronics and the six dot logo are registered trademarks of Interlink Electronics, Inc.
www.interlinkelectronics.com
Ring Sensor
Integration Guide
Table of Contents
1.0
Introduction ........................................................................................................................ 1
2.0 Scope .................................................................................................................................... 1
3.0 Theory of Operation ......................................................................................................... 2
4.0 Mounting and Connection .............................................................................................. 4
4.1 Mounting ................................................................................................................ 4
4.2 Connection ............................................................................................................ 5
5.0 Measurement Techniques and Algorithms .............................................................. 6
5.1 Measuring Angle and Position ............................................................................... 7
5.2 Averaging Multiple Samples ................................................................................ 10
6.0 Actuator ............................................................................................................................. 11
7.0 Orderable Part Numbers .............................................................................................. 11
8.0 Intellectual Property and Other Legal Matters .................................................... 11
9.0 Contact Interlink Electronics ..................................................................................... 12
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Ring Sensor
Integration Guide
1.0 Introduction
The Interlink Electronics Standard Ring Sensor is a radial position sensor for menu navigation . It
functions as a rotary potentiometer with the ability to detect a user's position for scrolling and allows
for a more intuitive user experience. The Ring Sensor is an easy to integrate, high resolution sensor
ideal for media players, mobile phones, control panels, medical instruments and home
entertainment devices. Measurement is simple enough that it can be accomplished directly with the
device's host processor without the need for a dedicated microprocessor.
The purpose of this document is to guide users through the successful integration of the Interlink
Electronics Ring Sensor.
2.0 Scope
This Integration Guide provides the OEM integrator with all of the necessary technical information
to successfully integrate the Ring Sensor into products such as:
Personal media players
Mobile phones
Control panels
Medical instruments
Home entertainment devices
Sensor part number is detailed in section 7.
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Ring Sensor
Integration Guide
3.0 Theory of Operation
The Interlink Ring Sensor is designed to overcome the shortcomings of a simple angular
potentiometer. A normal ring-shaped potentiometer would have a ring of resistor material broken
by a small gap. Electrodes on either side of the gap would be used to place a potential across the
ring. Touching a wiper against the ring would bring the wiper to a potential that is proportional to
angle. The problem with this arrangement is that when the wiper touches the gap, the potential is
undetermined. If the wiper is broad (as it is with a finger) the wiper could touch both electrodes at
once. In this case the potential on the wiper would be some kind of average of the two electrodes.
To solve this problem, the Interlink Ring Sensor has a continuous ring of resistor. Three electrodes
that are separated by 120° of arc drive this ring. The method is detailed on the next page.
Figure 1: Exploded view of the Ring Sensor.
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Ring Sensor
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2
Integration Guide
At any one time, the two drivelines furthest from the point of touch are being driven.
The Interlink sensor is shown schematically in Fig. 2. The sensor has four connections. Three of
the connections are drivelines for the resistor ring. The fourth line is the sense line.
The algorithm for measuring this sensor is straightforward and can be implemented in any small
microcontroller. Described in Section 5 is an algorithm using one eight-bit ADC and three generalpurpose I/O lines. Instead of an ADC, one could also use an op-amp voltage follower and a slope
converter.
D
SL
RR
3
1
R
D2D3
Figure 2: Schematic representation of the Ring Sensor.
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Ring Sensor
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3
.
5
5
Integration Guide
4.0 Mounting and Connection
4.1 Mounting
There are a few critical elements to consider when mounting the Ring Sensor:
The mounting surface should be free of any raised features (e.g. copper traces on a PCB
or dust contamination) as they will interfere with the sensor’s proper operation.
If the sensor is being mounted to a PCB, it should be installed after PCB assembly is
complete. Heat generated during the soldering of components can damage the Ring
Sensor.
When laminating the sensor, be sure to use a hard roller or other depres sion tool to ensure
proper bonding of the sensor’s pressure-sensitive adhesive and the removal of any air
bubbles.
42.7
43.0
13.3
5.0
0
GRAPHIC
FSR SUBSTRATE
STIFFENER ADHESIVEREAR ADHESIVE
STI F F EN ER SUBSTRATE
Figure 3: Overall dimensions of the Ring Sensor. All dimensions are in millimeters.
USER
ACTIVE AREA
GRAPHIC ADHESIVE
8.0 STIF FENER
0
0
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Ring Sensor
Integration Guide
4.2 Connection
Recommended Tail Connector:
JST 4-pin SMT connector (JST Part Number: 04FM-1.0SP-1.9-TF)
Interlink recommends the above connector, but any compatible connector may be use d. Note:
Tail is two sided which requires a connector that has both upper and lower connections. Refer
to Figures 4 for sensor tail dimensions.
Figure 4: Top view of sensor tail. All dimensions are in millimeters.
Stiffener
Adhesive
Conductor
Substrate
Adhesive
Conductor
Substrate
Stiffener
Figure 5: End view of sensor tail.
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Ring Sensor
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Integration Guide
5.0 Measurement Techniques and Algorithms
The Interlink Electronics Ring Senor can measure radial position. The connection to the measuring
microprocessor is relatively simple and requires only a few external components.
1
AVX #04-6227-004, or equivalent
Resonat orWit hC aps , 4MHz
X1
3
4
PIN # REF
1
2
3
4
Figure 6: Ring Sensor pin out.
R1
1K
Sense
1
D1
2
D2
J1
2
3
D3
4
1
3
VCC
PINOUT
SENSE LINE (SL)
DRI VE LINE 1 (D1)
DRIVE LINE 2 (D 2)
DRI VE LINE 3 (D3)
VCC
VSS1
VSS2
19
U1 PIC16F73
RC0/T1OSO/T1CKI
RC1/T1OSI
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
2
RA0/AN0
3
RA1/AN1
4
RA2/AN2
5
RA3/AN3/VREF
6
RA4/TOCKI
7
RA5/AN4/SS
9
OSC1/CLKIN
10
OSC2/CLKOUT
1
MCLR
20
VDD
8
R2
1M
RB0/INT
RB1
RB2
RB3
RB4
RB5
RB6
RB7
RC6/TX
RC7/RX
21
22
23
24
25
26
27
28
11
12
13
14
15
16
17
18
Figure 7: Circuit schematic.
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Ring Sensor
Integration Guide
5.1 Measuring Angle and Position
In the following discussion, the sensor is oriented with the tail at 12 o’clock. 0º is straight north (12
o’clock) and the angle increases clockwise. The drivelines are at 0°, 120° and 240°. It is useful to
think of the ring as divided into three zones, with zone 1 defined as the 120° segment centered on
D1, zone 2 as the 120° segment centered on D2, and zone 3 as the 120° segment centered on
zone 3.
Determining the Touched Zone
The first part of the algorithm is to determine which zone is being touched. The goal is to determine
which drive line is closest to the point of touch, so that the other two drive lines can be used in a
later step to determine the precise touch location.
Finding which drive line is closest to the point of touch can be done quickly with three
measurements. In each measurement, two of the drivelines are set high and one is set low. The
closest driveline will be the one with the lowest ADC reading. Here are the detailed steps required
for this measurement:
1. Set lines D2 and D3 high, and D1 low.
2. Measure the sense line using the ADC. Call this V1.
3. Set lines D1 and D3 high, and D2 low.
4. Measure the sense line using the ADC. Call this V2.
5. Set lines D1 and D2 high, and D3 low.
6. Measure the sense line using the ADC. Call this V3.
Then following code can be used to determine which zone is being touched:
if(v1<=v2)
{
if(v1<=v3)
zone=1;
else
zone=3;
}
else
{
if(v2<v3)
zone=2;
else
zone=3;
}
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Ring Sensor
Integration Guide
Measuring the Angle of Touch
Once the zone of touch is known, the actual angular touch position can be measured. The angle
can be measured with a single measurement.
If zone 1 is being touched, then drive lines two and three will be used for the angle measurement.
Similarly for zones 2 and 3; the rule is to drive the two lines that are furthest from the touch point to
perform the angle measurement. Here are steps for measuring the angle if zone 1 is being
touched:
1. Configure driveline D1 as an input, effectively disconnecting it from the sensor.
2. Set D3 low and D2 high (so that the potential increases clockwise).
3. Measure the sense line using the ADC. Call this Vtheta.
One could improve angular resolution by measuring Vtheta several times and accumulating the
results. The calculation of the touch angle from Vtheta will be described in a later section.
Because the ADC could have a few counts of noise on it, a second measurement could be
performed to “sanity check” the results. This would be exactly the same as the measurement
described above, but with the drive lines set so that the potential increases counterclockwise. In
the zone 1 example, this means that D2 would be set low and D3 would be set high. Call the
voltage from this measurement Vtheta_inverse. One can then check that Vtheta and
Vtheta_inverse are complements, as in this code example:
if(abs(255-v_theta-v_theta_inv)<10)
sensor_active_flag=1;
If Vtheta and the complement of Vtheta_inverse are not equal to within some tolerance, then no
valid touch was found.
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Ring Sensor
Integration Guide
Calculating the Touch Angle from Vtheta
The voltage Vtheta was measured by applying 256 counts over an angle of 240°. Voltage can
therefore be converted to angle by multiplying by 240/256. Conveniently, 240/256 reduces to the
ratio 15/16. If the touch was in zone 1, 120° must be added to the angle. If the touch was in zone 2,
240° must be added to the angle. And if the touch was in zone 3, no offset is required. Finally, the
result should be checked to see if it exceeds 359°. The following code example shows the complete
conversion of Vtheta to angle theta:
theta=(v_theta*15)/16; //convert from voltage to angle
if(1==zone) //Add necessary offsets
theta+=120;
else if (2==zone)
theta+=240;
else if (3==zone)
theta+=0;
if(theta>359) //sanity check angle
theta-=360;
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Ring Sensor
Integration Guide
5.2 Averaging Multiple Samples
Because consecutive samples may straddle the rollover from 359° to 0°, a special trick is required
for averaging multiple samples. For example a simple average of 358° and 2° would give 180° but
it should give 0°.
Although several methods are possible, the following is very simple and does not require much
code. In this description, theta[0] is the most recent measurement, theta[1] is the next most recent,
etc.
First, calculate the average. Then compare the average with theta[0]. If the average and theta[0]
are too different (we have chosen 50°), then we assume that we have encountered the zerocrossing problem. In that case, we then offset all small angles by 360 and re-calculate the average.
If the result ends up greater than 359°, then subtract 360°. Here is a code example:
tempint=(theta[0]+theta[1]+theta[2]+theta[3])/4;
if(abs(tempint-theta[0])>50) //If avg is weird
{
tempint=0;
for(loop=0; loop<4; loop++)
{
tempint+=theta[loop];
if(theta[loop] < 50) //If angle is small
tempint+=360; // add 360°
}
tempint/=4;
}
theta_out=tempint/4;
if(theta_out>359)
theta_out-=360;
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Ring Sensor
Integration Guide
6.0 Actuator
The Interlink Electronics Ring Sensor has a protective graphic overlay; this allows for a finger or
custom stylus to be used.
7.0 Orderable Part Numbers
Hardware Development Kit (54-00024)
o QTY 1 Ring Sensor Demo
o QTY 10 Ring Sensors
o QTY 5 Connectors, 4 Pin
o QTY 1 USB Flash Drive with Product Literature
Ring Sensor Data Sheet
Ring Sensor Integration Guide
Ring Sensor, (40-24131)
8.0 Intellectual Property and Other Legal Matters
Interlink Electronics holds several domestic and international patents for its Force Sensing Resistor
technology. FSR and Force Sensing Resistor are company trademarks. All other trademarks are
the property of their respective owners.
The product information contained in this document provides general information and guidelines
only and must not be used as an implied contract with Interlink Electronics. Acknowledging our
policy of continual product development, we reserve the right to change, without notice, any detail
in this publication. Since Interlink Electronics has no control over the conditions and method of use
of our products, we suggest that any potential user confirm their suitability for their own application.
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9.0 Contact Interlink Electronics
United States
Corporate Office
Interlink Electronics, Inc.
546 Flynn Road
Camarillo, CA 93012, USA
Phone: +1-805-484-8855
Fax: +1-805-484-9457
Web: www.interlinkelectronics.com
Sales and support: sales@interlinkelectronics.com
Japan
Japan Sales Office
Kannai-Keihin Bldg. 10F/1004
2-4-2 Ougi-cyo, Naka-ku
Yokohama-shi, Kanagawa-ken 231-0027
Japan
Phone: +81-45-263-6500
Fax: +81-45-263-6501
Web: www.interlinkelec.co.jp
Ring Sensor
Integration Guide
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