785 
ELECTRONIC RECEPTORS FOR TACTILE/HAPTIC* SENSING 
Andreas G. Andreou 
Electrical and Computer Engineering 
The Johns Hopkins University 
Baltimore, MD 21218 
ABSTRACT 
We discuss synthetic receptors for haptic sensing. These are based on 
magnetic field sensors (Hall effect structures) fabricated using standard 
CMOS technologies. These receptors, biased with a small permanent 
magnet can detect the presence of ferro or ferri-magnefic objects in the 
vicinity of the sensor. They can also detect the magnitude and direction 
of the magnetic field. 
INTRODUCTION 
The organizational structure and functioning of the sensory periphery in living beings has 
always been the subject of extensive research. Studies of the retina and the cochlea have 
revealed a great deal of information as to the ways information is acquired and 
preprocessed; see for example review chapters in [Barlow and Mollon, 1982]. 
Understanding of the principles underlying the operation of sensory channels can be 
utilized to develop machines that can sense their environment and function in it, much 
like living beings. Although vision is the principal sensory channel to the outside world, 
the "skin senses" can in some cases provide information that is not available through 
vision. It is interesting to note, that the performance in identifying objects through the 
haptic senses can be comparable to vision [Klatzky eL al, 1985]; longer learning periods 
may be necessary though. Tactually guided exploration and shape perception for robotic 
applications has been extensively investigated by [Hemami et. al, 1988]. 
A number of synthetic sensory systems for vision and audition based on physiological 
models for the retina and the cochlea have been prototyped by Mead and his coworkers in 
VLSI [Mead, 1989]. The key to success in such endeavors is the ability to integrate 
transducers (such as light sensitive devices) and local processing electronics on the same 
chip. A technology that offers that possibility is silicon CMOS; furthermore, it is 
readily available to engineers and scientists through the MOSIS fabrication 
services[Cohen and Lewicki, 1981]. 
Receptor cells, are structures in the sensory pathways whose purpose is to convert 
environmental signals into electrical activity (strictly speaking this is true for 
* Haptic refers to the perception of vibration, skeletal conformation or position and 
skin deformation. Tactile refers to the perceptual system that includes only the 
cutaneous senses of vibration and deformation. 
786 Andreou 
exteroceptors). The retina rods and cones are examples of receptors for light stimuli and 
the Pacinian corpuscles are mechanoreceptors that are sensitive to indentation or pressure 
on the skin, A synthetic receptor is thus the first and necessary functional element in any 
synthetic sensory system. For the development of vision systems parasitic bipolar devices 
can be used [Mead, 1985] to perform the necessary light to electrical signal transduction 
as well as low level signal amplification. On the other hand, implementation of synthetic 
receptors for tactile perception is still problematic [Barth et. al., 1986]. Truly tactile 
transducers (devices sensitive to pressure stimuli) are not available in standard CMOS 
processes and are only found in specialized fabrication lines. However, devices that are 
sensitive to magnetic fields can be used to some extend as a substitute. 
In this paper, we discuss the development of electronic receptor elements that can be used 
in synthetic haptic/tactile sensing systems. Our receptors are devices which are sensitive 
to steady state or varying magnetic fields and give electrical signals proportional to the 
magnetic induction. They can all be fabricated using standard silicon processes such as 
those offered by MOSIS. We show how our elements can be used for tactile and haptic 
sensing and compare its characteristics with the features of biological receptors. The 
spatial resolution of the devices, its frequency response and dynamic range are more than 
adequate. One of our devices has nano-watt power dissipation and thus can be used in large 
arrays for high resolution sensing. 
THE MAGNETIC-FIELD SENSORY PARADIGM 
In this section we show qualitatively how to implement synthetic sensory functions of 
the haptic and tactile senses by using magnetic fields and theft interaction with ferri or 
ferro-magnetic objects. This will motivate the more detailed discussion that follows on 
the transducer devices. 
DIRECT SENSING: 
In this mode of operation the transducer will detect the magnitude and direction of the 
magnetic induction and convert it into an electrical signal. If the magnetic field is 
provided by the fringing fields of a small permanent magnet, the strength of the signal 
will fall off with the distance of the sensor from the magnet. Such an arrangement for one 
dimensional sensing is shown in Figure 1. The experimental data are from the MOS 
Hall-voltage generator that is described in the next section. The magnetic field was 
provided by a cylindrical, rare-earth, permanent magnet with magnetic induction 
Bo=250mT, measured on the end surfaces.The vertical axis shows the signal from the 
transducer (Hall-voltage) and the horizontal axis represents the distance d of the sensor 
from the surface of the magnet. 
The above scheme can be used to sense the angular displacement between two fingers at a 
joint (inset b). By placing a small magnet on one side of the joint and the receptor on the 
other, the signal from the receptor can be conditioned and converted into a measure of the 
angle O between the two fingers. The output of our receptor would thus correspond to the 
output from the Joint Fibers that originate in the Joint Capsule [Johnson, 1981]. Joint 
angle perception and manual stereognosis is mediated in part by these fibers. The above is 
just one example of how to use our integrated electronic receptor element for sensing 
Electronic Receptors for TactileYHaptic Sensing 787 
skeletal conformation and position. Since there is no moving parts other than the joint 
itself, this is a reliable scheme. 
3.00 
E.63 
1 
1.13 
H89P n-tqpe (W=20m, L=800m) Uds=SV Bo=50mT 
Sensor 
_,,,,, - (a) 
Magnet e  
Sensor 
(b) 
0.00  , , , , , , , , 
O .GO 1.00 e.OO 3.00 q..00 5.00 &.00 ?.OO 8.00 9.eo 10. 
Distance of magne From sensor (mm) - 
Figure 1. Direct Sensing Using an Integrated Hall-Voltage Transducer 
PASSIVE SENSING 
In this mode of operation, the device or an array of devices are permanently biased with a 
uniform magnetic field whose uniformity can be disturbed in the presence of a ferro or 
ferri-magnefic object in the vicinity. Signals from an array of such elements would 
provide ifnormation on the shape of the object that causes the disturbance of the magnetic 
field. Non-magnetic objects can be sensed if the surface of the chip is covered with a 
compliant membrane that has some magnetic properties. Note that our receptors can 
detect the presence of an object without having a direct contact with the object itself. 
This may in some cases be advantageous. 
In this application, the magnetic field sensor would act more like the Ruffini organs 
which exist in the deeper tissue and are primarily sensitive to static skin stretch. The 
above scheme could also be used for sensing dynamic. stimuli and there is a variety of 
receptor cells, such as the Pacinian and Meissner's corpuscles that perform that function 
in biological tactile senses [Johnson, 1981]. 
788 Andreou 
SILICON TRANSDUCERS 
Magnetic field sensors can be integrated on silicon in a variety of forms. The transduction 
mechanism is due to some gaivanomagnetic effect; the Hall effect or some related 
phenomenon [Andreou, 1986]. For an up-todate review of integrated magnetic field 
sensors as well as for the fine points of the discussion that follows in the next two 
sections, please refer to [Baites and Popovic, 1986]. The simplest Hall device is the Hail- 
voltage sensor. This is a four terminal device with two current terminals and two voltage 
terminals to measure the Hail-voltage (Figure 2). A mag, letic field B in the direction 
perpendicular to the current flow, sets up the Hall-voltage in the direction indicated in the 
figure. The Hail-voltage is such that it compensates for the Lorentz force on the charge 
carders. In the experiment below, we have used a MOS Hall generator instead of a bulk 
device. The two current contacts are the source and drain of the device and the voltage 
contacts are formed by small diffusion areas in the channel. The Hall-voltage is linearly 
related to the magnetic induction B and the total current in the sample I between the 
drain and the source which is controlled by the gate voltage Vgs. 
V = KBI 
H 
3.00 
0.00 
MB9P n-tlpe (W=20Jm, L=OOm) Vds=SV 
ll=850mT 
J Ids VH G 
D 
 L 
11=-)50mT 
-3.00 
w 
Ug$ (V) 
Figure 2. An Integrated MOS Hall-Voltage Sensor and Measured Characteristics 
5.1 
Electronic Receptors for Tactile/Haptic Sensing 789 
The constant of proportionality K is related to the dimensions of the device and to 
silicon electronic transport parameters. The device dimensions and biasing conditions are 
shown in the figure above. Note that the Hall voltage reverses, when the direction of the 
magnetic field is reverse& The above device was fabricated in a standard 2-micron n-well 
CMOS process through MOSIS (production run M89P). The signal output of this 
sensor is a voltage and is relatively small. For the same biasing conditions, the signals 
can be increased only if the channel is made shorter (increase the transconductance of the 
device). On the otherhand, when the length of the device approaches its width the Hall- 
voltage is shorted out by the heavily doped source and drain regions and the signal 
degrades again. Some of the problems with the Hall-voltage sensor can be avoided if we 
use a device that gives a current as its output signal; this is discussed in the next section. 
THE HALL-CURRENT* SENSOR 
e_e M89P n-tgpe (W=12m, L=12m) Vds=350mV 
10-1o 
 0 
 Vds 
Vgs o yB= 50mT 
o J o 
D1 D2 
W 
Figure 3. The Hall-Current Sensor 
G 
i.0 
* Hall current is a misnomer (used also by this author) that exists in the literature to 
characterize the current flow in the sample when the HaH field is shorted out. 
790 Anareou 
This current is a direct consequence of the Lorentz force and is perpendicular to the 
direction of current flow without a magnetic field. The Loreritz force: 
depends on the velocity of the carriers in the sample and on the magnetic induction. Since 
this force is responsible for the transverse current flow a more appropriate name for this 
sensor is a Loreritz-current sensor. Obviously, given a magnetic field strength, if we 
want a maximum signal from our device we want the carriers to have the maximum 
velocity in the channel. We achieve that by operating the devices in the saturation region 
were the carders transverse the channel at what is called the "saturation velocity". In this 
configuration we can also use short channel devices (and consequently smaller devices) so 
that the high fields in the channel can be set with lower voltages. The geometry for such 
sensor as well as the biasing circuit is shown in the insets of Figure 3. As with the 
previous sensor, the magnetic field is applied in the direction perpendicular to the channel 
(also perpendicular to the plane of the gate electrode). 
This sensor is a MOS transistor with three terminals and the gate. It has a single source 
but a split drain. The polysilicon gate is extended in the region between the two drains so 
that they are electrically independent of each other. The device is biased with a constant 
drain-source voltage (with the two drains at the same potential) and the currents from the 
two sources are monitored. The current-mode circuitry for the synthetic neurons described 
by Kwabena [Kwabena et. al., 1989] can be employed for this function. We operate the 
device in the subthreshold region (denoted by the gate-source volrages between 0.5 and 0.9 
volts). On the application of a transverse magnetic field we observe the imbalance 
between the two drain currents. The Hall-current, plotted in Figure 3, is twice that 
current. 
Note that we can operate the device and observe an effect due to the magnetic field at very 
low currents in the nano-amp range. The graph in Figure 3 shows also the dependence of 
the Hall-current on the gate voltage. This is a logarithmic relationship because the Hall- 
current is directly related to the total current in the sample Ids through a linear 
relationship; it is also linearly related to the magnetic field with a proportionality 
constant Kh. The derivation of a formula for the Hall-current can be found in [Andreou, 
1986]. 
DISCUSSION 
The frequency response of the sensors described above is more than adequate for our 
applications. The frequency response of the Hall effect mechanism is many order of 
magnitude higher than the requirements of our circuits which have to work only the Hz an 
kHz range. Another important criterion for the receptors is their spatial resolution. We 
have fabricated and tested devices with areas as small as 144 square microns. These are 
comparable or even better with what is found in biological systems (10 to 150 receptors 
per square cm). On the otherhand, it is more likely that our final "receptor" elements will 
be larger, partly, because of the additional electronics. The experimental data shown 
above are only for stimuli that are static and are simply the raw output from our 
Electronic Receptors for Tactile/Haptic Sensing 791 
transducer itself. Clearly such output sigo_als will not be of much use without some local 
processing. For example, adaptation mechanisms have to be included in our synthetic 
receptors for cutaneous sensing. The sensitivity of our transducer elements may be a 
problem. In that case more sophisticated structures such as parasitic bipolar 
magnetotransistors or combination of MOS Hall and bipolar devices can be employed for 
low level signal amplification [Andreou, 1986]. Voltage offsets in the Hall-voltage sensor 
would also present some problem; the same is true for.current imbalances due to 
fabrication imperfections in the Hall current sensor. One of the most attractive properties 
of the HaH-current type sensor described in this paper is its ability to work with very 
lower voltages and very low currents; one of our devices can operate with bias voltage as 
low as 350mV and total current of lnA without compromising its sensitivity. Power 
dissipation may be a problem when large arrays of these devices are considered. 
Devices for sensing temperature can also be implemented on a standard silicon CMOS 
process. Thus a multisensor chip, could be designed that would respond to more than one 
of the somafie senses. 
CONCLUSIONS 
We have demonstrated how to use the magnetic field as a paradigm for haptic sensing. 
We have also reported on a silicon magnetic field sensor that operates with power 
dissipation as low as 350pW without any compromise in its performance. This is a dual- 
drain MOS Hall-current device operating in the subthreshold region. Our elements are 
only very simple "receptors" without any adaptation mechanisms or local processing; this 
will be the next step in our work. 
Acknowledgments 
This work was funded by the Independent Research and Development program of the 
Johns Hopkins University, Applied Physics Laboratory. The support and personal interest 
of Robert Jenkins is gratefully acknowledged. The author has benefited by the occasional 
contact with Ken Johnson of the Biomedical Engineering Department. 
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792 Andreou 
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Appendix 
Summaries of Invited Talks 
