Novel Temperature Compensation Structures for Multi-axis Force Sensors for Microassembly

Miniature force and torque sensors are a fast growing segment of MEMS devices. While accelerometers, gyroscopes, and pressure sensors are well developed and widely used, multi-axis sensors with high sensitivity are not yet established.

There are a wide ranging number of applications for force sensors including, but not limited to, minimally invasive surgery, 6 DOF accelerometers, contact surface mapping, and force-feedback assisted microassembly. Of particular interest is the use of force sensors in force-feedback assisted microassembly, an area where highly sensitive multi-axis sensors show promise. Microassembly is the process of manipulating micro-parts to construct an engineered device. On the microscale, the challenges to manipulating parts are far different than on the mesoscale. The reasons for this are:

   1. Part tolerances are generally rather poor.
   2. Optical feedback is limited because of the lack of depth at higher levels of magnification.
   3. Parts are easily overloaded by forces relatively small by normal standards.
   4. Adhesion forces have a significant effect on part handling.

To effectively overcome these problems, new multi-axis force sensors are being developed. Multi-axis force sensors have the capability to address the challenges facing microassembly through force-feedback control. Force-feedback could supplement optical techniques and reduce the effect of poor part tolerances by independently providing tactile perception data in a machine usable format. A machine could use the data provided by a sensor to determine positioning, part loading, and field effects upon a micropart.

In order to meet some of the demands required for accurate part manipulation and assembly, a force sensor must have a number of specific characteristics. Microparts weigh in the micro-Newton or even nano-Newton range. To detect the presence of such parts requires the ability to resolve forces at very low levels. In addition, parts are very fragile in comparison with mesoscale forces and can be easily broken. Therefore, a force sensor must have a high resolution and a high sensitivity at very low levels of force in order to be effective in the simplest tasks of micromanipulation.

 
Temperature Compensation

Piezoresistive devices are commonly used in force sensing applications due to their high sensitivity, good linearity, and absence of hysteresis. This combination of characteristics makes them appealing for force feedback control in microassembly. A piezoresistive sensor based on similar techniques employed in the fabrication of MEMS pressure sensors was developed. The sensor was intended to be used in conjunction with an electrostatic gripper, which would then pick up the parts for manipulation. A model of this sensor is shown below with an assembly view depicting the mounting of a thoerized gripper.

However, while piezoresistive devices have many suitable characteristics for multi-axis force sensors, piezoresistance is a function of a number of variables including device orientation along the crystalline structure, stress, temperature, and doping concentration. Temperature is of primary interest because of the difficulty in maintaing a precise, continuous temperature across the entire sensor. The maximum value of mechanical stress effects in piezoresistive elements are on the order of 1% of total resistance. In comparison, a temperature change of 1° C across an element can cause a resistance change of as much as 0.2% total resistance. Hence, minute variations in temperature across a piezoresistive element can cause an aberrant signal reducing the resolution of the sensor. This is not desirable in a sensor that needs to be sensitive on the microNewton scale.

Traditionally, the temperature effects are combated by using Wheatstone or Hall-like type piezoresistive elements. These elements mitigate the effects of temperature in the ideal case by changing temperature at the same rate but not changing in voltage output. However, in practice these elements are not ideal. Piezoresistors even microns apart have tracking errors inherent in the batch fabrication processes used in silicon processing. While the effects of temperature can be reduced by these structures, they cannot be eliminated. Other techniques to aid in temperature compensation have been developed, but none of them allow for temperature measurements in very close proximity to the affected structures.

In order to allow for the effects of fluctuating temperature in a variable stress environment, new resistors in the shape of an annulus were developed that are insensitive to stress under uniform stress conditions. These structures provide temperature measurements indpendent of stress in very close proximity to the stress sensitive elements, on structures which experience high levels of stress during measurements. This allows for the stress measurement devices to be thermally compensated to increase resolution and decrease the presence of drift and hysteresis.

Test Results

Testing of the annular resistors has showed the resistors have a near zero response to stress under multiple loading conditions. The resistors are still responsive to temperature changes as expected. Data tests of a single Wheatstone type element used in conjunction with an annular resistor are shown below.

Load tests showing the insensitivity to stress

Uncompensated load tests with thermal disturbance

Load tests with compensation from Annular Resistor