PicoSTRAIN Overview

PICOSTRAIN OVERVIEW

Strain gauge measurement is one of the big challenges for sensor technology, especially the weighing industry. Due to the small excitation of the strain gauges on a load cell of only 0.2 % (2000 ppm) full scale,

and due to the high resolution of 16 Bit and more, the resistors must be measured with a precision deeply in the ppb range. Reasonably, this can only be done measuring the ratio of the resistors.

Here the PICOSTRAIN measuring principle is breaking new grounds compared to A/D converters. The ratio of the resistors is ascribed to a time interval measurement, not to a voltage difference like in Wheatstone bridges. The sensor's resistors together with a capacitor act as low-pass filters. The capacitor, charged to Vcc, is discharged through the sensor's resistors. The discharge time to trigger level (selectable) is precisely measured by a TDC (Time-to-Digital Converter).

PICOSTRAIN Applications

  • Load Cells
  • Torque Wrenches
  • Solar Scales
  • Kitchen Scales
  • Pressure Transducers

General Circuit Features

The PICOSTRAIN integrated circuits are realized without any analog component. Again, acam uses its TDC technology to set new standards in circuit design and takes the advantages of this principle like:


  • No elaborate supply of the strain gage
  • No need for a full-bridge, 2 resistors (half-bridge) are sufficient
  • Reduction of the current into the strain gage (total system current down to a few µA)
  • Easy temperature compensation
  • No reference voltage
  • Flexibility in update rate, precision and current consumption
  • Wide temperature range up to process limits


DescriptionVersionSizeDownload
Datasheet PS09 Vol. 2:
CPU - Single-chip solution for strain gauges
V1.08.9MBDB_PS09_Vol2_en.pdf
Datasheet PS09 Vol. 1:
General Data and Front-end Description
V1.01.8MBDB_PS09_Vol1_en.pdf
Datasheet PS09 Eval System:
Evaluation Kit for PS09
V0.17.2MBDB_PS09-EVA_en.pdf
Datasheet PS09:
DLC Evaluation Kit for PS09
V0.14.8MBDB_PS09-DLC-EVA_en.pdf
Datasheet PS081:
Enhanced Single-chip solution for weigh scales
V0.97.5MBDB_PS081_en.pdf
Datasheet PS081 Eval System:
Evaluation kit for PS081
V0.15.4MBDB_PS081-EVA_en.pdf
Datasheet PS021:
PICOSTRAIN front-end for strain gauge sensors
 870kBDB_PS021_en.pdf
Datasheet PS021 Eval System:
Evaluation system for PS021
 6.6MBDB_PSA21_en.pdf
Datasheet ALCS350-V2:
Load cell simulator
V0.11.6MBDB_ALCS350-V2_en.pdf
Datasheet PicoProg081:
Production Programmer for PS081
V0.41.8MBDB_PicoProg081_en.pdf
DescriptionVersionSizeDownload
Application Note 030:
PS09: Using PICOSTRAIN with piezoresistive sensors
V0.0657kAN030_en.pdf
Application Note 025:
PS081: EMI Countermeasures for a Digital Load Cell
V1.01.9MBAN025_en.pdf
Application Note 023:
PS081: Design Guideline for Building a Solar Body Scale
V1.34MBAN023_en.pdf
Application Note 022:
PS081: Design Guideline for Building a Solar Kitchen Scale
V1.34MBAN022_en.pdf
Application Note 021:
General: Compensation of gain error for uncompensated load cells
V0.1658kBAN021_en.pdf
Application Note 018:
PS08: Metrological investigations of PS08, Determining Zero Drift and Gain Drift
V1.02.8MBAN018_en.pdf
Application Note 012:
General: Strain gage wiring with PICOSTRAIN
97kBAN012_en.pdf
Product Report 0909:
PS081: ESD reliability report
V1.0240kBPR0909_en.pdf
DescriptionVersionSizeDownload
White Paper 004:
PS081: How to build Digital Load Cells with PICOSTRAIN conveniently
V1.05.2MBWP004_en.pdf
White Paper 003:
PS08: Millikelvin Resolution with only a few Microampere
V1.01.1MBWP003_en.pdf
White Paper 002:
PS08 / PS081: How to lower gain and offset drift of a load cell
V1.12.8MBWP002_en.pdf
White Paper 001:
PS08: Construction guideline for solar driven scales
V2.06MBWP001_en.pdf
DescriptionVersionSizeDownload
Screencasts PS09:
38:11 min. webcast introduction into the PS09 evaluation kit
-27MBPS09_Evaluation-Software_en.zip
Screencasts PS09:
19:01 min. webcast shows how to do a correct temperature compensation of a load cell using PS09 using the load cell's Rspan
-17MBPS09_Temp-Compensation-ext-Rspan_en.zip
Screencasts PS09:
17:54 min. webcast shows how to do a correct temperature compensation of a load cell using PS09 using the PS09 internal temperature sensor
-17MBPS09_Temp-Compensation_en.zip
Screencasts PS081/PS09:
20:28 / 40:30 min. webcast introducing a new concept for digital load cells
-11MBPS081_DLC-Interweighing_en.zip
Screencasts PS08/PS081:
13:36 min. webcast shows how to do a correct scaling of the HB0 measurement result
-105MBPS081_Scaling-of-Result_en.zip
Screencasts PS08-EVA-KIT:
23:23 min. movie introducing the PS08 assembler software
-78MBASM-Screencast_en.zip
DescriptionVersionSizeDownload
Software PS09:
PS09-EVA-KIT, PICOPROG v2.0
V2.0281MBCD-PS09-EVA-PICOV20-XP-WIN7.zip
Software PS081:
PS081-EVA-KIT, PICOPROG v2.0
V2.0296MBCD-PS081-EVA-KIT-PICOV20-XP-WIN7.zip
Software PS081:
PS081-DLC-KIT, PICOPROG v2.0
V2.0296MBCD-PS081-DLC-KIT-PICOV20-XP-WIN7.zip
Software PSA021/PS021:
PSA021 / PS021-EVA-KIT, COM Port
99MBCD-PSA21-EVA-KIT-COM-XP.zip
Software PicoProg V3.0 (V2.0):
Driver Installer Stand-Alone
R115MBPicoProg_v2_Driver_Installer_Stand-Alone.zip
Software National Instruments:
NI VISA runtime engine for Windows 7
V4.6253MBvisa462runtime.exe
Software National Instruments:
NI VISA runtime engine for Windows Vista
V4.141MBvisa410full.exe
Software National Instruments:
NI VISA runtime engine for Windows XP
V3.214MBvisa320runtime.exe
PICOSTRAIN Measurement Principle

The capacitor is charged to the supply voltage and then discharged through one of the SG resistors. The discharge time down to an arbitrary trigger level is measured with ultra-high precision using a TDC (Time-to-Digital Converter). The discharge time is in the range 100 µs. The TDC unit used have a typical single-shot resolution of less than 20 ps.

This measuring process is repeated in time-multiplex with both resistors of a half-bridge, using the same capacitor and the same comparator. Calculating the ratio of the results will turn out the absolute values and temperature dependencies of the capacitor and the comparator.

Additional patented circuits and algorithms inside the products compensate for further error sources like the switch-on resistance of the output drivers (Rdson) and the propagation delay of the comparator. The result is very precise, nearly free of gain errors and very stable with temperature. In total each single measurement is made of 8 discharge/charge cycles to solve this compensation task.

Due to the measuring principle, does not need a full-bridge but a half-bridge is sufficient. The supply of the half-bridge is provided directly by the circuits. There is no need for a separate supply of the SG. Also the reference voltage is not required.

Thanks to the pulsed drive easily controls the current through the whole system and, even more important, reduces the current consumption to re-markably less than comparable ADC systems.

The measuring principle is showing a new approach to strain gage (SG) measurement. Contrary to the Wheatstone bridge, where the variation of resistance is transformed into a variation of voltage, solutions transfer it into a high-precision time interval measurement. For this purpose the SG resistors are connected to a capacitor, forming a low-pass filter.

Measurement Principle
(click to enlarge)

PicoStrain Background : Measurement Task

Metal strain gages (SG) change their value with mechanical deformation, especially a variation in length. The strain e designates the relative variation in length of the SG:

Strain (e) = dL/L

Common SG have a maximum strain of typical

e(max) = 1000 µ (1000 x 10-6 or 0.1 %).

The ratio of the resistance variation to the length variation is designated K-factor or strain gain.

dR/R = K x dL/

For metal SG the K-factor is typically of value 2. The maxim variation of the SG resistance is then given as:

dR(max)/R = e(max) x K = 2000 ppm

If the SG is connected in the manner of a Wheatstone bridge, this corresponds to a maximum signal output voltage of 2 mV/V. The resistance of common metal strain gauges is typically 350 Ohm or 1000 Ohm. The maximum variation in resistance and therefore the effective measurement range is within 0.7 Ohm to 2 Ohm. This small variation must be resolved according to the measurement task. The range of the resolution needed is very wide. It is between 10 ENOB (e.g. for pressure sensors) and 18 ENOB (e.g. calibrated scales). In the upper range the precision of the measurement has to be:

Resolution : 2000 ppm/218 = 0.008 ppm eff.

or 26.9 ENOB referenced to the full resistance.

The typical measurement rates are in between

2 - 8 Hz (e.g. scales) and

4 - 10 kHz (e.g. fast pressure sensors).

picostrain advantages

ADVANTAGE 1

Extremely low current consumption for your complete bridge sensor system

The ultra-low power PICOSTRAIN measuring principle shows an extremely low overall current consumption of the total system. It is possible to reduce the power consumption by a factor 10 to 20 compared to conventional AD-Converter solutions.

ADVANTAGE 2

One solution suits to (nearly) all applications, from low cost up to highest precision

PICOSTRAIN allows to build systems with 1.500 ... 150,000 peak to peak divisions (at 2 mV/V strain) and update rates up to 1 kHz. One and the same chip covers the requirements of many applications, just by variation of your software.

ADVANTAGE 3

Reduced overall system costs

PICOSTRAIN reduces or saves the costs of the power supply. Power plugs will be replaced by batteries, batteries will be reduced in size. In many applications a single coin cell battery may last for 10 years under normal operation. The number of external components is far below the usual.

ADVANTAGE 4

New products that could not be done up to now

There are new product options mainly due to the low current consumption and low operating voltage. First to mention are solar driven applications. Also 1.55 V silver-oxide batteries are possible although with some restrictions (e.g. re-programming). The measurement quality is impressive even at those low voltages and it might be difficult to find something comparable in the market.

ADVANTAGE 5

Improve your Quality and lower your production cost

PICOSTRAIN offers unique possibilities to simplify the production of your bridge sensor and to improve its quality. The bridge offset can be simply adjusted by setting a register, gain drift can be reduced through software correction of Rspan. This method is more precise than with standard solutions. At the same time costly mechanical trimming, wich is still common practice, becomes redundant.

TypePart numberPackageRoHS compl.Shipping package
PS09:
PS09MNR 1783DiceYesWaffle pack
PS09FNMNR 1840QFN40Yes
PS09-EVA-KITMNR 1785PCBYes
PS09-DLC-EVAMNR 1927PCBYes
PS081:
PS081MNR 1615DiceYesWaffle pack
PS081FNMNR 1612QFN56Yes
PS081-EVA-KITMNR 1525PCBYes
PS021:
PS021MNR 1002TQFP48YesTray
PS021FNMNR 1001QFN48YesTray
PSA21-STDMNR 984PCBYes
PSA21-WSBMNR 985PCBYes
PSA21mini-STDMNR 990PCBYes
PSA21mini-WSBMNR 991PCBYes
PicoProg:
PicoProg Uni System V2.0MNR 1723PCBYes

ADDITIONAL INFORMATION