Why the need for an Ergometer Calibrator?

By Andrew Huszczuk Ph.D. (Application Note No. 17800-A1)

Why Calibrate Ergometers

All cycle ergometers are essentially machines that convert pedaling motion or arm cranking into rotation of a mass in the from of a flywheel, upon which a braking force can be applied. This conversion is practically always associated with an increase of the speed of flywheel rotation with respect to the cranking rotational rate by a factor of 4 to 30 depending on the number of gear-up stages or principle of speed transmission utilized. Consequently, the following factors determine accuracy and the long-term performance of cycle ergometers:

Soundness of mechanical transmission determined by lubrication and storage conditions i.e. protection from corrosion and contamination by deposits of dust or other abrasive particles.
Consistency of the braking force applied to the flywheel by a variety of physical means such as friction, electromagnetism etc.
Accuracy of the braking force control from the point of view of temporal and thermal stability as well as utilization of feedback inputs such as temperature, torque and rotational speed.

All of the above factors can and usually do change the crucial performance parameters, although the resulting hindrance of the load setting accuracy may vary, usually from 5 to 20% of the intended one. Therefore, in research, diagnostic, health rehabilitation or even fitness training applications of cycle ergometry the correctness of the load setting and the pedaling cadence display becomes a matter of professional excellence, competence or neglect.

Some brands of cycle ergometers are equipped with simple means of periodic calibration, generally consisting of hanging known weights on a friction brake belt-tensioning gear or on a load cell torque feedback system. Still other, more advanced systems employ on-line monitoring and control of the transmission chain or belt tension as means of assuring the intended load setting. Although superior to no calibration at all, they do not and can not, however, verify the most important fact, namely, the amount of a rotational torque needed to be applied to the crankshaft to maintain rotation at any given cadence so that the amount of power expressed in Watts, Kilocalories/min etc. can be computed.

The most common causes of miscalibration appear to be:

Electronic control system out of tune; this can affect the slope of the calibration curve as well as its parallel shift.
Lack of maintenance, adverse storage or transport conditions or partial failure of bearing component usually shift the calibration curve upwards.
Programming mistakes, usually involving internal or external software, EPROM's etc. are most difficult to detect without a dynamic calibrator as it does incorporate the "reassuring" static weight hanging calibration procedure yielding proper readings with improper programming or computational assumption.

How the VacuMed Calibrator works

The Cycloergometer Calibrator is designed to measure a rotational torque applied to a crankshaft in order to maintain a chosen R.P.M. while a variety of braking loads is imposed upon a flywheel of a cycle ergometer by frictional, electromagnetic or other controllable means furnished by a manufacturer. This calibration process is intended to verify manufacturer's claims as to the accuracy of load settings expressed in Watts. It can also verify the constancy of a given setting across a range of cadence expressed in R.P.M. in ergometers, which posses a research grade mode of operation called "isopower" or "hyperbolic". This mode, much preferred by exercise scientists, automatically controls the amount of braking torque to yield a work rate independent of cadence according to the formula:

Torque x Cadence = Watts x k = const.

or:

Kg*m x R.P.M./k = Watts = const.,

where k is derived as follows, if:

1W = 6.118 kg*m/min.,

then for a rotational movement:

Power = Torque x R.P.M./6.118/2pi,

so: k = 6.118/6.2832 = 0.9737

Therefore, the calibration process consists of measurement of two variables - a rotational torque in kg*m and cadence in R.P.M., which are performed by means of a load cell and a tachometer respectively.

The source of rotary power consists of a D.C. motor and a worm-type speed reduction gear, rigidly mounted in a frame, which also hosts all additional electric and mechanical components of the apparatus. The output shaft of the reduction gear protrudes from one side of the frame in order to enable rigid coupling with the crankshaft of the cycle ergometer intended for calibration, whereas the input shaft is aligned and softly coupled with the shaft of the motor. The short rear end of this motor shaft holds the rotor of a tachometer, whereby the rotational speed (R.P.M.) can be measured and controlled.

The SCR type mains-powered motor speed controller allows adjustment of R.P.M. from 40 to 120 and regulation of the set R.P.M. regardless of the applied braking load.

The "S" beam load cell is mounted directly under the rear end of the motor. The whole load cell assembly can be either retracted into the underbelly of the calibrator for safety during transportation and storage, or locked in perpendicular position. In this perpendicular position it rests on the floor. A level adjuster is incorporated to position the calibrator horizontally using a water level as an indicator.

The control panel holds all the remaining electric and electronic components such as a load cell amplifier, an isolation amplifier with signal ratio circuits, a power supply, a panel LED display, adjustment potentiometers, fans, receptacles and switches.

The Calibration

Tools are included for removing the left crank of the ergometer. The user manual cautions that the cycle ergometer to be calibrated must be firmly secured in a steady position so that it cannot become unstable during calibration.

These are steps to follow:

Place a cycle ergometer on a firm even floor. Avoid carpeting. Provide ample space for approaching it from its left side.
Remove the left crank. (Detailed instructions are provided in the user manual).
Install the shaft coupler and firmly tighten the nut or bolt that originally held the crank. Again, detailed instructions are provided.
Insert the output shaft of the calibrator, with key and keyway aligned, into the coupler, thrust all the way in and tighten with the provided wrench. Level the calibrator as indicated by the water level.

Calibrating the Calibrator

To assure high accuracy of calibration, the calibrator itself needs to be briefly calibrated, which involves a few simple steps (detailed in the manual) that lead to the attachment of the provided calibration bar

Then:

Turn on the power switch and re-level the calibrator. Set the display selection switch to Watts or kg*m and adjust the reading to zero with the dial knob.

Hang a known weight (your own or the optional 5kg weight standard available from VacuMed) into the groove of the calibration bar. This groove has been machined at a distance of 66cm from the center of the output shaft, the resulting torque (using a 5kg calibration weight) can be computed as follows:<

Torque = 0.66 x 5 = 3.3 kg*m

which should appear on the display set to show torque values. If the reading differs from 3.30, adjust the trimpot labeled "CAL". Now, switch the display to Watts. It has been electronically ratioed to read Watts corresponding to a cadence of 60 RPM, which is widely regarded as a typical standard. The reading should reflect the following relationship:

Watts = Torque x 60/0.9737 = 3.3 x 60/0.9737 = 203 W

Remove the weight. Now the set-up is ready for calibration of the ergometer.

To ascertain the mechanical condition of the ergometer, begin calibration without turning its power on. Use the standard speed setting of 60 RPM as monitored by the panel meter, or via the output cable with your data acquisition system or just by an auxiliary voltmeter. (Instructions and calibration charts are in the user manual)

A typical power off, i.e. purely mechanical (frictional) wattage reading at 60 RPM is 10 - 20 W. If it reads more, your ergometer may need lubrication and cleaning. Now turn the power on and set the lowest available braking, which is usually from 25 to 30 Watts, as the lower settings are not reliable because of varying mechanical friction.

When setting different than 60 RPM cadence, derive actual Watts by multiplying the panel meter reading by the RPM/60 factor.

Example: Panel meter reads 100W at 90rpm,thus power = 100 * 90/60 = 150W

Proceed to higher settings at your preferred Watt increments. You may use the continuous ramp incrementation - a frequent experimental mode - to perform whole calibration by processing the data collected via the output cable and stored in your data acquisition system.

Conclusion

Ergometer calibration is essential in most research, clinical and fitness assessment settings, especially in applications such as:

Multi-center study involving many cycle ergometers, often of different make, age and maintenance history. Any comparison or pooling of data will be greatly compromised by nonuniformity of true versus intended load settings.
Health rehabilitation programs conducted in groups of patients with strictly prescribed exercise regimens requiring uniform performances of all available ergometers, whether "in house" or distributed to patient dwellings.
High-end athletic performance testing in training centers specializing in specific conditions (e.g. high altitude, hyperbaric or climatic chambers etc.) where lack of an ergometer's calibration will hinder interpretation.
Simple fitness tests required in military, police etc. to assess sustained qualifications for service. Often, rejected or disqualified individuals may challenge adverse decisions on the grounds of faulty measurement process.