There are two general types of tests to evaluate McKibben actuators:
- Actuator length held constant (isometric), and the force measured as the actuator is pressurized
- Actuator pressure held constant, and the force-contraction relationship measured
Both tests are carried out using tensile testing machines (i.e. Instron) which are typically used for evaluating material properties.
In isometric testing, actuators are clamped at both ends, with one clamp having a load cell to measure forces. The clamps are fixed, holding actuator length constant while the actuators are pressurized. Pressure is measured with a pressure transducer attached to the actuator air supply line.
Both quasistatic and dynamic tests can be performed:
- Quasistatic test: pressure is ramped from 0kPa to some maximum pressure and back using a pressure regulator, over the course of ~1 minute (slow).
- Dynamic test: a solenoid valve is opened rapidly (millisecond time scale) to quickly deliver air from a pressurized chamber to the actuator.
Quasistatic testing yields a force-pressure curve. McKibben actuators show a very linear force-pressure relationship, as seen in the below graph. The vertical line at the low pressure end of the graph is due to the fact that the pressure transducer was not sensitive enough.
Dynamic testing yields force-time curves and can be used to evaluate how quickly actuators respond to the pressure input.
Constant pressure test
Once again, the actuators are clamped at both ends, with one end attached to a load cell. However, this time one end is allowed to move. While pressure is held constant by an accumulator, actuators are allowed to contract until no load is measured at the supports, and then are stretched back to their original length. The displacement is measured and used to calculate percent contraction (displacement/initial active length).
As expected, force decreases with increasing contraction, regardless of the various constant pressures at which the tests were conducted.
It is important to determine the pressure limits of each actuator design, especially when it comes to the safety of using these actuators in various applications. A simple method for this is to gradually pressurize the actuator (with no load attached) until it fails. However, since failure happens very rapidly, it is helpful to film the actuator with a pressure readout (of a pressure sensor in-line with the actuator) placed next to it.
Fatigue testing can also be conducted by using an automated system like the control board to cycle the actuator continuously between relaxation and a set contraction amount while tracking the number of cycles. Depending on construction, McKibbens can last anywhere between a few hundred and a few million cycles.