One of the most common misconceptions when it comes to using our 300C-I Dual Mode Indenter, is that instructing a force profile in software via the Force In channel, will result in the device instantaneously eliciting a response and some sort of movement profile. This blog is meant to explain why this is not the case and serve as a guide to performing quality force control experiments.
The motor assembly of the Dual-Mode Indenter is a moving coil galvanometer. The amount of electrical current flowing through this coil is proportional to the load, or force. This force can be measured by the lever system, but can also be controlled, while instead simultaneously measuring the displacement. Thus, if there is no load to control, there will be no movement or displacement.
We can use the Force Offset dial on the front panel of the controller to set the amount of current which can flow through the coil contained in the motor itself. The dial therefore acts as a force limiter for the motor. This can be observed by gently touching the indenter probe with your forefinger while it is attached to the motor shaft. You will notice the difference in the amount of resistance from the probe as you turn the Force Offset Dial fully counterclockwise from 10 to 0.10 in this case 10 being the maximum holding current (and hence the stiffest) and 0 being no holding current (and hence the limpest). The indenter probe will begin to move once your forefinger overcomes the set resistance. In this way, when the probe is moved by your forefinger, the resistance remains the same, (thus force is controlled) and displacement can be measured.
This force offset knob is useful for a static test such as above, but it would be impossible to set any sort of dynamic force control profiles with it alone. Setting a force profile using a software tool would be much more powerful. This can be accomplished by sending a voltage waveform to the Force In BNC, which is proportional to the desired instantaneous resistance.
In order to properly execute a force control experiment we should follow this procedure.
- Firstly, turn the force offset knob on the controller of your 300C-I Dual Mode Indenter to 0. (See figure 1) We must perform this step as both the contribution from the Force In BNC and the Force Offset Dial are additive. Thus, any contribution from the knob will make the commanded force seem higher.
Figure 1: Force offset control knob set to zero.