Sensable

[Vygreif]

Hi,
I don't know if you guys are the right people to ask about this but here I go anyways. I'm using sensable to control a cursor, and I want it to become difficult to move the cursor at certain times without there being an object in the way. Or to be able to freeze up the joystick. As far as I can tell in the demos you have to have objects which have friction on their surface. Is there anyway around that?

Hope I was clear,
Yaron Greif

[farshizzo]

Hi Yaron,

The OpenHaptics API supports exactly what you want. You can add a constant force that moves against the movement of the pen. Unfortunately there is no interface for this feature within Vizard. I could probably add this feature to the plugin, I'm not sure how soon I could get to it. Do you need it before a certain date?

[Vygreif]

Hi,
We want to start using this code for experiments Friday. So if possible could it be done by Wendsday or Thursday afternoon?

-Yaron Greif

[farshizzo]

Hi,

I've just finished adding these features. I'm currently making some samples script . It should be ready by tomorrow.

[farshizzo]

Hi,

The new plugin is on the Vizard download page under the "Vizard - Developer Tools" section, titled "Haptic Effects".

[Vygreif]

Hi,
Thanks for the quick response time. I'll take a look at it tomorrow.

-Yaron

[Vygreif]

Hi,
So I was using the plug-in you supplied to apply random forces of magnitude
1-5 in random directions in overlapping time periods and the device ended up breaking, we think a cable snapped. Probably it just couldn't handle the jerkiness of the program, but can you think of any problems from the software side? What happens when you exert force a for 5 seconds, and force b only 3 seconds later? Just putting this up to warn future similarly foolhearty programmers.

-Yaron Greif

[farshizzo]

Wow, that sucks!

I would have assumed that the device would have some mechanism for ensuring the force doesn't exceed a certain limit. Here is a paragraph from the OpenHaptics documentation regarding simultaneous effects:

HLAPI allows for multiple force effects to be rendered at the same time. All
of the effects will be combined to produce one force to be rendered to the
device. Also note that the built-in effects utilize the proxy position as input, since this allows for stable effect force rendering while feeling shapes.
Therefore, it is possible to contact a shape while an ambient friction effect is
active or to be constrained to a shape while resisting a spring effect.

Also, here is a section of the documentation on force kicking:

Kicking is caused by large discontinuities in force magnitude.

...

Unintentional kicking is usually a result of the application conveying an
overly large instantaneous force to the device; in other words, a force
magnitude that exceeds the physical capabilities of the device. Safety
mechanisms such as enabling: HD_MAX_FORCE_CLAMPING,
HD_SOFTWARE_FORCE_LIMIT, or
HD_SOFTWARE_VELOCITY_LIMIT,
HD_SOFTWARE_FORCE_IMPULSE_LIMIT can help catch these errors
and either signal an error or put a ceiling on the amount of force that can be
commanded.

...

During development, it is important that the developer take care in
commanding forces and handling the device during operation. The device has
both software and hardware safety mechanisms for maximum force, but this
does not guarantee that it will never give a substantial kick. In particular,
applications typically assume the device is being held. Small forces that are
felt when the user is holding the device may generate a large velocity if the
device is free. To mitigate this, either artificially scale down forces when
experimenting with force effects, or use
HD_CURRENT_SAFETY_SWITCH to prevent forces from being
commanded inadvertently.
It is strongly advised that the developer not disable safety routines such as
HD_FORCE_RAMPING and HD_SOFTWARE_VELOCITY_LIMIT, and
HD_SOFTWARE_FORCE_IMPULSE_LIMIT unless there is some
compelling reason to do so. Those provide a level of safeguard for protection
of the device and user.

FYI, the hd plugin does not explicitly disable any of this safety limits, so I'm not sure why this happened. You should contact Sensable about this.