PsychPowerMate - A driver for the Griffin Technology PowerMate USB knob.


This driver is currently only for the PowerMate USB edition, not for the
bluetooth product. It best supports modern Linux systems, e.g.,
Ubuntu 14.04.2 LTS or later, Debian-8 or later, or other similar modern
systems with version 2.9.0 or later of the “evdev” X11 input driver.

Support on Apple OSX and Microsoft Windows is much more limited. Expect
worse reliability, higher cpu load, higher latencies, less accurate
timing. While on Linux, the current driver can internally record up to
104 full knob rotations by default, or an unlimited number with suitable
options set, without losing information, the Windows variant can at most
record 5 rotations, and the OSX driver will probably not even manage to
record one full rotation. To avoid loss of position data, you will need
to call PsychPowerMate(‘Get’) frequently on OSX and Windows, even if you
do not need the information at that moment.

On Windows, setting LED brightness does not work.

Linux setup:

You must install a specific xorg input configuration file for the PowerMate
to be recognized. In a terminal window do:

  1. cd into the Psychtoolbox working directory (output of PsychtoolboxRoot))
  2. sudo cp PsychHardware/50-evdev-powermate.conf /usr/share/X11/xorg.conf.d/
  3. Logout and login again.

Interesting technical background info:

Technical background info, copied from the Linux powermate.c device driver,
written by William R Sowerbutts, the author of the Linux driver:

“Testing with the knob I have has shown that it measures approximately 94 “clicks”
for one full rotation. Testing with my High Speed Rotation Actuator (ok, it was
a variable speed cordless electric drill) has shown that the device can measure
speeds of up to 7 clicks either clockwise or anticlockwise between pollings from
the host. If it counts more than 7 clicks before it is polled, it will wrap back
to zero and start counting again. This was at quite high speed, however, almost
certainly faster than the human hand could turn it. Griffin say that it loses a
pulse or two on a direction change; the granularity is so fine that I never
noticed this in practice.”

  • The rotary resolution is therefore about 3.8298 degrees per “click” or unit.

  • The PowerMate can lose a click or two on a direction change, therefore can
    accumulate rotational error on each turn direction change.

  • The PowerMate wants to be sampled at 10 msec intervals according to its
    USB HID descriptor. Low-speed USB interrupt endpoint sampling interval
    duration must be a power of two, so it will actually get sampled at
    8 msec intervals, for a maximum update rate of 125 Hz.


deviceIds = PsychPowerMate(‘List’);
– Return a list of all ‘deviceIds’ of all connected PowerMates. Items from
this list can be passed as ‘deviceId’ parameter to the ‘Open’ function to
select a specific PowerMate. Please note that the PowerMates themselves do
not provide any identifying information, like a serial number, so the computer
can’t distinguish them. The ‘deviceIds’ you get back from this function are
therefore only guaranteed to be valid during a given session. Unplugging and
replugging PowerMates may change the ids assigned to them, based on operating
system specific criteria. On MS-Windows the deviceIds don’t change even
if you plug the PowerMate into a different USB port, so this function may
be useless for device selection and you may have to figure out something
specific to your computer hardware setup. On Linux you can assign fixed
ids to fixed USB ports. See the Technical note at the end of this help
text for how to do it. On OSX the deviceIds seem to be associated with
fixed USB ports, with numbers at least persistent during a session.

handle = PsychPowerMate(‘Open’ [, deviceId][, historySize=10000])
– Open a connected Griffin PowerMate, return a handle to it.

The optional ‘deviceId’ selects which of multiple connected PowerMates should
be used. PsychPowerMate(‘List’) returns a list of all possible ‘deviceId’s.
If ‘deviceId’ is omitted then the first found PowerMate is opened.

Linux only:

The optional ‘historySize’ parameter allows to select how many events the
knob press and motion history functions ‘StartHistory’ and ‘GetHistory’ can
collect in the background. It defaults to 10000 events if omitted, ie. up to
5000 knob presses + releases, or alternatively about 104 full knob rotations
can be recorded in the background before you either need to get the recorded
data via the ‘GetHistory’ subfunction to empty the internal storage, or the
internal storage will be full and stop collecting new data.

If you need to record more than 10000 such events in total, just specify a
larger ‘historySize’: Each button press or release creates 2 events (1 press +
1 release). Each turn of the knob by 1 unit (about 3.8 degrees) creates one

PsychPowerMate(‘Close’, handle);
Close previously opened PowerMate specified by ‘handle’.

PsychPowerMate(‘SetBrightness’, handle, level);
– Change brightness of the LED of the PowerMate specified by ‘handle’
to ‘level’. Level can be between 0 and 255. ‘handle’ is currently ignored
on Linux.

[button, dialPos] = PsychPowerMate(‘Get’, handle);
– Poll the PowerMate specified by ‘handle’, return the status
of its ‘button’ 1 = Pressed, 0 = Released. Also return the current
‘dialPos’ dial position of its dial - its turning knob. Please note
that the knob does not have a defined zero position. You can turn it
endlessly in one direction. Turning in one direction will simply
increment the ‘dialPos’ position, turning into the other direction
will decrement it. See above comments on the accuracy and resolution
of position reporting.

secs = PsychPowerMate(‘WaitButton’, handle [, waitforbuttonpress=1][, untilTime=inf]);
– Wait for button press or release on the PowerMate specified by ‘handle’,
return the GetSecs() time of press in ‘secs’.

If the optional ‘waitforbuttonpress’ flag is set to 0 then the function
waits for a button release, otherwise it waits for a button press.

If the optional ‘untilTime’ parameter is given, the function times out
at time ‘untilTime’, otherwise it waits forever if nobody presses or releases
the button.

secs = PsychPowerMate(‘WaitRotate’, handle [, untilTime=inf]);
– Wait for knob rotation on the PowerMate specified by ‘handle’,
return the GetSecs() time of rotation in ‘secs’.

If the optional ‘untilTime’ parameter is given, the function times out
at time ‘untilTime’, otherwise it waits forever if nobody moves the knob.

The following functions are only supported on Linux

PsychPowerMate(‘StartHistory’, handle [, clearHistory=0]);
– Start recording of a knob press and knob motion history in the background.
If the optional ‘clearHistory’ flag is set to 1 then the current history is
cleared/discarded before start of recording of a new one.

PsychPowerMate(‘StopHistory’, handle);
– Stop recording of knob press and knob motion history.
You can resume recording via ‘StartHistory’ or start recording a new one,
discarding old data via ‘StartHistory’ with the flag clearHistory=1 set.

history = PsychPowerMate(‘GetHistory’, handle);
– Retrieve currently recorded knob press and knob motion history.
history is a 4-by-n matrix with n columns for n recorded events.
Row 1 contains knob position. Row 2 contains knob button state: 0 = Released,
1 = Pressed. Row 3 contains a GetSecs timestamp of the element. Row 4 tells
if the given column records a knob press/release (0) or a knob movement (1).

numItems = PsychPowerMate(‘GetHistorySize’, handle);
– Return number of items currently recorded in history.

Technical note: How to map physical USB bus location to a /dev/input/powermateXX
symlink via a udev rule, then how to map that device file to a xinput device
name for enumeration/selection by our driver. Follow the approach outlined
at the bottom of this thread for dual-touchscreen setups with identical touchscreen:

Path   Retrieve current version from GitHub | View changelog