Strategies For Controlling Several Powered Devices

When it is not possible to control two devices independently, mode selection schemes are used to shift one good control source like two good myoelectric sites so that two or duet devices can be controlled. The Boston Elbow II, the Utah Artificial Arm and, Otto Bock 2-Channel and 4-Channel controls all use simple mode selection. Selection schemes may be operated bythe signals themselves or by external switches. They may revert to a default state, to a ready state or they may cycle tluough states Liberty Teclutology VariGrip circuits permit practitioners to apply mode selection to devices from many manufacturers of wrists, elbows and hands. MODE SELECTION SCHEMES A method for shifting one control source so that several devices are contr olled is a mode selection scheme These schemes fall into two categories, those that use the control signals themselves to select the mode and those that use some other mechanism such as a separate switch Each has its advantage and disadvantages Both categories have certain criteria in common. Design criteria. For analyzing which mode selection schemes are best, design criteria ate required Listed below are six aiteria applicable to any mode selection scheme. These criteria can be used to examine the schemes currently in use and new schemes as they occur. Criteria for a Good Mode Selection Scheme 1. Selection is easy and quick 2 Incorrect selection has a low probability 3. The user knows which mode is in use 4 Selection does not interfere with the control task The scheme is easy for the user to learn The technician can set up the system easily Change initiated by an unchanging signal. The original Utah Artificial Arm is a good example of the use of an unchanging signal to change mode In this case the myoelectric signal is held constant for a defined period of time to shift control from the elbow to the terminal device. Another scheme is used to retum control to the elbow. This same scheme is used in the Boston Elbow II with either the single Touch Pad input or the positional servo 112 IInstitute of Biomedical Engineering input However, the unchanging signal merely parks the elbow rather than shifting control in a scheme best called "set it and forget it " It is important to note that with any single-channel servo scheme the device being controlled must produce a position reference signal Motion Control modifies Bock hands to produce such a signal so that in the Servo Pro Arm an unchanging signal shifts mode Change initiated by a lack of signal. A special case of the unchanging signal is a lack of signal. Here a mode change can be initiated whenever the signal stays below an arbitrary level for a predetermined interval This level is set based on the noise level of the signal Lack of signal is usually used to reset the system Change initiated by a fixed time delay. If there is a default control mode, there are several ways to return to the default The simplest merely returns control after a fixed interval in the selected mode A fixed delay may be too short so the user has not finished the selected task or too long so the wait for default return is too long or a given delay may present both problems The reset problem. There are two reset conditions which can be used in system design In the fix st the system resets to one of the control modes called the default mode Here the system will respond immediately to signals commanding motion of the default device in either direction In the second the system tesets into a "ready" mode This is the normal case when the signals themselves are to be used to initiate mode selection These two reset schemes will be referred to as "reset to default" and "reset to ready " RATE-OF-RISE SELECTION SCHEMES From a ready state one or two signals can be use,d to select several modes by using the initial rate of increase in the signals to select a mode Consider Figure 1 where two signal thresholds are shown,. The lower threshold LI depends critically on the noise in the signal when the user is gene' sting no signal OT the signal level that is achieved immediately after the user stops commanding a signal,. With myoelecuic signals this threshold is relatively high A quick-rise signal will remain in the area between LI and L2 for a time less than To and will select the "quick" mode while a slow-rise signal will remain in this region longer than To and will result in selection of the "slow" mode This scheme is used by Otto Bock in the One-Site Double Channel hand (8E38) and is also used to choose between wrist and hand control in the Fout-Channel Control Unit (13E103) These Bock systems offer fixed-speed control To date the simple quick-slow scheme has been used to select a device and direction simultaneously,. As soon as the signal goes below threshold L 1, the system returns to ready. Once selection has been made, any signal from a second muscle or signal generator is ignored This may not be desirable. Users often over shoot especially when positioning the forearm with the elbow. Over shoot is most easily corrected if the mode for the degree of freedom remains selected until both signals remain below tlueshold for a reset interval TR which may differ from TO Rapid coactivation. With two muscles or signal sources a third mode can readily be added by using rapid coactivation The selection criterion is that both sources meet the quick-rise criterion within a short interval in the order of To With myoelectric signals rapid cocontaction is easy if the two muscles are an agonistantagonist pair like biceps-triceps, but difficult or impossible for other muscle combinations From "MEC 95," Proceedings of the 1995 MyoElectric Controls/Powered Prosthetics Symposium Fredericton, New Brunswick, Canada: August, 1995. Copyright University of New Brunswick. Distributed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License by UNB and the Institute of Biomedical Engineering, through a partnership with Duke University and the Open Prosthetics Project. University ofNew Brunswick With biceps and triceps it is easy to select one mode (grasp) with quick contractions, a second mode (flex-extension) with slow contractions, and a third (wrist rotation) with rapid cocontractions. Once the mode is selected, control may be fixed-speed or proportional Fig.. 1., In a rate-of-rise circuit, mode selection is done using a window about 60 msec long SWITCH INITIATED MODE SELECTION If the signals themselves do not do the mode selection, some other device must be used Typically this is a momentarily or continuously activated switch. For many years the older Boston Elbowused a continuously activated switch to select elbow operation with TD control the default This system has been used with myoelectric and switch control, and has recently been retrofitted to use Liberty Touch Pads which are force sensing resistors (FSRs). This scheme is also available with the Boston Elbow U. It is often rejected apriori by fitters, but has worked surprisingly well for many users. Cyclic mode selection. It is easy to design a selector which cycles through two or three modes a, b; a, b or a, b, c; a, b, c This is currently the mostpopular selector for Boston Elbow Select and rmert. This scheme was first used for multimode control of the Boston Elbow by engineers at H Steeper in London in the mid eighties. Control starts in a default mode and is cycled to a second mode by one rapid switchclosure and to a third mode by two rapid-closures. Either of these modes revert to the default when the control signals go away or after a predetermined time. Note that this scheme will also work if the switch closure is replaced by a rapid coactivation This is an ideal way to control mode selection if the user can generate a reliable rapid cocontraction Clearly not all of the schemes discussed above are how commercially available. All have been tried in the process ofdeveloping Boston Elbow IL Those with experience in high level upperextremity powered devices will be well aware that each scheme has some major flaw for some patients 'Thus the long term goal withBoston Elbow ll is to make it easy for the fitter to work with the patient to easily choose a mode selection scheme appropriate to that user. In the meantime, some of the ways several rate-of-rise schemes can be used together are discussed below PO (a) (b) Two Independent VariGrips One Input Controls TD and One Input Controls Wrist Rotator Two Independent VariGrips One Input Controls TD and One Input Controls Elbow Fig. 2. Independent control of two devices V = VariGrip controller. M Motor 115 Institute of Biomedical Engineering